Mil/Thou (mil) - Unit Information & Conversion

Quick Answer

One mil equals exactly one thousandth of an inch (0.001″ = 25.4 μm). A human hair is approximately 70-100 mils thick, a standard sheet of printer paper is about 4 mils, and a credit card measures roughly 30 mils. The mil provides precision measurement for manufacturing tolerances, coating thickness, and electronic component spacing in industries where imperial units remain dominant.

Quick Comparison Table

Measurement	Mils	Inches	Millimeters	Common Item
Human hair (average)	70-100 mil	0.070-0.100″	1.8-2.5 mm	—
Sheet of paper (20 lb)	4 mil	0.004″	0.1 mm	Standard printer paper
Business card	10-14 mil	0.010-0.014″	0.25-0.36 mm	Standard cardstock
Credit card	30 mil	0.030″	0.76 mm	ISO standard
Plastic wrap (food)	0.5-1 mil	0.0005-0.001″	12-25 μm	Kitchen cling film
Garbage bag (standard)	0.7-1.5 mil	0.0007-0.0015″	18-38 μm	13-gallon kitchen bag
Aluminum foil (standard)	0.6-1 mil	0.0006-0.001″	15-25 μm	Reynolds Wrap thickness
PCB copper trace (1 oz)	1.4 mil	0.0014″	35 μm	Standard circuit board
Paint coating (automotive)	4-6 mil	0.004-0.006″	100-150 μm	Base + clear coat
Typical machining tolerance	±1-5 mil	±0.001-0.005″	±25-127 μm	General manufacturing

Definition

What is a Mil?

A mil (also called thou) is a unit of length in the imperial measurement system equal to one thousandth of an inch. The mathematical definition is:

1 mil = 0.001 inch = 1/1000 inch = 25.4 micrometers (μm) = 0.0254 millimeters

The mil is not a metric unit and should not be confused with the millimeter (mm), despite similar-sounding names. This confusion is one of the most common measurement errors in international manufacturing:

• 1 mil (imperial) = 0.0254 mm = 25.4 μm
• 1 millimeter (metric) = 39.37 mils = 1,000 μm

The mil exists as a natural subdivision of the inch, providing a convenient decimal-based measurement for very small imperial dimensions without resorting to awkward fractions like 1/1000″.

Mil vs. Thou: Regional Terminology

The terms "mil" and "thou" are completely interchangeable and refer to the exact same measurement:

• Mil: Predominant in American English, technical documentation, industry standards (ASTM, ANSI), and scientific contexts. The name derives from Latin "mille" (thousand).
• Thou: Common in British English shop-floor terminology, machining, and colloquial engineering speech. It's an abbreviated form of "thousandth" (of an inch).

Example usage:

• American engineer: "The coating should be 5 mils thick."
• British machinist: "Machine this to within 2 thou tolerance."
• Both mean the same thing: 0.005″ and 0.002″ respectively.

Decimal Imperial vs. Fractional Imperial

The mil represents a decimal approach to imperial measurement, contrasting with the traditional fractional approach:

Fractional imperial (traditional):

• 1/2″, 1/4″, 1/8″, 1/16″, 1/32″, 1/64″
• Awkward for very small measurements
• Difficult to add/subtract without common denominators

Decimal imperial (mils/thousandths):

• 0.500″ (500 mils), 0.250″ (250 mils), 0.125″ (125 mils)
• Easy arithmetic: 5 mils + 3 mils = 8 mils
• Natural compatibility with digital calipers and micrometers

Engineering drawings often show both: 0.250″ (1/4″) for clarity, but critical tolerances are always specified in decimal thousandths: ±0.005″ (±5 mils), never ±1/200″.

Mils in Wire Gauge Systems

American Wire Gauge (AWG) uses diameter in mils as the basis for its numbering system, though the relationship is logarithmic rather than linear:

AWG Size	Diameter (mils)	Diameter (mm)	Common Use
0000 (4/0)	460 mil	11.7 mm	Heavy power cables
00 (2/0)	365 mil	9.3 mm	Welding cable
10 AWG	102 mil	2.6 mm	Household 30A circuits
14 AWG	64 mil	1.6 mm	Household 15A circuits
18 AWG	40 mil	1.0 mm	Lamp cords, doorbells
22 AWG	25 mil	0.64 mm	Solid hookup wire
30 AWG	10 mil	0.25 mm	Wire-wrap wire
40 AWG	3 mil	0.08 mm	Magnet wire (coils)

The AWG system was established in the 1850s based on the number of times wire was drawn through reducing dies, with each gauge step representing approximately a 20% reduction in diameter (factor of √2 for cross-sectional area).

History

Ancient and Medieval Precision (Before 1700)

Before the Industrial Revolution, precision measurement was limited by available tools. Ancient civilizations measured small lengths using:

• Barleycorns: 1 inch = 3 barleycorns (still referenced in shoe sizing)
• Hair breadth: Approximately 1/48 of an inch (about 21 mils)
• Poppy seed: Medieval unit ≈ 1/72 inch (14 mils)

These natural standards were inconsistent but adequate for carpentry, masonry, and traditional crafts where tolerances of 1/16″ to 1/32″ were acceptable. Precision metalworking (locksmithing, clockmaking, weapon-smithing) relied on master craftsmen's skill and direct comparison to reference standards rather than numerical measurement.

The finest medieval measurements were in clock and instrument making, where tolerances approaching 1/100″ (10 mils) were occasionally achieved, but not reliably measured or documented.

Industrial Revolution and the Birth of Precision (1750-1850)

The transformation of manufacturing from craft to industry created an urgent need for precision measurement:

1775-1800: John Wilkinson's cylinder boring machine (1775) achieved tolerances within 1/20″ (50 mils) for steam engine cylinders—revolutionary at the time, enabling James Watt's efficient steam engines.

1800-1840: Interchangeable parts manufacturing emerged. Eli Whitney's 1798 contract to produce 10,000 muskets with interchangeable parts required consistent tolerances around 1/32″ (31 mils), though he struggled to achieve this consistently.

1840s: Joseph Whitworth revolutionized precision measurement:

• Invented the first practical micrometer (1840s), capable of measuring to 0.001″ (1 mil)
• Developed precision surface plates and scrapers
• Standardized screw threads with tolerances of a few thousandths
• Established the "thou" as the standard British precision unit

Whitworth's work meant that for the first time, machinists could reliably measure and verify work to within one thousandth of an inch, making the mil/thou a practical working unit rather than a theoretical ideal.

1850s: American System of Manufacturing (Colt, Robbins & Lawrence, Singer) adopted decimal measurements in thousandths for gauging and inspection, establishing "mils" in American technical vocabulary.

Standardization and Proliferation (1850-1920)

1860s-1880s: Development of precision measuring instruments:

• Vernier calipers graduated in thousandths
• Micrometer calipers (marketed by Brown & Sharpe from 1867)
• Dial indicators (from 1890s) showing 0.001″ divisions

1857: American Wire Gauge (AWG) standardized by J. R. Brown, using wire diameter in mils as the basis.

1880s-1900s: Paint and coating industries adopted mils for dry film thickness:

• Paint manufacturers specified coverage rates based on mils of dry film
• Wet film thickness gauges calibrated in mils
• Quality standards (e.g., U.S. Navy specifications) required minimum coating thickness in mils

1900-1920: Sheet metal gauges proliferated (ironically, these use arbitrary numbering systems, but actual thickness is specified in decimal inches/mils):

• Manufacturer's Standard Gauge (MSG) for steel
• Birmingham Wire Gauge (BWG)
• Confusion from multiple gauge systems led to increasing use of direct mil specifications

1916: SAE (Society of Automotive Engineers) founded, establishing thousands of specifications with tolerances in thousandths of an inch for automotive parts.

Electronics Age and Micro-Precision (1920-1980)

1930s-1940s: Electronics manufacturing adopted mils for:

• Wire insulation thickness (enamel on magnet wire: 0.5-2 mils)
• Capacitor dielectric thickness
• Vacuum tube electrode spacing

1943: Printed Circuit Board (PCB) invention (Paul Eisler, 1943; mass production 1950s):

• Copper foil thickness: 1.4 mils (1 oz/ft²), 2.8 mils (2 oz/ft²)
• Trace width: typically 5-15 mils for signal traces
• Spacing between traces: 5-8 mils minimum in early designs
• PCB substrate thickness: 62 mils (1.57 mm) became standard

1950s-1960s: Plastic film industry standardized thickness in mils:

• Polyethylene film (bags, wrap): 0.5-6 mils
• Polyester film (Mylar): 0.5-10 mils
• Vinyl flooring underlayment: 6-20 mils

1960s: Integrated circuit (IC) manufacturing pushed precision far beyond mils:

• Early transistors: features measured in tens of mils
• By 1970: features approaching 100 microinches (0.1 mil)
• The mil remained relevant for IC package dimensions, lead spacing, and encapsulation thickness

1975: ANSI/ASTM standards codified mil-based specifications for coating thickness, plastic film, and adhesive tape across dozens of industries.

Modern Era and Metrication Resistance (1980-Present)

1980s-1990s: Global metrication movement:

• Britain officially metricated in 1965-1995, but "thou" persisted in machine shops
• U.S. industries largely resisted metrication in favor of established decimal-inch (mil) systems
• Aerospace, defense, and automotive industries retained imperial specs due to:
  • Enormous legacy of existing designs, tooling, and parts libraries
  • Cost of converting decades of technical documentation
  • Risk of conversion errors (Mars Climate Orbiter lost in 1999 due to metric/imperial confusion)

1990s-2000s: PCB design and microelectronics:

• PCB CAD software (OrCAD, Altium, Eagle) defaulted to mil grid spacing (5 mil, 10 mil, 25 mil grids)
• Modern PCB fabrication: 3-4 mil traces/spaces achievable in standard processes
• Advanced processes: sub-mil features (0.5 mil), but package and board dimensions still in mils

2000s-Present: Hybrid metric/imperial usage:

• American manufacturing: Part dimensions often in inches/mils, tolerances in mils
• International suppliers: Provide both metric and mil equivalents
• 3D printing: Software often accepts both (mil settings for layer height, nozzle diameter)
• Coating industry: Paint thickness gauges typically show both mils and microns

Present day: The mil remains dominant in:

• American PCB and electronics manufacturing
• U.S. aerospace and defense (military specifications)
• Coating and painting (ASTM standards)
• Plastic film and flexible packaging
• American Wire Gauge system
• Machining tolerances in U.S. shops

Real-World Examples Across Different Scales

Ultra-Thin Films (0.1-2 mils)

Plastic food wrap (0.5-1 mil = 12-25 μm):

• Saran Wrap, Glad Cling Wrap: 0.5-0.6 mils
• Provides barrier against moisture while maintaining transparency
• Thinner than human cell (typical cell: 10-30 μm)

Aluminum foil (0.6-1 mil = 15-25 μm):

• Reynolds Wrap standard: 0.6 mil (0.00063″)
• Heavy-duty foil: 0.9-1.0 mil
• Thickness optimized for balance between flexibility and tear resistance

Mylar/polyester film (0.5-1 mil = 12-25 μm):

• Metallized Mylar (space blankets): 0.5 mil
• Food packaging barrier films: 0.8-1.5 mil
• Electrical insulation in capacitors: 0.25-1 mil

Everyday Items (2-15 mils)

Paper products (3-5 mils = 75-125 μm):

• Standard copy paper (20 lb): 4 mils (0.004″ = 0.1 mm)
• Cardstock (110 lb): 10-14 mils (0.010-0.014″)
• Tissue paper: 1-2 mils
• Paper thickness critical for printer/copier jamming prevention

Plastic bags (0.7-6 mils):

• Grocery bags (single-use): 0.7-1 mil (becoming rare due to bans)
• Kitchen garbage bags (13-gal): 0.9-1.5 mil
• Heavy-duty contractor bags (55-gal): 3-6 mil
• Zipper storage bags (Ziploc): 1.5-2 mil
• Specification ensures tear resistance and load capacity

Credit cards and ID cards (30 mils = 0.76 mm):

• ISO/IEC 7810 standard: 30 mils (0.030″ = 0.76 mm) exactly
• Driver's licenses: 20-30 mils depending on state
• Gift cards: 10-15 mils (thinner, non-embossed)
• Critical dimension for card reader slot compatibility

Coatings and Platings (1-10 mils)

Paint and protective coatings (2-6 mils dry film):

• Automotive paint (total system):
  • Primer: 1-1.5 mils
  • Base coat: 0.8-1.2 mils
  • Clear coat: 1.5-2 mils
  • Total: 4-6 mils (100-150 μm)
• House paint (exterior): 4-5 mils wet, 1.5-2.5 mils dry
• Industrial coatings (tanks, bridges): 8-15 mils for corrosion protection

Electroplating and galvanizing (0.1-2 mils):

• Chrome plating (decorative): 0.5-1 mil (12-25 μm)
• Nickel plating: 0.3-2 mils depending on application
• Gold plating (electronics connectors): 0.05-0.1 mil (1.3-2.5 μm) = 50-100 microinches
• Hot-dip galvanizing (steel): 2-5 mils (50-125 μm)
• Measured with magnetic thickness gauges or X-ray fluorescence

Powder coating (2-5 mils):

• Appliances and outdoor furniture: 2.5-4 mils
• Automotive wheels: 3-5 mils
• Applied as dry powder, cured in oven
• Thickness critical for durability and appearance

Precision Manufacturing (0.5-50 mils)

Machining tolerances:

• General machining: ±5 mils (±0.005″ = ±0.13 mm)
• Precision machining: ±1 mil (±0.001″ = ±25 μm)
• High-precision grinding: ±0.1 mil (±0.0001″ = ±2.5 μm) = ±100 microinches
• Ultra-precision (diamond turning): ±0.01 mil (±10 microinches = ±0.25 μm)

Examples:

• Engine piston ring gap: 10-20 mils (0.010-0.020″)
• Bearing clearances: 1-5 mils depending on application
• Hydraulic valve spool clearance: 0.5-2 mils (leakage control)
• Gauge block accuracy (Grade 0): ±0.05 mil (±50 microinches)

Sheet metal forming:

• Aluminum sheet (beverage cans): 4-6 mils body, 8-10 mils ends
• Steel roofing: 10-30 mils (26-gauge to 20-gauge)
• Stainless steel (kitchen appliances): 35-48 mils (20-gauge to 18-gauge)

Electronics and PCBs (1-62 mils)

Printed Circuit Board dimensions:

• Standard PCB thickness: 62 mils (1.57 mm) for rigidity
• Thin PCBs (mobile devices): 31 mils (0.8 mm)
• Thick PCBs (power electronics): 93 mils (2.4 mm)

Copper layer thickness (PCB):

• 1 oz copper (most common): 1.4 mils (35 μm)
• 0.5 oz copper (fine traces): 0.7 mil (18 μm)
• 2 oz copper (power planes): 2.8 mils (70 μm)
• 4 oz+ copper (high-current): 5.6+ mils

PCB trace geometry:

• Standard manufacturing capability: 5 mil trace / 5 mil space (5/5)
• Advanced capability: 3 mil trace / 3 mil space (3/3)
• High-density PCBs: sub-1 mil traces possible
• Via drill diameter: 8-20 mils typical

Component lead spacing:

• Through-hole resistor leads: 100 mils (0.1″) = 2.54 mm
• Dual in-line package (DIP) pin spacing: 100 mils
• Surface-mount fine-pitch: 25 mil (0.025″) = 0.635 mm pitch
• Ball grid array (BGA): 50 mil pitch common

Human and Biological Scale (10-100 mils)

Human hair (70-100 mils diameter = 1.8-2.5 mm):

• Asian hair: 80-120 mils (thickest)
• Caucasian hair: 70-100 mils
• African hair: 60-90 mils (variable due to curl)
• Fine baby hair: 30-50 mils
• Reference standard in precision comparison ("finer than a hair" ≈ <50 mils)

Paper and fingernail:

• Fingernail thickness: 20-30 mils (0.5-0.75 mm)
• Skin thickness (epidermis): 2-6 mils (50-150 μm)
• Business card: 10-14 mils

Common Uses

1. Precision Manufacturing and Machining

The mil is the fundamental tolerance unit in American manufacturing:

Engineering drawings and blueprints:

• Nominal dimensions in decimal inches: 1.250″, 0.375″
• Tolerances in thousandths: ±0.005″ (±5 mils), ±0.002″ (±2 mils)
• Geometric dimensioning and tolerancing (GD&T): Position tolerance ⌖ 0.010″ (10 mils)

Quality control and inspection:

• Calipers and micrometers graduated in 0.001″ (1 mil) divisions
• Dial indicators show 0.001″ or 0.0001″ (0.1 mil) graduations
• Coordinate measuring machines (CMM) measure to ±0.1 mil or better
• Pass/fail gauges: "Go" and "No-Go" plugs/rings differ by a few mils

Real-world tolerances by industry:

• General fabrication: ±5-10 mils (±0.005-0.010″)
• Automotive production: ±2-5 mils typical
• Aerospace critical parts: ±0.5-2 mils
• Medical implants (hip, knee): ±0.2-1 mil
• Semiconductor equipment: ±0.1 mil (±100 microinches)

Example: A shaft specified as Ø 1.000″ ± 0.002″ must measure between 0.998″ and 1.002″ (within ±2 mils). A shaft measuring 1.003″ (3 mils over) would be rejected.

2. Coating Thickness Measurement

Paint, plating, and protective coating thickness is universally measured in mils in American industry:

Paint inspection (wet film thickness gauges):

• Notch gauges (comb-style): Teeth graduated in mils (2, 4, 6, 8, 10 mils)
• Painter inserts gauge into wet paint; highest clean tooth indicates thickness
• Ensures proper coverage to meet specification (e.g., "apply to 6 mils wet film")

Dry film thickness (DFT) measurement:

• Magnetic gauges (ferrous substrates): Reed sensor or Hall effect, calibrated in mils
• Eddy current gauges (non-ferrous): For aluminum, brass, copper substrates
• Ultrasonic gauges: For non-metallic coatings on any substrate

Industry specifications:

• SSPC-PA 2 (paint inspection): Requires DFT measurement in mils
• U.S. Navy painting specs: Minimum DFT requirements (e.g., 5-7 mils for topcoat)
• Automotive OEM specs: Total paint system 4-6 mils
• Industrial maintenance coatings: Often 8-15 mils for severe environments

Example: A steel bridge specification might require: "Apply epoxy primer to 5-8 mils DFT, followed by polyurethane topcoat to 3-5 mils DFT." Inspectors measure at multiple points; any reading below minimum triggers rework.

Galvanizing and plating:

• Hot-dip galvanizing: ASTM A123 requires minimum 2.0 mils (for <1/4″ thick steel)
• Chrome plating: 0.5-1 mil decorative, 8-10 mils hard chrome (wear resistance)
• Anodizing (aluminum): Type II = 0.2-0.7 mils, Type III (hard anodize) = 2-4 mils

3. Plastic Film and Flexible Packaging

Plastic film thickness is specified in mils in the U.S. packaging industry:

Food packaging films:

• Shrink wrap: 2-4 mils (60-100 μm)
• Stretch wrap (pallet wrap): 12-20 gauge = 0.5-0.8 mil (gauge ≠ mils, confusingly)
• Vacuum seal bags (FoodSaver): 3-4 mils
• Frozen food bags: 2-3 mils
• Thicker film = better puncture resistance, higher material cost

Construction and industrial films:

• Vapor barriers (under-slab): 6-10 mils required by building codes
• Construction tarps: 5-12 mils depending on duty
• Agricultural greenhouse film: 4-6 mils (UV-stabilized)
• Geomembrane liners (landfills, ponds): 20-100 mils

Adhesive tapes:

• Scotch tape (Transparent): 1-1.5 mils
• Masking tape: 5-7 mils total thickness
• Duct tape: 10-12 mils
• Double-sided foam tape: 30-60 mils
• Specification includes backing + adhesive thickness

Industry standards:

• ASTM D6988: Standard specification for polyethylene film (thickness in mils)
• ASTM D374: Method for measuring tape thickness (to 0.1 mil)

Example: A customer ordering "6-mil poly sheeting" for a concrete vapor barrier receives polyethylene film exactly 0.006″ (6 mils = 0.15 mm) thick, ensuring compliance with IRC building code.

4. Electronics and Printed Circuit Boards

PCB design and manufacturing is dominated by mil-based dimensions in the United States:

PCB design software (Altium, Eagle, KiCAD, OrCAD):

• Default grid spacing: 50 mils, 25 mils, 10 mils, 5 mils, 1 mil
• Component libraries use 100-mil pin spacing for through-hole parts
• Trace width and clearance rules specified in mils

Design rules and capabilities:

• Standard PCB fabrication (budget services):
  • Minimum trace width: 6 mils
  • Minimum spacing: 6 mils
  • Via drill: 8-10 mils
  • Copper weight: 1 oz (1.4 mils)
• Advanced PCB fabrication:
  • Minimum trace/space: 3 mil / 3 mil
  • Microvias: 4-6 mil drill
  • HDI (high-density interconnect): sub-mil features

Copper thickness selection:

• 1 oz copper (1.4 mils): Standard for signal layers, adequate for currents <3A
• 2 oz copper (2.8 mils): Power planes, higher current capacity (5-10A per trace)
• 3-4 oz copper (4.2-5.6 mils): Power electronics, automotive
• Current-carrying capacity increases with thickness (more copper = less resistance = less heat)

PCB stackup (multilayer boards):

• 4-layer board example:
  • Top copper: 1.4 mils (1 oz)
  • Prepreg: 7 mils
  • Inner layer 1 (ground): 1.4 mils
  • Core: 47 mils
  • Inner layer 2 (power): 1.4 mils
  • Prepreg: 7 mils
  • Bottom copper: 1.4 mils
  • Total: 67 mils (1.7 mm)

Soldermask and silkscreen:

• Soldermask (green coating): 0.8-1.2 mils thickness
• Silkscreen (white text): 0.5-1 mil
• Surface finish (ENIG gold): 1-2 microinches (0.001-0.002 mil)

5. Wire Gauge and Cable Manufacturing

American Wire Gauge (AWG) is fundamentally based on diameter in mils:

AWG diameter formula:

• Each gauge step represents approximately a 1.123× change in diameter (10 steps = 1 decade = 10× area change)
• AWG 10 = 102 mils diameter
• AWG 20 = 32 mils diameter
• AWG 30 = 10 mils diameter

Wire insulation thickness (in mils over conductor diameter):

• Thin-wall insulation: 4-8 mils
• Standard insulation: 15-30 mils
• High-voltage insulation: 30-60+ mils
• Specification: "22 AWG with 10 mil insulation" = 25 mil conductor + 20 mil insulation total = 45 mil overall diameter

Magnet wire (enameled copper for motors, transformers):

• AWG 28-40 common for small coils
• Enamel coating: 0.5-2 mils depending on temperature rating
• Grade 1 (thinnest): 0.5-0.8 mil enamel
• Grade 2 (standard): 1-1.5 mil enamel
• Grade 3 (heavy build): 2-3 mil enamel
• Thicker enamel = better insulation, lower packing density in coil

Cable jacket thickness:

• Ethernet Cat6 cable jacket: 20-30 mils
• Power cord (SJOOW): 60-80 mils
• Mining cable: 125-250 mils (extreme abrasion resistance)

6. Sheet Metal and Gauge Systems

Sheet metal thickness is specified in gauge numbers, but actual thickness is in decimal inches (mils):

Manufacturer's Standard Gauge (steel):

• 10-gauge = 135 mils (0.1345″)
• 14-gauge = 75 mils (0.0747″)
• 20-gauge = 36 mils (0.0359″)
• 24-gauge = 24 mils (0.0239″)

Confusion and standardization issues:

• Gauge numbers are not universal: steel gauge ≠ aluminum gauge ≠ galvanized gauge
• Modern practice: Always specify in decimal inches (mils) to avoid ambiguity
• Example: "0.060″ aluminum" (60 mils) instead of "14-gauge aluminum"

HVAC ductwork:

• Residential ducts: 24-26 gauge (18-30 mils)
• Commercial ducts: 20-22 gauge (33-42 mils)
• Galvanized coating adds 2-5 mils to thickness

Automotive body panels:

• Hood outer skin: 28-30 mils (0.7-0.8 mm)
• Door outer skin: 26-28 mils
• Roof panel: 28-32 mils
• Frame rails: 90-120 mils (2.3-3.0 mm)

7. Quality Control and Standards Compliance

Mil-based specifications appear in thousands of ASTM, ANSI, SAE, and military standards:

ASTM coating standards:

• ASTM D3359: Adhesion of coatings (measured after cutting through coating, thickness in mils)
• ASTM D1186: Paint film thickness (magnetic gauges calibrated in mils)
• ASTM D6132: Nondestructive measurement of dry film thickness (mils)

Military specifications (MIL-SPEC):

• MIL-DTL-53030: Primer coating, epoxy, 2-3 mils DFT
• MIL-PRF-85285: Coating, polyurethane, aircraft, 1.5-2.5 mils
• Compliance requires thickness measurement at specified intervals

Building codes and construction:

• International Residential Code (IRC): Vapor retarders minimum 6 mils
• Roofing underlayment: Minimum thickness specifications in mils
• Window film (safety/security): 4-14 mils for various protection levels

Automotive and aerospace:

• SAE J1960: Brake hose specifications (wall thickness in inches/mils)
• AS9100 (aerospace quality): Dimensional inspections recorded in mils
• Engine bearing clearances: Specified in mils (e.g., 1.5-2.5 mils for main bearings)

Conversion Guide

Basic Conversion Formulas

Mils ↔ Inches:

• Mils to inches: Divide by 1,000

  • Formula: inches = mils ÷ 1,000
  • Example: 50 mils = 50 ÷ 1,000 = 0.050″

• Inches to mils: Multiply by 1,000

  • Formula: mils = inches × 1,000
  • Example: 0.125″ = 0.125 × 1,000 = 125 mils

Mils ↔ Millimeters:

• Mils to millimeters: Multiply by 0.0254

  • Formula: mm = mils × 0.0254
  • Example: 100 mils = 100 × 0.0254 = 2.54 mm

• Millimeters to mils: Multiply by 39.37 (or divide by 0.0254)

  • Formula: mils = mm × 39.37
  • Example: 5 mm = 5 × 39.37 = 196.85 mils ≈ 197 mils

Mils ↔ Micrometers (microns, μm):

• Mils to micrometers: Multiply by 25.4

  • Formula: μm = mils × 25.4
  • Example: 5 mils = 5 × 25.4 = 127 μm

• Micrometers to mils: Divide by 25.4

  • Formula: mils = μm ÷ 25.4
  • Example: 250 μm = 250 ÷ 25.4 = 9.84 mils ≈ 10 mils

Mils ↔ Microinches (μin):

• Mils to microinches: Multiply by 1,000

  • Formula: μin = mils × 1,000
  • Example: 0.5 mil = 0.5 × 1,000 = 500 microinches

• Microinches to mils: Divide by 1,000

  • Formula: mils = μin ÷ 1,000
  • Example: 100 μin = 100 ÷ 1,000 = 0.1 mil

Comprehensive Conversion Table

Mils	Inches	Millimeters (mm)	Micrometers (μm)	Common Reference
0.1 mil	0.0001″	0.00254 mm	2.54 μm	100 microinches (precision machining)
0.5 mil	0.0005″	0.0127 mm	12.7 μm	Plastic food wrap, thin enamel
1 mil	0.001″	0.0254 mm	25.4 μm	Aluminum foil, standard tolerance
2 mil	0.002″	0.0508 mm	50.8 μm	Tight machining tolerance
4 mil	0.004″	0.1016 mm	101.6 μm	Standard printer paper
5 mil	0.005″	0.127 mm	127 μm	General machining tolerance, PCB trace
10 mil	0.010″	0.254 mm	254 μm	Business card, thin cardstock
15 mil	0.015″	0.381 mm	381 μm	PCB trace (high current)
30 mil	0.030″	0.762 mm	762 μm	Credit card (ISO standard)
50 mil	0.050″	1.27 mm	1,270 μm	1/20 inch (machining)
62 mil	0.062″	1.575 mm	1,575 μm	Standard PCB thickness
100 mil	0.100″	2.54 mm	2,540 μm	1/10 inch (0.1″ pin spacing)
125 mil	0.125″	3.175 mm	3,175 μm	1/8 inch
250 mil	0.250″	6.35 mm	6,350 μm	1/4 inch
500 mil	0.500″	12.7 mm	12,700 μm	1/2 inch
1,000 mil	1.000″	25.4 mm	25,400 μm	1 inch exactly

Quick Mental Approximations

For fast estimates without a calculator:

Mils to millimeters:

• 1 mil ≈ 0.025 mm (actually 0.0254, so slightly more)
• 10 mils ≈ 0.25 mm (actually 0.254 mm)
• 40 mils ≈ 1 mm (actually 39.37 mils = 1 mm)
• 100 mils ≈ 2.5 mm (actually 2.54 mm)

Millimeters to mils:

• 1 mm ≈ 40 mils (actually 39.37 mils, so round to 40 for quick estimate)
• 0.1 mm ≈ 4 mils (100 μm ≈ 4 mils)
• 2.5 mm ≈ 100 mils (close to 0.1 inch)

Example estimation:

• Question: "What is 6 mils in millimeters?"
• Quick calculation: 6 mils × 0.025 = 0.15 mm
• Exact: 6 × 0.0254 = 0.1524 mm (estimation very close!)

AWG to Mils Diameter

AWG	Diameter (mils)	Diameter (mm)
0000 (4/0)	460	11.68
000 (3/0)	410	10.40
00 (2/0)	365	9.27
0 (1/0)	325	8.25
2	258	6.54
4	204	5.19
6	162	4.11
8	128	3.26
10	102	2.59
12	81	2.05
14	64	1.63
16	51	1.29
18	40	1.02
20	32	0.81
22	25	0.64
24	20	0.51
26	16	0.40
28	13	0.32
30	10	0.25
32	8	0.20
36	5	0.13
40	3	0.08

Coating Thickness Conversion (Mils ↔ Microns)

Paint and coating industry often uses mils (U.S.) and microns/μm (international):

Mils (DFT)	Microns (μm)	Application
0.5 mil	12.7 μm	Thin electroplating
1 mil	25.4 μm	Chrome plating, anodizing
2 mils	50.8 μm	Powder coating (light)
3 mils	76.2 μm	Automotive clear coat
4 mils	101.6 μm	House paint (dry film)
5 mils	127 μm	Industrial coating
8 mils	203.2 μm	Heavy-duty coating
10 mils	254 μm	Galvanizing (thick)
15 mils	381 μm	Protective coating (severe)

Common Conversion Mistakes

1. Confusing Mils with Millimeters

The Problem: The similar-sounding names "mil" and "millimeter" (abbreviated "mil" in some languages) cause frequent confusion, especially in international contexts.

Reality:

• 1 mil (imperial) = 0.001 inch = 0.0254 mm = 25.4 μm
• 1 millimeter (metric) = 0.001 meter = 39.37 mils

Confusion factor: 1 millimeter is approximately 40 times larger than 1 mil!

Real-world consequence: A specification calling for "5 mil paint thickness" (0.127 mm) mistakenly interpreted as "5 millimeters" (5,000 μm = 197 mils) would result in a coating 40 times too thick—wasting material and potentially causing application problems (cracking, poor adhesion, excessive cost).

Example error:

• Specification: "Apply coating to 6 mils dry film thickness"
• Incorrect interpretation: 6 millimeters (240 mils!) = massive over-application
• Correct interpretation: 6 mils = 0.15 mm = 152 μm

Prevention: Always write units explicitly:

• "6 mils (0.15 mm)" or "0.15 mm (6 mils)" in international specifications
• Never abbreviate "millimeter" as "mil"—use "mm" only

2. Incorrect Decimal Point Placement

The Problem: Forgetting that "mils" already represents thousandths leads to incorrect decimal conversions.

Correct conversion:

• 5 mils = 0.005 inches (move decimal 3 places left)
• 125 mils = 0.125 inches (1/8 inch)

Common error:

• Incorrectly thinking "5 mils = 0.5 inches" (actually 500 mils!)
• Incorrectly thinking "0.5 inches = 5 mils" (actually 500 mils)

Real consequence: A machinist told to remove "5 mils" (0.005″) who accidentally removes 0.5″ (500 mils) has destroyed the part—the error is 100 times too much material removal.

Example:

• Blueprint tolerance: ±0.002″ (±2 mils)
• Machinist misreads as: ±0.02″ (±20 mils) = 10× too loose
• Part may assemble incorrectly or fail inspection

Prevention:

• Always write both forms when critical: "±0.002″ (±2 mils)"
• Double-check decimal point placement (thousandths = 3 decimal places)

3. Using Wrong Metric Conversion Factor

The Problem: Using approximate or rounded conversion factors introduces significant errors in precision work.

Correct exact conversion:

• 1 mil = 25.4 μm exactly (or 0.0254 mm)
• 1 mm = 39.3701 mils (often rounded to 39.37)

Common approximation errors:

• Using "1 mil ≈ 25 μm" (0.4 μm error per mil)
  • For 10 mils: 250 μm vs. actual 254 μm = 4 μm error (1.6%)
• Using "1 mm ≈ 40 mils" (0.63 mil error per mm)
  • For 5 mm: 200 mils vs. actual 196.85 mils = 3.15 mil error (1.6%)

When approximations are OK:

• Rough estimates, quick mental math
• Non-critical dimensions with loose tolerances (±10 mils or more)

When exact conversions are required:

• Precision machining (tolerances ±1-5 mils)
• Coating thickness specifications (regulatory compliance)
• Electronics (PCB design, exact stackup calculations)
• Quality control documentation

Example error:

• Spec: 100 μm coating thickness required
• Approximate conversion: 100 ÷ 25 = 4 mils
• Exact conversion: 100 ÷ 25.4 = 3.94 mils
• If painter targets 4 mils instead of 3.94, result is 101.6 μm (1.6% over spec, possibly failing tight tolerances)

Prevention: Use exact conversion factors (0.0254 mm/mil or 25.4 μm/mil) for all precision work.

4. Mixing Up Mils and Microinches

The Problem: Precision machining uses microinches (μin) for ultra-fine tolerances, easily confused with mils.

Relationship:

• 1 mil = 1,000 microinches
• 1 microinch = 0.001 mil = 0.000001 inch = 0.0254 μm

Context differences:

• Mils (0.001″): General machining, tolerances, material thickness
• Microinches (0.000001″): Surface finish, ultra-precision grinding, gauge blocks, optical components

Example specifications:

• General tolerance: ±5 mils (±0.005″ = ±127 μm)
• Surface finish: 32 microinches Ra (0.032 mil = 0.000032″ = 0.81 μm) = roughness average

Common error:

• Surface finish spec: "Finish to 63 microinches Ra"
• Misread as: "63 mils Ra" = 1,000× too coarse! (would be visibly rough, unacceptable)
• Correct: 63 microinches = 0.063 mil = 1.6 μm Ra (smooth machined finish)

Real consequence: Ordering a part with "125 microinch tolerance" but machinist interprets as "125 mil tolerance" results in a part 1,000 times less precise than required—useless for precision applications.

Prevention:

• Always write units: "125 μin" vs. "125 mil"
• Context check: Surface finish? Probably microinches. Dimension tolerance? Probably mils.

5. Gauge Number Confusion

The Problem: Assuming gauge numbers directly correlate to mils, or that gauge systems are universal.

Reality:

• Gauge numbers are arbitrary and system-specific
• Steel gauge ≠ aluminum gauge ≠ wire gauge
• Larger gauge number = thinner material (counterintuitive)

Examples of confusion:

• 10-gauge steel (MSG) = 135 mils
• 10-gauge aluminum = 102 mils (different!)
• 10 AWG wire diameter = 102 mils (coincidentally same as aluminum sheet, but unrelated)

Common error:

• Ordering "20-gauge" without specifying material or standard
• Supplier interprets as steel (36 mils), customer expected aluminum (32 mils) or galvanized (40 mils)

Prevention: Always specify thickness in decimal inches (mils) for sheet metal:

• Instead of: "20-gauge steel"
• Use: "0.036″ steel" or "36 mil steel (20-gauge MSG)"

6. Forgetting Mils in Calculations

The Problem: Performing arithmetic with mixed units (mils and inches) without converting.

Example error:

• Adding tolerances: 1.000″ ± 5 mils + 0.500″ ± 2 mils
• Incorrect: 1.000 + 5 + 0.500 + 2 = 8.5 (nonsense!)
• Correct: Convert mils to inches first:
  • 1.000″ ± 0.005″ + 0.500″ ± 0.002″ = 1.500″ ± 0.007″ (7 mils combined tolerance)

Stacking tolerances:

• Part A: 2.000″ ± 3 mils
• Part B: 1.500″ ± 2 mils
• Part C: 0.750″ ± 1 mil
• Total stack: 4.250″ ± (3 + 2 + 1) = ±6 mils (worst-case tolerance stack)
• Must convert all to same unit before adding

Prevention:

• Convert everything to decimal inches before calculating
• Or convert everything to mils, calculate, then convert back if needed