Unit converter
Millimeters to Inches Converter
Turn millimetres into inches in a moment. An inch is exactly 25.4 millimetres, since the 1959 definition of 2.54 cm per inch means 25.4 mm per inch, so dividing any millimetre figure by 25.4 gives inches with no approximation in the factor itself. Enter a measurement in millimetres and the tool returns inches, plus a table of the values searched most. This is the conversion for engineering and machining tolerances, matching a metric drill or bolt to an inch chart, reading rainfall given in millimetres, or sizing screen bezels and small hardware quoted in millimetres.
1 millimeter = 0.03937 inches
Common millimeters to inches values
| Millimeters | Inches |
|---|---|
| 1 mm | 0.03937 in |
| 2 mm | 0.07874 in |
| 5 mm | 0.19685 in |
| 10 mm | 0.393701 in |
| 20 mm | 0.787402 in |
| 50 mm | 1.9685 in |
| 100 mm | 3.937 in |
How to convert
- Begin with the measurement in millimetres.
- Divide by 25.4, because one inch is exactly 25.4 millimetres.
- The quotient is that length expressed in inches.
- Multiply inches by 25.4 to convert the other way into millimetres.
- For a rough check, divide by 25: 100 mm is close to 4 inches, with the precise answer just under at 3.94 inches.
inches = millimetres / 25.4
Since one inch is exactly 25.4 millimetres, dividing a millimetre length by 25.4 produces the same length in inches. Multiplying inches by 25.4 reverses the step. The divisor is a fixed constant, so this is one exact division.
- mm
- the length in millimetres you start with
- 25.4
- millimetres in one inch, by definition
Worked example
A 50 mm bolt length is 50 divided by 25.4, which is 1.9685 inches, near enough 1.97 inches or just under two. A 152 mm pipe diameter converts to 152 divided by 25.4, giving 5.9843 inches, effectively a 6 inch pipe.
Key facts
- One inch is exactly 25.4 mm; ten millimetres equal about 0.3937 inches.
- A 19 mm spanner is just over three quarters of an inch at 0.748 inches.
- Rainfall of 25 mm is almost exactly one inch.
- A 10 mm socket measures roughly 0.394 inches.
Tips
- For fastener sizes, divide by 25.4 and then match to the nearest standard fractional inch such as 1/4 or 3/8.
- When swapping between metric and imperial drill bits, convert first to spot the closest available size.
- Retain extra decimal places during the division so a fine tolerance is not blurred by early rounding.
Frequently asked questions
How many inches in 1 mm?+
One millimetre is 0.0393701 inches, just under four hundredths of an inch. It is the reciprocal of 25.4.
How many mm in 1 inch?+
Exactly 25.4 millimetres, which follows from 2.54 centimetres per inch.
How many inches is 25 mm?+
25 mm divides by 25.4 to give 0.9843 inches, fractionally under one inch.
Why are tolerances often in millimetres?+
Millimetres give fine resolution without fractions, so engineering drawings frequently use them even where inch parts are involved.
Things to watch
- Rounding millimetres to the nearest inch can lose nearly half an inch of accuracy, which is far too coarse for machined parts.
Sources
Last updated: 2026-01-01
Conversions use internationally defined factors. Provided for general use; verify critical measurements independently.