Inch to MM converter, inches to millimeters, fractions in.

To convert inches to mm, multiply the inches by 25.4. So 1 inch is 25.4 mm, 2 inches is 50.8 mm and 1/4 inch is 6.35 mm. The factor is exact, so an inch fraction with a power of two below the line always gives a mm answer that ends. 5/16 inch is 7.9375 mm, with no digits cut off. This page takes fractions as typed.

How to use the converter

1. Type the size in the Inches (in) box. Decimals, fractions and mixed numbers such as 3 1/2, 5/16 and ¾ all work.
2. The big result shows millimeters. The cards under it show centimeters, thousandths of an inch (mils) and feet.
3. Use Decimals to set the places. 2 places suits most parts, 3 suits precision work.
4. Pick Inch fractions if you type a decimal and want to know the fraction it matches.
5. Click a row in Common values to load it. Copy table copies the whole list. Print chart prints the fraction chart.

The definition: 25.4 millimeters

One inch is exactly 25.4 mm. NIST Handbook 44 (2026), Appendix C gives 1 inch as 2.54 centimeters exactly, and 0.001 inch as 0.0254 millimeter exactly. NIST Special Publication 811, Appendix B.8 has no inch to millimeter row, but its inch to meter factor of 2.54 E-02 is marked exact, and 1 m is 1,000 mm. Both were checked 18 Oct 2026. Taken together, 1 in = 0.0254 m = 25.4 mm with no rounding, so inch to mm is a clean multiplication.

A mental shortcut and how far off it is

Multiply by 25 and add 1.6 percent, or by 25 and add 0.4 for each inch. For 3 inches: 3 × 25 = 75, plus 3 × 0.4 = 1.2, gives 76.2 mm, which is exact here. Without the add-on, 25 per inch is 1.57 percent low: a 12 inch rule is 300 mm by the shortcut and 304.8 mm in truth, a gap of 4.8 mm.

Worked examples

• A 3/8 inch drill bit. 3/8 is 0.375. 0.375 × 25.4 = 9.525 mm. Metric bits come in steps of 0.5 mm or 1 mm, so check the nearest size in your set against 9.525 mm.
• A 1/2 inch socket or bolt head. 0.5 × 25.4 = 12.7 mm. A 13 mm spanner opening is 0.3 mm larger, so a 13 mm spanner fits a 1/2 inch head loosely.
• A 2 1/4 inch pipe. 2 1/4 is 2.25. 2.25 × 25.4 = 57.15 mm.
• A 0.040 inch sheet. 0.04 × 25.4 = 1.016 mm, or 40 mils (thousandths of an inch).
• A 6 inch caliper reading. 6 × 25.4 = 152.4 mm, which is 15.24 cm.

A mistake specific to this pair

Rounding 25.4 to 25. On a short part it hides, on a long one it shows. A 40 inch run is 1,016 mm; at 25 per inch it is 1,000 mm, a full 16 mm short. The second mistake is reading a fraction as a decimal: 7/16 is 0.4375 inch, not 0.716. And a thou is not a mm: 40 thou is 0.04 inch or 1.016 mm, a common mix-up when a gauge is in mils and a drawing is in mm.

The inch fraction chart in millimeters

Every common inch fraction has an exact millimeter value. The factor is 25.4, so a fraction with a power of two below the line gives a finite decimal. The rows follow the marks on a tape and the sizes in a drill index.

Use the table to see how the steps run. Each 1/16 inch is 1.5875 mm, and so a 1/2 inch step is eight of them, which is 12.7 mm. Metric sizes fall between them, which is why a 6 mm, 8 mm or 10 mm bit never quite matches an inch bit.

Working to a tolerance

If a drawing gives 2.000 in plus or minus 0.005, the limits are 50.673 mm and 50.927 mm, a total band of 0.254 mm. Convert both limits, not just the middle, and keep three decimals. Rounding the middle to 51 mm and then adding a rounded tolerance can leave a part slightly out. The Decimals setting keeps every row of the table at the same precision.

Who uses inches to mm

Mechanics converting socket and bolt sizes. Plumbers and electricians matching pipe and conduit between a US catalogue and a European one. Machinists who work in both systems. Designers and 3D printers who get a model in inches and a printer in mm. Photographers checking sensor and filter sizes. For longer lengths use the Inches to CM converter, and for the reverse direction the MM to inches converter.