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MM to Inches Converter

How to Convert MM to Inches

To convert millimeters to inches, divide by 25.4. One inch equals exactly 25.4 millimeters by international definition (since 1959). So 1 mm = 0.03937 inches, or approximately 1/25 of an inch. This conversion is essential in engineering, manufacturing, 3D printing, CNC machining, and any field where metric and imperial specifications intersect. Many bolts, screws, and hardware come in both metric (mm) and imperial (inches) sizes.

The MM to Inches Formula

The formula is: inches = millimeters ÷ 25.4. For fractional inches (common in US construction and hardware), find the nearest fraction: divide the decimal inches by the fraction increment (1/16, 1/32, or 1/64) and round. For example, 10 mm = 0.3937" ≈ 25/64" (0.3906") or approximately 3/8" (0.375"). The 'mil' or 'thou' (thousandth of an inch) is useful for thin materials: 1 mm = 39.37 mils.

Frequently Asked Questions

How many inches is 1 mm?

1 millimeter equals 0.03937 inches, or approximately 1/25 of an inch. To convert mm to inches, divide by 25.4. To convert inches to mm, multiply by 25.4.

How do I convert mm to fractional inches?

Divide mm by 25.4 to get decimal inches. Then multiply by the denominator you want (16 for 16ths, 32 for 32nds, 64 for 64ths) and round. Example: 10 mm = 0.3937" × 32 = 12.6/32 ≈ 13/32".

What is 25.4 mm in inches?

25.4 mm equals exactly 1 inch. This is the exact definition — 1 inch = 25.4 mm was established by international agreement in 1959.

Is a 10mm wrench the same as 3/8 inch?

Very close but not exact. 10mm = 0.3937" while 3/8" = 0.375" — a difference of 0.019" (0.47 mm). A 3/8" wrench can usually fit a 10mm bolt, but it may be slightly loose. For precision work, use the correct metric or imperial tool.

What is a mil or thou?

A mil (also called thou) is 1/1000 of an inch = 0.0254 mm. It's used in manufacturing for thin materials like sheet metal, wire gauge, paint thickness, and PCB traces. 1 mm = 39.37 mils.

How do I measure mm without a metric ruler?

If you only have an imperial ruler: 1/16" ≈ 1.6 mm, 1/8" ≈ 3.2 mm, 1/4" ≈ 6.4 mm, 1/2" ≈ 12.7 mm, 1" = 25.4 mm. For precise work, use a digital caliper that displays both mm and inches.