Inches to Millimeters Converter

Drill bits, sockets, sheet thicknesses and fittings sized in inches are usually written as fractions. If you are matching them to metric stock or entering them into a metric drawing, here are the exact values. Because 1 in is defined as 25.4 mm, none of these are rounded:

The converter takes these sizes as written. Type 5/16 and it returns 7.9375 mm; type a mixed size with a space, such as 1 3/8, and it returns 34.925 mm. Drill charts sometimes print sizes like 1-3/8 with a hyphen, which the field can't read, so replace the hyphen with a space.

Engineering drawings from inch-based shops give dimensions as decimals, often to three places. Each thousandth of an inch is 0.0254 mm, so a tolerance of ±0.005 in is ±0.127 mm, and 0.010 in is 0.254 mm.

How many decimals to keep depends on the work:

• Woodwork and building: one decimal place of a millimeter is already finer than a pencil line.
• Machined parts: two or three decimals, matching the tolerance on the drawing.
• Screens and product specs: whole millimeters are normal.

The converter shows four decimal places for results below 100 mm and two from 100 mm up. A 6.1 in value displays as 154.94 mm, and nothing is lost there, because 6.1 × 25.4 has only two decimals to begin with.

A useful anchor is that 4 inches is 101.6 mm, so four inches is roughly a tenth of a meter. Rounding 25.4 down to 25 for mental math keeps you within 1.6%, but on 2.5 in that already loses a full millimeter: 62.5 mm instead of 63.5 mm.

The most common slip is using the centimeter factor by mistake. 2.54 gives centimeters and 25.4 gives millimeters. If a 3/4 in board comes out as 1.905, you have centimeters. The millimeter answer is 19.05.

Also watch for inch sizes that are names rather than measurements. Some products keep an inch label that no longer matches the real part, so measure the part itself before converting.