MM to Inches Converter
Convert a workshop or product measurement written in millimetres — fastener length, phone thickness, pipe OD — into inches for a US customary caliper or drill chart.
Updated
How to use
- Enter the millimetre value from the drawing or caliper. Keep the decimal places you measured.
- Read decimal inches. 25.4 mm is exactly 1 inch.
- If you need a fraction (1/4, 3/8), compare the decimal to a fraction chart after converting.
- Use the centimetre converter only when the print is already in cm, not when it says mm.
How it works
Millimetres are the unit on most engineering drawings, phone specs and furniture hardware. US shops still talk in inches and fractional drill bits. This page divides millimetres by 25.4 because one international inch is defined as 25.4 millimetres exactly (ten times the 2.54 cm inch).
Fasteners are the usual reason to open it. An M6 bolt is named by metric thread, but a 20 mm length is 0.7874 in. A 50 mm cabinet screw is 1.9685 in. The converter does not tell you thread pitch or whether a #8 wood screw is a substitute — it only changes the length unit.
Consumer gadgets are another case. A phone listed as 7.8 mm thick is 0.3071 in. A laptop listed as 16.9 mm is 0.6654 in. Those figures are useful when a US review quotes thickness in inches and you only have the metric spec sheet.
Pipe and tubing confuse people because “1/2 inch pipe” is a nominal trade size, not the measured outside diameter. Converting 12.7 mm gives the true half-inch length. It does not give NPS pipe OD. If the drawing says Ø 21.3 mm, convert that number; do not assume it is “1/2 inch pipe”.
Tolerance matters more here than on the centimetre page. A 0.1 mm error is about 0.004 in. If you are fitting a bearing or a printed part, keep the millimetre measurement and convert only for communication — do not round to the nearest 1/16 in and then machine to that fraction.
Formula
inches = millimetres ÷ 25.4
Worked examples
One defined inch
25.4 mm → 1 in
A 50 mm furniture screw
50 mm → 1.9685 in
Phone thickness
7.8 mm → 0.3071 in
Workshop millimetres in inches
| mm | in |
|---|---|
| 1 | 0.0394 |
| 5 | 0.1969 |
| 10 | 0.3937 |
| 25.4 | 1 |
| 50 | 1.9685 |
| 100 | 3.937 |
Useful notes
- 1 inch = 25.4 mm exactly. 1 mm ≈ 0.03937 in.
- Nominal pipe and lumber names are not the same as measured millimetres.
- For centimetres, use CM to Inches. Dividing mm by 10 first is optional, not required.
FAQ
- How many inches are in 1 mm?
About 0.03937 inches (1 ÷ 25.4). Four decimal places is 0.0394 in on this page.
- Is 25.4 mm exactly one inch?
Yes. The international inch is defined as 25.4 millimetres. Entering 25.4 returns 1.
- How many inches is 10 mm?
10 ÷ 25.4 ≈ 0.3937 in — the same as 1 cm, because 10 mm is 1 cm.
- Can I use this for drill bit sizes?
You can convert a metric bit diameter to inches. Matching a fractional US bit is a nearest-size problem; 6 mm is 0.2362 in, close to 15/64 (0.2344 in), not 1/4 in.
- Should I use mm or cm?
Use millimetres when the drawing is dimensioned in mm. Centimetres are the same conversion scaled by 10.
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