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Millimeter (mm) - Unit Information & Conversion

Symbol:mm
Plural:millimeters
Category:Length

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What is a Millimeter?

The millimeter (mm) is a metric unit of length equal to one-thousandth of a meter or one-tenth of a centimeter. Used worldwide for precision measurements in engineering, manufacturing, jewelry, and everyday measurements. Approximately 0.0394 inches or about the thickness of a credit card.

History of the Millimeter

Defined as a subunit of the meter when the metric system was established in late 18th century France. The millimeter provides a standard unit for fine measurements required in science, engineering, and manufacturing. Became essential with industrial precision machining in the 19th century.

Quick Answer

What is a millimeter? A millimeter (mm) is one-thousandth of a meter or one-tenth of a centimeter - about the thickness of a credit card. It's the standard unit for precision measurements in engineering, manufacturing, and technical work worldwide. Use our length converter to convert mm to inches, cm, meters, and other units instantly.

Quick Comparison Table

Millimeters Centimeters Inches Meters Common Example Convert Now
1 mm 0.1 cm 0.039 in 0.001 m Thickness of credit card Convert →
10 mm 1 cm 0.39 in 0.01 m Width of fingernail Convert →
25.4 mm 2.54 cm 1 in 0.0254 m One inch (exactly) Convert →
100 mm 10 cm 3.94 in 0.1 m Width of smartphone Convert →
1,000 mm 100 cm 39.4 in 1 m 1 meter = 1,000 mm Convert →

Need a different conversion? Try our length converter for all units.

Definition

The millimeter (symbol: mm) is a unit of length in the metric system equal to one-thousandth of a meter (1/1,000 m) or one-tenth of a centimeter (1/10 cm). It is used for precise measurements in engineering, manufacturing, and everyday applications.

Key relationships:

  • 1 millimeter = 0.001 meters (m)
  • 1 millimeter = 0.1 centimeters (cm)
  • 10 millimeters = 1 centimeter
  • 1,000 millimeters = 1 meter
  • 1 millimeter ≈ 0.03937 inches (about 1/25th inch)
  • 25.4 millimeters = 1 inch (exactly)

The prefix "milli-":

  • From Latin "mille" meaning "thousand"
  • SI prefix denoting 10⁻³ (one-thousandth)
  • Also used in: milligram (mg), milliliter (mL), millisecond (ms)

Visual perspective (how big is it?):

  • 1 mm = Thickness of a credit card
  • 2 mm = Thickness of a US dime
  • 5 mm = Thickness of a pencil lead (mechanical)
  • 10 mm = Width of an adult fingernail
  • 25.4 mm = Exactly one inch

Convert between length units: mm converter

History

  • Metric System Origins: The millimeter was defined as a subunit of the meter when the metric system was established in France during the late 18th century (1790s). As measurement standardization progressed, the need for decimal subdivisions of the meter became clear.

  • Industrial Revolution: The millimeter became essential during the 19th century Industrial Revolution as:

    • Precision machining required fine tolerances
    • Interchangeable parts manufacturing emerged
    • Engineering drawings needed standard units
    • Technical specifications became international

  • Engineering Adoption: By the mid-19th century, engineers and machinists worldwide adopted millimeters as the standard for:

    • Machine tool specifications
    • Tolerance requirements (±0.1 mm common)
    • Technical drawing dimensions
    • Quality control measurements

  • Scientific Standardization: The millimeter became standard in scientific research for:

    • Laboratory equipment calibration
    • Microscopy measurements
    • Specimen documentation
    • Experimental apparatus dimensions

  • SI Formalization: When the International System of Units (SI) was established in 1960, the millimeter was confirmed as an official subdivision of the meter, forming part of the coherent decimal system.

  • Modern Manufacturing: The 20th century saw millimeters become universal in:

    • Automotive engineering specifications
    • Electronics manufacturing (PCB design)
    • Aerospace tolerances
    • Medical device specifications
    • Consumer product dimensions

  • Global Standard: Today, millimeters are the primary unit for precision measurements in virtually every country except the United States (which uses both mm and inches). Even in the US, many technical fields have adopted millimeters as standard.

Real-World Examples

Engineering and Machining

Tolerance Specifications:

  • Tight tolerance: ±0.01 mm (10 micrometers) - precision parts
  • Standard tolerance: ±0.1 mm - typical machined parts
  • Loose tolerance: ±0.5 mm - rough cuts
  • Very tight: ±0.001 mm (1 micrometer) - optical components

Common Engineering Measurements:

  • Bolt diameter: M6 = 6 mm diameter
  • Thread pitch: 1.5 mm (distance between threads)
  • Bearing clearance: 0.02-0.05 mm typical
  • Sheet metal thickness: 0.5-5 mm range
  • Drill bit sizes: 1 mm, 1.5 mm, 2 mm, etc.

Manufacturing Standards:

  • ISO metric threads measured in mm
  • DIN standards specify mm dimensions
  • CNC machining programs use mm coordinates
  • Quality control measured to 0.01 mm precision

Electronics and Technology

Component Dimensions:

  • Resistor (standard): 3.2 × 1.6 mm (0805 size)
  • Microchip (small): 5 × 5 mm
  • LED (5mm standard): 5 mm diameter
  • PCB thickness: 1.6 mm (standard)
  • PCB trace width: 0.25-1 mm typical

Connector Specifications:

  • USB-C port: 8.4 × 2.6 mm
  • Headphone jack (3.5mm): 3.5 mm diameter
  • Lightning connector: 6.7 × 1.5 mm
  • HDMI port: 13.9 × 4.45 mm

Display Technology:

  • Smartphone thickness: 7-10 mm
  • Tablet thickness: 5-7 mm
  • Laptop thickness: 15-20 mm (varies widely)
  • Bezel width: 2-5 mm (modern phones)

Rainfall and Weather

Rainfall Measurement:

  • Light rain: 0-2.5 mm per hour
  • Moderate rain: 2.5-10 mm per hour
  • Heavy rain: 10-50 mm per hour
  • Violent rain: >50 mm per hour
  • Annual rainfall: Cities typically 500-2,000 mm per year

Weather Records:

  • Driest place (Atacama Desert): <1 mm per year
  • Wettest place (Mawsynram, India): ~11,872 mm per year
  • Heavy snowfall: 100-500 mm (equivalent water content)

Why millimeters for rain:

  • Precise measurement of small amounts
  • International standard (meteorology)
  • Easy accumulation tracking
  • Standardized gauge readings

Jewelry and Precious Items

Gem and Diamond Sizes:

  • Small diamond: 2-3 mm diameter
  • Medium diamond: 4-6 mm diameter
  • Large diamond: 7-10 mm diameter
  • Pearl sizes: 6-12 mm diameter (cultured pearls)

Ring Measurements:

  • Ring width: 2-8 mm (thin to wide bands)
  • Ring thickness: 1-2 mm typical
  • Stone setting depth: 3-5 mm

Watch Specifications:

  • Case diameter: 36-46 mm (men's watches)
  • Case thickness: 8-15 mm
  • Strap width: 18-24 mm
  • Crown diameter: 6-8 mm

Medical and Healthcare

Medical Measurements:

  • Needle gauge: 25G = 0.5 mm outer diameter
  • Kidney stone: 2-10 mm (small to large)
  • Tumor size: Measured in mm for precision
  • Surgical incision: Length in mm
  • Stent diameter: 2-4 mm typical

Diagnostic Imaging:

  • CT scan slice: 1-5 mm thickness
  • MRI slice: 3-7 mm typical
  • Lesion measurement: Documented in mm
  • Growth tracking: Changes of 1-2 mm significant

Everyday Objects

Common Measurements:

  • Credit card thickness: 0.76 mm
  • US dime thickness: 1.35 mm
  • US quarter thickness: 1.75 mm
  • Smartphone screen glass: 0.5-1 mm
  • Paper thickness: 0.1 mm (standard)
  • Cardboard: 2-5 mm thickness
  • Coin thickness: 1-3 mm typically

Office and School Supplies:

  • Pencil lead diameter: 0.5 mm, 0.7 mm, 0.9 mm
  • Pen tip: 0.5-1 mm
  • Staple wire: 0.5 mm diameter
  • Paper clip wire: 1 mm diameter

Hardware and Fasteners:

  • Screw sizes: M3 (3mm), M4 (4mm), M5 (5mm), M6, M8, M10
  • Nail diameter: 2-6 mm
  • Wire diameter: 1-10 mm range

Common Uses

The millimeter is the standard unit for precise measurements across multiple fields:

Engineering & Machining

Precise measurement requirements for manufacturing, quality control, and technical specifications. Essential for maintaining tolerances in mechanical parts.

Why millimeters in engineering:

  • Decimal system (easy calculations)
  • International standard (ISO)
  • Suitable precision range
  • Compatible with CNC machinery
  • Standard drawing units

Applications:

  • Machine tool specifications
  • Part tolerances (±0.05 mm common)
  • Technical drawing dimensions
  • CAD/CAM software units
  • Quality inspection reports

Convert for technical work: mm to inches | mm to cm

Technical Design & Drafting

Standard measurement unit for architectural, mechanical, and technical drawings worldwide.

Drawing Standards:

  • ISO standards use millimeters
  • Engineering drawings in mm
  • Architecture plans (many countries)
  • Product design specifications
  • Patent drawings (international)

Why mm for drawings:

  • Eliminates fractions (unlike inches)
  • Scales easily (1:10, 1:100, etc.)
  • International communication
  • CAD software standard

Electronics & PCB Design

Measuring component sizes, trace widths, and circuit board dimensions in electronic manufacturing.

PCB Design:

  • Trace width: 0.25-1 mm
  • Component footprints: mm specifications
  • Hole diameters: 0.6-1.2 mm
  • Board thickness: 1.6 mm standard
  • Component spacing: mm grid

Component Specs:

  • All modern ICs specified in mm
  • Resistor/capacitor sizes (mm codes)
  • Connector dimensions
  • Display module sizes

Meteorology & Climate

Measuring rainfall amounts with precision, essential for weather forecasting, agriculture, and climate research.

Rain Gauges:

  • Standard measurement unit worldwide
  • Collected in graduated cylinders (mm scale)
  • Automated weather stations use mm
  • Historical records in mm
  • Climate data standardized

Importance:

  • Drought assessment
  • Flood prediction
  • Agricultural planning
  • Water resource management
  • Climate change tracking

Use our mm converter for scientific calculations.

Manufacturing & Quality Control

Inspecting product dimensions and ensuring parts meet specifications within required tolerances.

QC Measurements:

  • Caliper readings in mm
  • Micrometer measurements
  • Coordinate measuring machines (CMM)
  • Surface profile measurements
  • Dimensional inspection reports

Industries:

  • Automotive parts
  • Aerospace components
  • Medical devices
  • Consumer electronics
  • Precision instruments

Jewelry & Watchmaking

Specifying sizes of gems, watch cases, and jewelry components with precision.

Why jewelers use mm:

  • International gem standard
  • Precise size communication
  • Watch industry standard
  • Setting specifications
  • Customer communication

Measurements:

  • Gemstone dimensions
  • Ring widths and thicknesses
  • Watch case diameters
  • Bracelet link sizes
  • Clasp dimensions

Medical Measurements

Documenting precise measurements in diagnostics, surgery planning, and treatment monitoring.

Clinical Uses:

  • Imaging measurements (CT, MRI, X-ray)
  • Tumor size tracking
  • Surgical planning
  • Needle specifications
  • Medical device dimensions

Why mm in medicine:

  • International standard
  • Appropriate precision
  • Imaging system default
  • Research compatibility
  • Treatment standardization

Common Conversion Mistakes to Avoid

❌ Mistake #1: Wrong mm to Inches Conversion

  • Wrong: 1 mm = 1 inch ÷ 10 = 0.1 inches
  • Correct: 1 mm ≈ 0.03937 inches (or 1 inch = 25.4 mm)
  • Remember: 25.4 mm = 1 inch (memorize this!)
  • Example: 50 mm = 50 ÷ 25.4 ≈ 1.97 inches, NOT 5 inches
  • Tool: mm to inches converter

❌ Mistake #2: Confusing mm with cm

  • Wrong: 1 mm = 1 cm (thinking they're the same)
  • Correct: 10 mm = 1 cm (mm is 10× smaller)
  • Example: 5 mm = 0.5 cm, NOT 5 cm
  • Visual: Fingernail width is ~10 mm = 1 cm
  • Convert: mm to cm

❌ Mistake #3: Adding/Subtracting Wrong Units

  • Wrong: 10 mm + 5 cm = 15 (forgetting units differ)
  • Correct: 10 mm + 5 cm = 10 mm + 50 mm = 60 mm (or 6 cm)
  • Rule: Always convert to same unit first!
  • Example: 25 mm + 3 cm = 25 mm + 30 mm = 55 mm

❌ Mistake #4: Decimal Point Errors

  • Wrong: 1,000 mm = 10 meters
  • Correct: 1,000 mm = 1 meter (three zeros!)
  • Remember: "milli" = 1/1,000, so 1,000 mm = 1 m
  • Example: 2,500 mm = 2.5 m, NOT 25 m

❌ Mistake #5: Using Fractions with Millimeters

  • Wrong: Writing "3/4 mm" or "1½ mm"
  • Correct: Use decimals: "0.75 mm" or "1.5 mm"
  • Why: Metric system is decimal-based
  • Exception: Verbal shorthand only ("half a millimeter" = 0.5 mm in writing)

❌ Mistake #6: Tolerance Notation Confusion

  • Wrong: ±0.1 means 0.1 mm to 0.2 mm range
  • Correct: ±0.1 means 0.1 mm below to 0.1 mm above (0.2 mm total range)
  • Example: 10 mm ±0.1 mm = 9.9 mm to 10.1 mm
  • Important in engineering specs!

Millimeter Conversion Formulas

To Meter:

1 mm = 0.001 m
Example: 5 millimeters = 0.005 meters

To Kilometer:

1 mm = 0.000001 km
Example: 5 millimeters = 0.000005 kilometers

To Hectometer:

1 mm = 0.00001 hm
Example: 5 millimeters = 0.00005 hectometers

To Decimeter:

1 mm = 0.01 dm
Example: 5 millimeters = 0.05 decimeters

To Centimeter:

1 mm = 0.1 cm
Example: 5 millimeters = 0.5 centimeters

To Inch:

1 mm = 0.03937 in
Example: 5 millimeters = 0.19685 inches

To Foot:

1 mm = 0.003281 ft
Example: 5 millimeters = 0.016404 feet

To Yard:

1 mm = 0.001094 yd
Example: 5 millimeters = 0.005468 yards

To Mile:

1 mm = 6.2137e-7 mi
Example: 5 millimeters = 0.000003 miles

To Nautical Mile:

1 mm = 5.3996e-7 NM
Example: 5 millimeters = 0.000003 nautical miles

To Micrometer:

1 mm = 1000 μm
Example: 5 millimeters = 5000 micrometers

To Nanometer:

1 mm = 1000000 nm
Example: 5 millimeters = 5000000 nanometers

To Light Year:

1 mm = 1.0570e-19 ly
Example: 5 millimeters = 5.2849e-19 light years

To Astronomical Unit:

1 mm = 6.6845e-15 AU
Example: 5 millimeters = 3.3422e-14 astronomical units

To Parsec:

1 mm = 3.2408e-20 pc
Example: 5 millimeters = 1.6204e-19 parsecs

To Angstrom:

1 mm = 10000000 Å
Example: 5 millimeters = 50000000 angstroms

To Point (Typography):

1 mm = 2.834644 pt
Example: 5 millimeters = 14.173219 points

To Mil/Thou:

1 mm = 39.370079 mil
Example: 5 millimeters = 196.850394 mils

To Fathom:

1 mm = 0.000547 fath
Example: 5 millimeters = 0.002734 fathoms

To Furlong:

1 mm = 0.000005 fur
Example: 5 millimeters = 0.000025 furlongs

To Link (Gunter's):

1 mm = 0.004971 li
Example: 5 millimeters = 0.024855 links

To Pace:

1 mm = 0.001312 pace
Example: 5 millimeters = 0.006562 paces

To Span:

1 mm = 0.004374 span
Example: 5 millimeters = 0.021872 spans

To Digit:

1 mm = 0.052493 digit
Example: 5 millimeters = 0.262467 digits

To Cable Length:

1 mm = 0.000005 cb
Example: 5 millimeters = 0.000027 cable lengths

To Ell:

1 mm = 0.000875 ell
Example: 5 millimeters = 0.004374 ells

To Finger:

1 mm = 0.008749 finger
Example: 5 millimeters = 0.043745 fingers

To Roman Mile:

1 mm = 6.7568e-7 m.p.
Example: 5 millimeters = 0.000003 Roman miles

To Stadion:

1 mm = 0.000005 stadion
Example: 5 millimeters = 0.000027 stadia

To Chi (Chinese):

1 mm = 0.003 chi
Example: 5 millimeters = 0.015002 chi

To Shaku (Japanese):

1 mm = 0.0033 shaku
Example: 5 millimeters = 0.016502 shaku

To Li (Chinese):

1 mm = 0.000002 li
Example: 5 millimeters = 0.00001 li

To Toise:

1 mm = 0.000513 toise
Example: 5 millimeters = 0.002565 toise

To Bolt:

1 mm = 0.000033 bolt
Example: 5 millimeters = 0.000164 bolts

To Rope:

1 mm = 0.000164 rope
Example: 5 millimeters = 0.00082 ropes

To Smoot:

1 mm = 0.000588 smoot
Example: 5 millimeters = 0.002938 smoots

To Sajene:

1 mm = 0.000469 sajene
Example: 5 millimeters = 0.002343 sajenes

To Ken:

1 mm = 0.00055 ken
Example: 5 millimeters = 0.00275 ken

To Wa:

1 mm = 0.0005 wa
Example: 5 millimeters = 0.0025 wa

To Vara:

1 mm = 0.001193 vara
Example: 5 millimeters = 0.005965 varas

To Aln:

1 mm = 0.001684 aln
Example: 5 millimeters = 0.008418 alnar

To Cubit (Royal/Egyptian):

1 mm = 0.001912 cubit
Example: 5 millimeters = 0.00956 cubits

To Versta:

1 mm = 9.3738e-7 versta
Example: 5 millimeters = 0.000005 versts

To Arpent:

1 mm = 0.000017 arpent
Example: 5 millimeters = 0.000086 arpents

To Ri (Japanese):

1 mm = 2.5465e-7 ri
Example: 5 millimeters = 0.000001 ri

To Klafter:

1 mm = 0.000527 klafter
Example: 5 millimeters = 0.002636 klafter

To Yojana:

1 mm = 7.4074e-8 yojana
Example: 5 millimeters = 3.7037e-7 yojanas

To Skein:

1 mm = 0.000009 skein
Example: 5 millimeters = 0.000046 skeins

Frequently Asked Questions

There are exactly 10 millimeters (mm) in 1 centimeter (cm). Conversion:

  • 1 cm = 10 mm
  • To convert cm to mm: multiply by 10
  • To convert mm to cm: divide by 10 Examples:
  • 5 cm = 50 mm
  • 2.5 cm = 25 mm
  • 75 mm = 7.5 cm Memory aid: "Centi-" means 1/100 meter, "milli-" means 1/1,000 meter, so 10 mm = 1 cm. Use our cm to mm converter for instant conversions.

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Millimeter Quick Info

mm
Symbol:mm
Category:Length
System:Metric (SI)
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