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.