Mm to Inches Converter

Quick Answer

Last updated: 11 November 2026 ·How we calculate this: NIST exact inch definition

How This Mm to Inches Converter Works

Bolts, drill bits, wrenches, jewelry, camera lenses and 3D prints are often sized in millimeters while tools and plans in the US use inches. Choose the direction, pick how fine the fraction should be, enter the size and press Calculate. You get the decimal inch to four places, the nearest fraction, how far that fraction is from the exact size in millimeters, centimeters and thousandths of an inch.

How to Use the Mm to Inches Converter

1. Choose millimeters to inches or inches to millimeters.
2. Choose how fine the fraction should be (1/8 to 1/64 inch).
3. Enter the size.
4. Press Calculate.
5. Read the decimal inch, the nearest fraction and its error, cm and thousandths.

Mm to Inches Formulas

• Definition: 1 inch = 25.4 millimeters exactly
• Inches from millimeters: in = mm ÷ 25.4 = mm × 0.0393701
• Millimeters from inches: mm = in × 25.4
• Nearest fraction: round (in × 16) to a whole number, then divide by 16 (or 32 or 64) and reduce
• Thousandths: 1 thou (mil) = 0.001 in = 0.0254 mm

Key Terms

• Millimeter (mm): one thousandth of a meter; 10 mm make a centimeter.
• Inch (in): exactly 25.4 mm.
• Fractional inch: an inch size written as halves, quarters, eighths, sixteenths and so on.
• Thou or mil: one thousandth of an inch, 0.0254 mm.
• Tolerance: how far a real size may be from the size on the drawing.

Worked Example

Will a 10 mm bolt fit a 3/8 inch hole?

• 10 ÷ 25.4 = 0.3937 in
• 3/8 in = 0.375 in = 9.525 mm
• The bolt is 0.475 mm larger than the hole, so it will not fit; a 25/64 in hole (0.3906 in, 9.922 mm) would.

Going the other way, a 1/4 in drill bit is 0.25 × 25.4 = 6.35 mm.

Real-Life Examples

• Drill bits: a 6 mm bit is 0.2362 in, about 15/64 in.
• Plywood: 18 mm sheet is 0.709 in, slightly under 3/4 in.
• Jewelry: an 8 mm bead is 0.315 in, about 5/16 in.
• Rain and snow: 25.4 mm of rain is exactly 1 inch.

Millimeter vs Inch

• Millimeter (mm): one thousandth of a meter, the main unit in metric engineering.
• Inch (in): exactly 25.4 mm, used for US hardware and lumber.
• Size: 1 mm = 0.03937 in, about 1/25 of an inch.

Millimeters to Inches Chart

Exact factor: 1 in = 25.4 mm.

Fractions, Decimals and Tolerances

Woodworkers read tape measures in sixteenths of an inch, while machinists and engineers use decimal inches to three or four places. A sixteenth of an inch is 1.5875 mm, so rounding to the nearest 1/16 can be off by up to 0.79 mm. That is fine for framing, but not for bolts and bearings. Choose 1/32 or 1/64 inch for drill bits, and check the fraction error line before you cut or drill.

Metric and inch sizes rarely match exactly. A 13 mm wrench (0.512 in) is close to 1/2 inch (12.7 mm), and a 19 mm socket (0.748 in) is close to 3/4 inch (19.05 mm), but on a tight fastener the small difference can round off the corners. Use the size the part was made in when you can, and use this converter to find the closest substitute when you cannot.

Common Mistakes to Avoid

• Dividing by 2.54. That is for centimeters; millimeters use 25.4.
• Trusting a rough fraction for tight fits. 1/16 in rounding can be off by 0.79 mm.
• Assuming 13 mm equals 1/2 in. 1/2 in is 12.7 mm.
• Mixing mils and millimeters. A mil is 0.0254 mm, not 1 mm.
• Reading 0.375 in as 3.75 mm. 0.375 in is 9.525 mm.

Frequently Asked Questions

References & Sources