How to Convert mm to Inches (Formula + Quick Trick)

The mm to inches formula

Use inches = millimeters ÷ 25.4, or mm × 0.0393701. The factor is exact because one inch is defined as 25.4 mm. To go back, mm = inches × 25.4.

Example: 25 mm ÷ 25.4 = 0.98425 in, which the converter shows as 0.9842519685. For workshop use, three decimals (0.984) are plenty. For a spec sheet, keep four.

From a decimal inch to a fraction

Tool sizes are usually in sixteenths, thirty-seconds or sixty-fourths of an inch. Multiply the decimal inches by the denominator you want and round: 0.984 × 16 = 15.7, so 25 mm is about 16/16, or 1 inch, within a fraction of a millimeter. For a smaller size, 8 mm is 0.315 in, and 0.315 × 32 = 10.08, so it is very close to 10/32, which reduces to 5/16 in.

Exact matches are rare. 6.35 mm is a true 1/4 in, and 12.7 mm is a true 1/2 in, but most metric sizes fall between imperial ones. That is why a 6 mm bit will not fit a hole meant for a 1/4 in bit without a small gap.

Common sizes in millimeters and inches

Computed with the exact factor:

• 1 mm = 0.039 in
• 3 mm = 0.118 in
• 6.35 mm = 0.25 in (exactly 1/4 in)
• 8 mm = 0.315 in
• 10 mm = 0.394 in
• 12.7 mm = 0.5 in (exactly 1/2 in)
• 25 mm = 0.984 in
• 50 mm = 1.969 in

Camera and phone specs give more examples: a 35 mm lens focal length is 1.378 in, and a 2.5 mm audio jack is about 0.098 in. Whenever a metric size is standardized, it is rarely a round number in inches.

Working with tolerances

If a part is specified as 10 mm ± 0.1, the limits are 9.9 to 10.1 mm, which is 0.3898 to 0.3976 in. Convert each limit separately rather than converting the tolerance and adding it, and keep at least four decimals so you do not lose the small margin you are trying to check. When two parts have to fit together, convert both to the same unit before comparing them, and prefer to do the whole comparison in millimeters if either drawing already uses metric.

Reading a drill index and a caliper

In a workshop the two systems often sit side by side: a drill index lists fractional inch sizes while a digital caliper can switch between millimeters and inches at the press of a button. When you are matching a part to a hole, measure it in the unit the caliper shows first, then convert once, rather than converting a printed drawing value and a measured value separately. Keep track of which unit each number came from, because a mistaken unit is a far more common cause of a bad fit than an arithmetic slip.

If you often move between the two, note the handful of conversions you use most, such as the sizes of your favorite bits, on a small card kept with the tools. Write the millimeter value and the inch value together on the drawing, and note which one is the master, so that the person cutting the part knows which number to trust.

Where millimeters and inches collide

Metric parts used in an inch-based setup are common in cycling, plumbing, photography and 3D printing. A 50 mm lens filter is 1.97 in, a 6 mm hex key is about 0.236 in, and a 3 mm filament is about 0.118 in. The sizes are close to fractional inches but not identical, so check before substituting.

• Bike and car parts: a 10 mm socket does not fit a 3/8 in bolt perfectly, so use the correct size to avoid rounding off the head.
• Photography: sensor and lens measurements use millimeters, while print sizes use inches.
• 3D printing: slicers use millimeters, while many US models are designed in inches.
• Home projects: lumber is often named in inches, while hardware lists millimeters.

For larger measurements, converting cm to inches and converting inches to cm apply the same factor, while converting units correctly covers other kinds of measurement.

Keep enough digits for precision work

For machining or fitting parts, keep four decimals in inches and verify with a caliper. Rounding to two decimals can hide the difference between a loose and a tight fit.

A short practice set

Try these to check your method. A 5 mm drill bit is 0.197 in. A 20 mm pipe is 0.787 in. A 100 mm ruler section is 3.937 in. Each comes from dividing by 25.4, and the last digits rarely matter outside precision work. If you can estimate each one in your head first and then confirm with a calculator, you will quickly gain a feel for the numbers, which helps when reading product listings, checking spare parts or following a workshop tutorial written for another system.

Keep a small card with a few reference sizes near your bench, such as 6.35 mm for a quarter inch and 12.7 mm for half an inch. Having them at hand speeds up matching hardware and reduces mistakes caused by doing arithmetic with greasy hands. When precision matters, measure with a caliper, convert once and write the result in both systems on the drawing so everyone works from the same value.

Step-by-step

Common mistakes to avoid

Pro tips

Frequently asked questions