Millimeters to Inches

American shop drawings and tape measures use fractions of an inch rather than decimals, so a millimeter value often needs a second step. Divide by 25.4, then find the nearest sixteenth by multiplying the decimal by 16. 10 mm is 0.394 in; 0.394 × 16 is 6.3, so it sits just above 3/8 in (6/16, which is 9.525 mm). For finer work use thirty-seconds: 10 mm is closest to 13/32 in.

Common pairs are worth memorizing. 3 mm is close to 1/8 in (3.175 mm), 6 mm to 1/4 in (6.35 mm), 13 mm to 1/2 in (12.7 mm) and 19 mm to 3/4 in (19.05 mm). They are near-misses, not equals, which is why a metric bolt wobbles in an inch-sized hole.

Rainfall is reported in millimeters in most of the world and in inches in the US: 25.4 mm of rain is exactly one inch, and a 50 mm storm is just under two inches. Phone and laptop specifications list thickness in millimeters, so a 7.8 mm phone is 0.31 in thick and a 15.6 mm laptop is 0.61 in.

Paper is the third case. A4 is 210 × 297 mm, which is 8.27 × 11.69 in, a little narrower and longer than US Letter at 8.5 × 11 in. If you need to fit a document to the international size, the PDF to A4 converter rescales the pages for you.

The most frequent error with this pair is dividing by 2.54 instead of 25.4. That is the factor for centimeters, and it makes every answer ten times too large: 50 mm would come out as 19.7 in instead of 1.97 in. A sanity check helps: an inch is about the width of a thumb, so anything under 25 mm must be under an inch.

The factor 25.4 itself is exact, a result of the 1959 international yard agreement that fixed the inch at 2.54 cm. For the opposite direction see inches to millimeters.