MM to inches converter, millimeters to inches, with the fraction.

To convert mm to inches, divide the millimeters by 25.4. So 1 mm is 0.03937 inches, 10 mm is 0.3937 inches and 25.4 mm is exactly 1 inch. In a workshop you want the fraction as well: this converter shows the inch answer as a decimal and as the nearest 1/2, 1/4, 1/8, 1/16, 1/32, 1/64 or 1/128, and tells you how far that fraction is from the real size.

How to use the converter

1. Type a size in the Millimeters (mm) box, for example 8 for an 8 mm drill bit.
2. Read the decimal inches in the big result. The line under it gives the nearest fraction, for 8 mm that is 5/16 inch.
3. Change Inch fractions to the finest division your tool uses. The note after the fraction says how many thousandths of an inch it is off.
4. Type inches in the Inches (in) box to go back. You can type a fraction there.
5. Click a row in Common values to load it. Use Print chart to put a mm to inch sheet on the workshop wall.

The definition: 25.4 millimeters

An inch is exactly 25.4 millimeters, because it is exactly 2.54 centimeters. NIST Handbook 44 (2026), Appendix C lists "1 inch (in) 2.54 centimeters (exactly)" and "0.001 inch (exactly) 0.025 4 millimeter (exactly)" for the mil. NIST Special Publication 811, Appendix B.8 gives the inch to meter factor as 2.54 E-02 and marks it exact. Both were checked 18 Oct 2026. Going from inches to millimeters the answer always ends. Going from millimeters to inches it usually does not, because you divide by 25.4, so the last digit is rounded and nothing before it is.

A mental shortcut and its error

Divide by 25. That is the same as multiplying by 0.04. For 50 mm: 50 ÷ 25 = 2 inches. The exact value is 1.9685 inches. The shortcut is always 1.6 percent high, so it is fine for a rough idea of a long part and wrong for a fit: 10 mm is 0.3937 inches, but the shortcut says 0.4. A few anchors are worth knowing: 6.35 mm is 1/4 inch, 12.7 mm is 1/2 inch and 25.4 mm is 1 inch.

Worked examples

• An 8 mm drill bit. 8 ÷ 25.4 = 0.315 inches. The nearest 1/16 is 5/16 inch, which is 0.3125 inches. The gap is 0.0025 inches, so an 8 mm bit is a little larger than a 5/16 bit.
• An M6 bolt, 6 mm across the threads. 6 ÷ 25.4 = 0.2362 inches. A 1/4 inch bolt is 6.35 mm, so the two are not the same size, though they are close.
• A 13 mm nut. 13 ÷ 25.4 = 0.5118 inches. A 1/2 inch spanner opening is 12.7 mm, which is 0.3 mm smaller than the nut, so it will not slide on.
• Sheet metal 1.5 mm thick. 1.5 ÷ 25.4 = 0.0591 inches, or 59.1 thousandths of an inch.

A mistake specific to millimeters and inches

Close does not mean equal. Metric and inch sizes sit near each other, which makes it tempting to swap one for the other. 8 mm and 5/16 inch are 0.315 and 0.3125 inches. That is 2.5 thousandths apart. On a drill that is a loose hole, on a press fit it is a failure. Use the fraction line to see the gap and choose a finer fraction if it matters. The other common mistake is typing centimeters as millimeters: 2 cm is 20 mm, not 2.

Common metric sizes against inch fractions

Metric hardware and tools are close to some inch sizes without matching them. The table lists common metric sizes, their exact inch value to four places, the nearest 1/64 inch, and the gap. A gap under a thousandth of an inch is close enough for most work, and a gap of several thousandths is not a match.

The table shows why a 6 mm drill is not a 1/4 inch drill, and why 19 mm is near 3/4 inch but a little smaller. For a clearance hole, pick the next size up. For a press fit, use the tolerance on the drawing and not the nearest fraction.

Small values: thousandths and microns

Machinists talk in thousandths of an inch, called mils or thou. One mil is exactly 0.0254 mm, so 1 mm is 39.37 mils. A sheet 0.5 mm thick is 19.7 mils, and a 3D printer layer of 0.2 mm is 7.87 mils. The card under the result gives the same length in mils, so you do not need a second conversion.

Who uses mm to inches

Mechanics and DIY users with a metric car and inch tools. Woodworkers with a metric plan and an inch tape. People buying screws, drill bits, bearings and pipe from abroad. 3D printing and photography users who see lens or filament sizes in millimeters. Anyone reading a spec sheet made for a metric market. For larger lengths use the CM to inches converter. For the other way round, from inches to millimeters, see the Inch to MM converter.