MM to Inches Converter
Convert millimeters to inches with precision. Enter a length in millimeters and the tool returns inches, based on the exact definition that one inch is 25.4 millimeters. Millimeters are the workhorse unit of engineering drawings, machining, and hardware specs, so this is the conversion you reach for when a metric drawing has to be read in inches.
Calculate
Default result: 1.0000 in
MM to Inches Converter · Result
calculators.dev
Inches
25.4 mm
Reviewed by the calculators.dev team · Last updated 2026-06-23
How to calculate
Enter the millimeter value and read the inches result below. To convert by hand, divide millimeters by 25.4: an M6 bolt with a 6 mm shank is 6 ÷ 25.4 = 0.236 inches. To convert inches back to millimeters, multiply by 25.4 instead. Because machining tolerances are tight, the tool keeps several decimal places.
inches = millimeters ÷ 25.4, where 25.4 is the exact number of millimeters in one inch. Equivalently, millimeters = inches × 25.4. There are 1000 millimeters in a meter, so this is the same ratio as centimeters scaled by ten.
Example calculation
25.4 millimeters divided by 25.4 millimeters per inch is exactly 1 inch. Because one inch is defined as exactly 25.4 mm, this is the cleanest possible example of the ratio.
- result
- 1 in
Assumptions
- One inch is defined as exactly 25.4 millimeters, so the conversion is exact and suitable for engineering work.
- Tolerances are not applied — the tool converts the nominal value you enter, not a dimension with a plus-or-minus band.
- Displayed inches are rounded for readability; the underlying value keeps full precision for tight-tolerance use.
Common mistakes
- Confusing millimeters with centimeters: a centimeter is 10 millimeters, so a value entered in the wrong unit is off by a factor of ten.
- Rounding 25.4 to 25 by hand, which throws off precision machining and fastener sizing.
- Mixing up the direction — divide by 25.4 for inches, multiply by 25.4 for millimeters.
Frequently asked questions
How many inches is 1 mm?
One millimeter is about 0.0394 inches, because 1 inch is exactly 25.4 mm and 1 ÷ 25.4 = 0.0394.
What is 25.4 mm in inches?
Exactly 1 inch, because the inch is defined as 25.4 millimeters.
Why are drawings often in millimeters?
Millimeters give whole-number precision for small features without decimals, which is why most engineering and machining drawings use them.
How do I convert a fractional inch to mm?
Turn the fraction into a decimal first — 1/8 inch is 0.125 inch — then multiply by 25.4. So 0.125 × 25.4 = 3.175 mm.