How to Convert MM to Inches

How it works

One inch is defined as exactly 25.4 mm, so inches = mm ÷ 25.4, and mm = inches × 25.4.

Method 1: divide by 25.4

This is the exact method and the one to use whenever accuracy matters. An inch is 25.4 mm, so the number of inches is the number of millimetres divided by 25.4.

Method 2: multiply by 0.03937

One millimetre is 0.0393701 inches. Multiplying by 0.03937 gives the same answer as dividing by 25.4 to four significant figures, and some people find multiplication easier on a phone calculator. 30 mm × 0.03937 = 1.181″.

Method 3: go through centimetres

If you already know the centimetre factor, move the decimal point one place left to turn millimetres into centimetres, then divide by 2.54. 85 mm = 8.5 cm, and 8.5 ÷ 2.54 = 3.346″. This is exactly equivalent — 25.4 mm and 2.54 cm are both one inch. The cm to inches converter covers that step.

Estimating in your head

For a rough figure, divide by 25: drop two zeros and multiply by 4. 150 mm → 1.5 × 4 = 6″ (exact: 5.91″). The estimate is always slightly high, by about 1.6%. Another quick check: 1 inch is “two and a half centimetres”, so 10 mm is a bit under half an inch.

Turning the answer into a fraction

Multiply the decimal part of the inches by 16 and round: 12 mm = 0.472″, and 0.472 × 16 = 7.6 → 8/16 = 1/2″. Because 7.6 is not close to a whole number, 1/2″ is only an approximation — check the error with the mm to inches fraction calculator.

How to check your answer

Multiply your answer by 25.4; you should get back the millimetres you started with. Also sanity-check the size: the inch value must always be much smaller than the millimetre value (about 1/25 of it). If it is bigger, you multiplied instead of dividing.

Practice questions

Why both systems are still used

Almost every country measures in millimetres, but the United States still uses inches for construction, woodworking and many consumer products. Global manufacturing means you will often meet both on the same project: a metric screw in an American cabinet, or a phone screen sized in inches but built in millimetres. Knowing that an inch is exactly 25.4 mm is enough to move between them reliably. For conversions in the other direction, see the inches to mm pages.