Millimeters to Inches conversion

Millimeters and inches

The millimeter is the smallest unit of length most people use day to day. It belongs to the metric system: 10 mm make a centimeter and 1,000 mm make a meter. The inch belongs to the imperial and US customary systems and is used for screen sizes, pipe diameters, bolts, tyres and countless product specifications. The two meet all the time in technical work, so the conversion is worth knowing precisely.

The exact factor

Since the international agreement of 1959, one inch equals exactly 25.4 millimeters. That makes the conversion a single division: millimeters ÷ 25.4 = inches. The number is exact, so any error in your result comes only from rounding at the end.

For a quick estimate, remember that 25 mm is roughly one inch and 100 mm is roughly four inches. For precise work — machining, 3D printing, engineering drawings — use the full 25.4.

Handy values

• 1 mm = 0.039 in (a little over 1/32 in)
• 5 mm = 0.197 in (about 3/16 in)
• 10 mm = 0.394 in (about 3/8 in)
• 12.7 mm = 0.5 in exactly
• 25.4 mm = 1 in exactly
• 50 mm = 1.97 in
• 100 mm = 3.94 in

Where millimeters meet inches

Screens and photos. Phone and monitor diagonals are quoted in inches, while the physical width of the device is given in millimeters. Hardware. Metric bolts (M6, M8) live next to imperial sizes (1/4", 5/16") in the same drawer; 6 mm is 0.236 in, which is why a 1/4-inch wrench almost, but not quite, fits. Jewellery and watches. Ring widths, case diameters and strap lugs are in millimeters, while some catalogues still use inches.

Reading the result on a tape

Imperial tape measures divide the inch into sixteenths. To use a decimal result, multiply the fraction after the decimal point by 16 and round: 0.394 in × 16 = 6.3, so 10 mm is about 6/16 in, or 3/8 in.

Worked examples

A 42 mm watch case. 42 ÷ 25.4 = 1.65 in — a mid-size men's watch; 38 mm is 1.5 in, 46 mm is 1.81 in.

An 18 mm plywood sheet. 18 ÷ 25.4 = 0.71 in, which is what American suppliers sell as 3/4-inch plywood (nominally 19.05 mm, actually often 18.3 mm).

A 700 mm bike wheel. 700 ÷ 25.4 = 27.6 in — hence the old "27-inch" road wheels that 700C rims replaced.

Frequently asked questions

How many millimeters are in an inch? Exactly 25.4 since 1959. Before that the US and UK inches differed by a few millionths.

What is 10 mm in fractions of an inch? 10 ÷ 25.4 = 0.394 in. Times 16 is 6.3, so the nearest sixteenth is 6/16, which reduces to 3/8 in. A 10 mm socket and a 3/8-inch socket are nearly interchangeable — nearly.

Why do rulers show both? Most rulers sold outside the US have millimeters on one edge and inches on the other; the mm edge is finer (1 mm ≈ 1/25 in) and better for small measurements.

Related conversions

Ten millimeters make a centimeter, so for larger objects the centimeters to inches converter gives the same answer with fewer digits. Going the other way, inches to millimeters multiplies by 25.4.