mm en pouces
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. With the site set to metric the big readout is the millimetres instead, and the tape mark sits under it.
Exemple détaillé
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.
Exercez-vous
Quick referencefor mm and inches
- The definition
- 1 in = 25.4 mm exactly · 1 mm = 0.03937 in
- Fractions in mm
- 1/16 = 1,6 · ⅛ = 3,2 · ¼ = 6,35 · ⅜ = 9,5 · ½ = 12,7 · ⅝ = 15,9 · ¾ = 19,05 · 1 in = 25,4
- Round numbers
- 10 mm ≈ 25/64 · 20 mm ≈ 25/32 · 50 mm ≈ 1 31/32 · 100 mm = 3 15/16 · 300 mm ≈ 11 13/16
- Pieds
- 1 ft = 304,8 mm · 1 m = 39,37 in = 3′ 3⅜″
- Quick check
- mm ÷ 25 is close enough to spot a wrong answer; ÷ 25.4 to cut
Mistakes to catchbefore you trust the number
- Dividing by 25 instead of 25.4 in your head — over a metre that is a 16 mm slip, which is more than half an inch.
- Reading the tool's tape fraction as exact: it is the nearest sixteenth, and the decimal beside it is the true figure.
- Confusing centimetres with millimetres on a plan; a dimension with no units and a decimal point is usually metres, not mm.
La formule
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.
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