MM to Inches Converter
This mm to inches converter divides your figure by exactly 25.4 for the decimal result, then rounds it to the nearest 1/16 or 1/32 inch and shows how far off each one lands. It runs both ways, so inches to mm is a single click.
Inches
1 in
Nearest fraction of an inch
To the nearest 1/16
1 in
Off by 0 mm
To the nearest 1/32
1 in
Off by 0 mm
Common conversions
| Millimeters | Inches |
|---|---|
| 1 | 0.0394 |
| 5 | 0.1969 |
| 10 | 0.3937 |
| 25.4 | 1 |
| 50 | 1.9685 |
| 100 | 3.937 |
Examples
How to use
- 1
Enter a length in millimeters, or switch the direction to start from inches.
- 2
Read the decimal result, rounded to four useful places.
- 3
Check the nearest 1/16 and 1/32 inch fraction and the error on each.
- 4
Copy the result or scan the reference table below the tool.
What is the mm to inches formula?
One inch is 25.4 millimeters, exactly, by international agreement since 1959. So the conversion is a single division:
inches = millimeters / 25.4
Run 100 mm through it and you read 3.937 inches. Going the other way you multiply, which is how a 3/4 inch socket (0.75 x 25.4 = 19.05 mm) ends up almost interchangeable with a 19 mm one. The gap is only 0.05 mm.
Why does the result show a fraction?
US rulers, drill bits and wrenches are marked in fractions, not decimals. No shop stocks a 0.394 inch bit, it stocks 25/64 or 13/32. So the tool rounds your decimal to the nearest usable fraction at two precisions:
- 1/16 inch, or 1.5875 mm per step, the marks on a standard tape measure.
- 1/32 inch, or 0.79375 mm per step, for drill charts and finer machining work.
Each fraction is shown with its error in mm, so you can tell whether it is close enough for the job. Take 10 mm, which works out to 0.3937 in. The nearest 1/16 is 3/8 in, sitting 0.48 mm short, while the nearest 1/32 is 13/32 in, only 0.32 mm over. On a wrench, 3/8 does the job.
What are the common mm to inches values?
| Millimeters | Inches (decimal) | Nearest fraction |
|---|---|---|
| 1 mm | 0.0394 in | 3/64 in |
| 5 mm | 0.1969 in | 13/64 in |
| 10 mm | 0.3937 in | 13/32 in |
| 25.4 mm | 1 in | 1 in exact |
| 50 mm | 1.9685 in | 1 31/32 in |
| 100 mm | 3.937 in | 3 15/16 in |
How do you convert mm to inches in your head?
Multiply the millimeters by 4, then drop two zeros. For 50 mm: 50 x 4 is 200, so about 2 inches. The exact answer is 1.9685 in, so the shortcut runs roughly 1.6 percent high. Close enough to sanity check an order, not to mark a cut.
Where this conversion comes up
Woodworkers turn European plans into imperial stock, cyclists match metric bolts to inch-labeled tools, and anyone reading a phone spec sees the screen quoted in inches but the body in mm. The math never changes, it stays the same division by 25.4. The fraction panel is what spares you the second lookup on a ruler.
Why is it easy to confuse mm and cm?
Plans come in millimeters, but habits run in centimeters, and that mismatch causes tenfold errors. There are 10 mm in 1 cm, so a 45 cm cabinet depth is 450 mm, and 450 / 25.4 = 17.72 in. When a result looks ten times too big or too small, check the input unit first. This tool reads millimeters, so multiply centimeters by 10 before you type, and turn meters into mm as well (1 m = 1000 mm).
Parameters
Every field of this tool can be prefilled from the URL. Use these query parameters:
| Parameter | Type | Default |
|---|---|---|
| value | number | 25.4 |
| direction | mmToIn | inToMm | mmToIn |
Example : https://www.veltotools.com/conversion/mm-to-inches-converter?value=25.4
API
The same tool is available as a free JSON API, with the same parameters as above. No key, no sign-up.
Frequently asked questions
Updated Jul 22, 2026