Square Millimeter to Square Inch Conversion Calculator: Free Online Tool

Convert square millimeters to square inches with our free online area converter.

From:

Square Millimeter

To:

Square Inch

How to Use the Calculator:

Enter the value you want to convert in the 'From' field (Square Millimeter).
The converted value in Square Inch will appear automatically in the 'To' field.
Use the dropdown menus to select different units within the Area category.
Click the swap button (⇌) to reverse the conversion direction.

How to Convert Square Millimeter to Square Inch

Converting Square Millimeter to Square Inch involves multiplying the value by a specific conversion factor, as shown in the formula below.

Formula:

1 Square Millimeter = 0.00155 square inches

Example Calculation:

Convert 10 square millimeters: 10 × 0.00155 = 0.0155 square inches

Disclaimer: For Reference Only

These conversion results are provided for informational purposes only. While we strive for accuracy, we make no guarantees regarding the precision of these results, especially for conversions involving extremely large or small numbers which may be subject to the inherent limitations of standard computer floating-point arithmetic.

Not for professional use. Results should be verified before use in any critical application. View our Terms of Service for more information.

What is a Square Millimeter and a Square Inch?

The Square Millimeter (symbol: mm² or sq mm) is a unit of area in the International System of Units (SI). It represents the area of a square whose sides measure exactly one millimeter (mm) in length.

Key relationships:

1 mm² = 1 millimeter × 1 millimeter
1 meter (m) = 1,000 millimeters (mm)
1 square meter (m²) = (1,000 mm) × (1,000 mm) = 1,000,000 mm²
1 square centimeter (cm²) = (10 mm) × (10 mm) = 100 mm² (or 1 mm² = 0.01 cm²)

In terms of imperial/US customary units:

1 inch = 25.4 mm exactly
1 square inch (in²) = (25.4 mm)² = 645.16 mm²
Therefore, 1 mm² ≈ 0.00155 square inches (in²)

The Square Inch (symbol: in² or sq in) is a unit of area in both the British Imperial and United States customary systems of measurement. It is defined as the area of a square whose sides each measure exactly one inch in length.

Mathematically: 1 in² = 1 inch × 1 inch

It is related to other imperial/US customary area units:

1 square foot (ft²) = 144 in² (since 1 ft = 12 in)
1 square yard (yd²) = 1,296 in² (since 1 yd = 3 ft = 36 in)

In terms of metric units, based on the international inch defined as exactly 25.4 millimeters (mm):

1 in² = (25.4 mm)² = 645.16 mm²
1 in² = (2.54 cm)² = 6.4516 cm²
1 in² = (0.0254 m)² = 0.00064516 m²

Note: The Square Millimeter is part of the metric (SI) system, primarily used globally in science and trade. The Square Inch belongs to the imperial/US customary system.

History of the Square Millimeter and Square Inch

Origin (Millimeter): The square millimeter is derived directly from the millimeter, which itself is derived from the meter, the base unit of length in the metric system established during the French Revolution. The prefix "milli-" denotes one-thousandth (1/1000).
Metric System Coherence: The square millimeter arose naturally within the decimal-based metric system as the appropriate unit for measuring very small areas, by squaring the millimeter length unit.
SI Standardisation: With the formal establishment of the International System of Units (SI) in 1960, the square meter (m²) was confirmed as the derived unit for area. The square millimeter, formed using an SI prefix (milli-) and the base unit (meter) squared, became a standard SI unit for area, suitable for fine-scale measurements.
Technological Need: The widespread use of the square millimeter grew significantly with advancements in science and technology requiring precise measurement of very small objects and features, such as in microscopy, electronics, and precision engineering.

Origin: The square inch originates from the inch, a fundamental unit of length in the English (later British Imperial and US customary) systems. The concept of measuring area based on squaring a length unit dates back to ancient measurement practices.
Evolution of the Inch: The precise length of the inch itself varied significantly over centuries and by location before the 20th century. Early definitions were often based on barleycorns or thumb widths.
Standardization (International Inch): In 1959, the United States and countries of the Commonwealth of Nations agreed upon the definition of the international yard as exactly 0.9144 meters. This agreement consequently defined the international inch as exactly 25.4 millimeters (mm). This standardization fixed the exact metric equivalent of the square inch as 6.4516 square centimeters (cm²).
Continued Use: Despite widespread global metrication, the square inch remains a common unit in the United States, and to a lesser extent in the United Kingdom and Canada, particularly in certain trades and industries.

Common Uses for square millimeters and square inches

Explore the typical applications for both Square Millimeter (metric) and Square Inch (imperial/US) to understand their common contexts.

Common Uses for square millimeters

The square millimeter is used for measuring extremely small areas, primarily in technical and scientific contexts:

Engineering and Manufacturing: Specifying cross-sectional areas of fine wires, optical fibers, and small mechanical components; defining surface areas of micro-electromechanical systems (MEMS); measuring surface roughness features.
Material Science: Quantifying the area of microscopic structures like crystal grains, pores, or defects in materials under microscopic analysis; measuring the area of test indentations (e.g., in hardness testing).
Electronics: Defining the area of contact pads, traces, vias, and components on printed circuit boards (PCBs) and integrated circuits (ICs).
Optics: Measuring the effective area of small apertures, detector pixels, laser beam cross-sections, or optical fiber cores.
Medicine and Biology: Measuring the cross-sectional area of fine capillaries, nerve fibers, or cells under a microscope; quantifying the area of small lesions or tissue samples.
Physics: Used in calculations involving small surfaces, pressures on small areas, or flux densities.

It is generally far too small for everyday measurements like room sizes or land areas.

Common Uses for square inches

The square inch is primarily used for measuring relatively small areas, especially in specific fields:

Engineering and Manufacturing (US/UK): Specifying the cross-sectional area of wires, beams, or pipes; surface area of small mechanical parts; material stress calculations (often in conjunction with psi - pounds per square inch).
Electronics: Measuring the surface area of components like microchips or circuit boards.
Material Science: Quantifying the area of material samples for testing (e.g., fabric swatches, paper sheets).
Printing and Graphic Design (US): Sometimes used for small image dimensions or layout areas, although points and picas are also common.
Pressure Measurement: Implicitly used as the denominator in the widely used unit of pressure, pounds per square inch (psi).
Retail (Specific Goods): Occasionally used for pricing small, specialized items sold by area (e.g., certain textiles, craft supplies).
Crafts and Hobbies: Measuring small patches of fabric, paper dimensions, or design elements in quilting, scrapbooking, etc.

It is generally too small for measuring room dimensions (square feet are used) or land (acres or square miles are used).

Frequently Asked Questions

Questions About Square Millimeter (mm²)

How many square millimeters are in a square centimeter?

There are exactly 100 square millimeters (mm²) in one square centimeter (cm²). This is because 1 cm = 10 mm, so 1 cm² = (10 mm) × (10 mm) = 100 mm².

How many square millimeters are in a square meter?

There are exactly 1,000,000 square millimeters (mm²) in one square meter (m²). This is because 1 m = 1000 mm, so 1 m² = (1000 mm) × (1000 mm) = 1,000,000 mm².

How do you convert square millimeters to square inches?

To convert square millimeters to square inches, multiply the area in square millimeters by approximately 0.00155 (or divide by 645.16).

Conversion formula: Area [in²] ≈ Area [mm²] × 0.00155
Example: 500 mm² ≈ 500 × 0.00155 in² ≈ 0.775 in²

Is the square millimeter an official SI unit?

Yes, the square millimeter (mm²) is an SI unit of area. It is derived from the SI base unit of length, the meter, using the standard SI prefix "milli-". While the square meter is the fundamental SI unit of area, square millimeters are standard for measuring very small areas.

Is mm² commonly used for paper sizes?

No. Standard paper sizes (like A4, Letter) are typically defined using millimeters (mm) for their dimensions (length and width), but their area is usually expressed in square meters (m²) or square centimeters (cm²) if needed, not typically square millimeters.

About Square Inch (in²)

How many square inches are in one square foot?

There are exactly 144 square inches in one square foot. This is because 1 square foot is the area of a square with 12-inch sides (12 inches × 12 inches = 144 in²).

How do you convert square inches to square centimeters?

To convert square inches to square centimeters, multiply the area in square inches by 6.4516.

Conversion formula: Area [cm²] = Area [in²] × 6.4516
Example: 10 in² = 10 × 6.4516 cm² = 64.516 cm²

Is the square inch the same as "inches squared"?

Yes, the terms "square inch" (in²) and "inches squared" refer to the same unit of area.

Is the square inch an SI unit?

No, the square inch is not part of the International System of Units (SI). It belongs to the British Imperial and US customary systems. The corresponding SI units for area are the square meter (m²) and its derived units like the square centimeter (cm²) and square millimeter (mm²).

Why is psi (pounds per square inch) so common?

Pounds per square inch (psi) became a standard unit for measuring pressure (force per unit area) in many English-speaking countries, particularly in engineering and industrial applications, due to the established use of the pound for force and the inch for dimensions within the Imperial and US customary systems. While the SI unit for pressure is the Pascal (Pa), psi remains deeply ingrained in many technical fields in the US.

Conversion Table: Square Millimeter to Square Inch

Square Millimeter (mm²)	Square Inch (in²)
1	0.002
5	0.008
10	0.016
25	0.039
50	0.078
100	0.155
500	0.775
1,000	1.55

All Area Conversions

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