Inches to millimeters converter

How many millimeters are in an inch?

One inch is exactly 25.4 millimeters, so multiply inches by 25.4 to get mm. 2 in = 50.8 mm, 1/2 in = 12.7 mm and 10 in = 254 mm. The factor is exact, so the answers are too; it applies to any inch dimension, from drill bits to paper sizes.

Where 25.4 comes from

The international inch has been defined as exactly 0.0254 m since 1959 (NIST). A meter is 1,000 mm, so 0.0254 × 1,000 = 25.4 mm, with nothing rounded.

Fractional inches to mm

Turn the fraction into a decimal, then multiply. For 1 3/8 in: 1 + 3/8 = 1.375, and 1.375 × 25.4 = 34.925 mm. Rounding the input to 1.38 first gives 35.052 mm, a different size. Mixed numbers such as 1 3/8 can be typed as written.

Paper uses the same factor: a US Letter sheet, 8.5 × 11 in, is 215.9 × 279.4 mm.

Feet and inches to mm

Convert to total inches first. 5 ft 11 in is 5 × 12 + 11 = 71 in, which is 1,803.4 mm. Entering 5.11 instead means 5.11 inches, or 129.794 mm.

Common mistakes

• Rounding the factor to 25. It loses 0.4 mm per inch, so 10 in comes out as 250 mm instead of 254 mm.
• Multiplying by 2.54. That factor gives centimeters (1 in = 2.54 cm). Writing the result in mm makes it ten times too small.
• Converting a nominal size. A 2×4 board actually measures 1.5 × 3.5 in, or 38.1 × 88.9 mm, not 50.8 × 101.6 mm.
• Reading a screen size as a width. A 27-inch monitor's 685.8 mm is its diagonal.
• Dropping the tolerance. 2 ± 0.01 in is 50.8 ± 0.254 mm; convert both numbers with the same factor.
• Using the length factor for area. 1 in² is 645.16 mm², so a 2 × 3 in card (50.8 × 76.2 mm) covers 3,870.96 mm².

Related conversions

For the reverse direction, use mm to inches; for centimeters, inches to cm. The length converter starts from feet, yards or meters, and the area converter handles square inches.