How do I convert mm to inches?

Divide the millimetres by 25.4. An inch is defined as exactly 25.4 mm, so 100 mm ÷ 25.4 ≈ 3.937 inches. This tool does the division live, and it also reads the answer back as a tape fraction so you are not squinting at a decimal on the saw.

What is 1 inch in millimetres?

Exactly 25.4 mm — that is the definition of the inch, not a rounded figure. So 2 inches is 50.8 mm, half an inch is 12.7 mm, and a sixteenth is 1.5875 mm. Type either box and the other follows instantly.

Why does the fraction not match the decimal exactly?

A tape is only marked in sixteenths (or thirty-seconds), so the fraction here is the nearest mark, not the exact number. The exact decimal is shown beside it. On a rip you usually want the mark; on a layout you may want the decimal.

Is this the same as converting cm to inches?

Same idea, one decimal place over — a centimetre is 10 mm, so multiply your cm figure by 10 and drop it in the mm box. For metres, multiply by 1000. The inch definition does the rest.

mm to inches

This is the exact 25.4 mm-per-inch definition worked both ways. The tape fraction is rounded to the nearest mark, so trust the decimal when a job is fussy about the last few thousandths.

Type a figure in either box and the other updates as you go. The big readout is the inch value snapped to the nearest sixteenth — the mark you actually cut to — with the exact decimal underneath for when a sixteenth is not close enough.

Worked example

A drawing calls out 100 mm. Divide by 25.4: 100 ÷ 25.4 ≈ 3.937 in. On the tape that rounds to 3 15/16″ — a thirty-second under 4 inches. Going the other way, 3/4″ is 0.75 × 25.4 = 19.05 mm.

The formula

inches = mm ÷ 25.4 · mm = inches × 25.4

The inch was fixed at exactly 25.4 millimetres by international agreement in 1959, so this is a definition rather than a measurement — the conversion never drifts and needs no rounding until you snap it to a tape mark.

Questions

How do I convert mm to inches?: Divide the millimetres by 25.4. An inch is defined as exactly 25.4 mm, so 100 mm ÷ 25.4 ≈ 3.937 inches. This tool does the division live, and it also reads the answer back as a tape fraction so you are not squinting at a decimal on the saw.
What is 1 inch in millimetres?: Exactly 25.4 mm — that is the definition of the inch, not a rounded figure. So 2 inches is 50.8 mm, half an inch is 12.7 mm, and a sixteenth is 1.5875 mm. Type either box and the other follows instantly.
Why does the fraction not match the decimal exactly?: A tape is only marked in sixteenths (or thirty-seconds), so the fraction here is the nearest mark, not the exact number. The exact decimal is shown beside it. On a rip you usually want the mark; on a layout you may want the decimal.
Is this the same as converting cm to inches?: Same idea, one decimal place over — a centimetre is 10 mm, so multiply your cm figure by 10 and drop it in the mm box. For metres, multiply by 1000. The inch definition does the rest.

Related tools

New tools, now and then

We add calculators most weeks. Drop your email and get a short note when a new one lands — nothing else, no selling your address, one-click out.