MM to Inches Converter
Convert millimeters to inches instantly. Type a value in the field below and the equivalent number of inches appears as you type. The formula is a single division by 25.4, the exact number of millimeters in one international inch as fixed by the 1959 yard and pound agreement.
Millimeters → Inches
Instant conversion. No sign-up. Works offline after the page loads.
Note: Negative values are converted mathematically. Physical lengths are normally positive.
The millimeters to inches formula
inches = millimeters ÷ 25.4
The divisor 25.4 is the exact number of millimeters in one international inch. This value follows directly from the 1959 yard and pound agreement, which fixed the international inch at exactly 2.54 centimeters. Since one centimeter contains ten millimeters, one inch contains exactly 25.4 millimeters. Because the relationship is a definition, no measurement uncertainty is introduced by the factor itself.
To reverse the calculation and go from inches back to millimeters, multiply the inch value by 25.4. The two operations are exact inverses, so converting a value in one direction and then back in the other always returns the original number, subject only to the limits of floating-point representation.
A millimeter is one thousandth of a meter, making it the smallest unit that appears on most everyday rulers. It is the working unit for engineering drawings, machined parts, electronic components, and anything manufactured to a tolerance finer than a centimeter. The inch, meanwhile, remains the working unit for many trades in the United States, and conversions between the two appear constantly in cross-border manufacturing, imported tools, and product specifications.
Worked examples
Each example below shows the arithmetic step by step, using the exact factor and full internal precision.
Example 1 — Convert 1 mm to inches
Formula: inches = millimeters ÷ 25.4 Substitute: inches = 1 ÷ 25.4 Divide: inches = 0.03937007874015748 Displayed: 0.03937 in (6 decimals, trailing zero trimmed)
One millimeter is a hair over 39 thousandths of an inch. To put that in perspective, a standard sheet of 80-gram office paper is about 0.1 millimeter thick, which is 0.003937 inches — a value well below what most everyday rulers can show.
Example 2 — Convert 10 mm to inches
Formula: inches = millimeters ÷ 25.4 Substitute: inches = 10 ÷ 25.4 Divide: inches = 0.3937007874015748 Displayed: 0.393701 in (6 decimals)
Ten millimeters is a common metric dimension — roughly the width of a fingernail or the thickness of a phone. In imperial units it is 0.393701 inches, which is close to the fraction 25/64 inch. This value appears frequently when metric drawings are converted for use with imperial tooling.
Example 3 — Convert 25 mm to inches
Formula: inches = millimeters ÷ 25.4 Substitute: inches = 25 ÷ 25.4 Divide: inches = 0.984251968503937 Displayed: 0.984252 in (6 decimals)
A very common misconception is that 25 millimeters equals one inch. It does not. One inch is exactly 25.4 millimeters, so 25 millimeters is short by 0.4 millimeters — approximately 0.0157 inches. Over a single measurement the difference is small, but across a long run of parts it accumulates and can cause a misfit.
Example 4 — Convert 50 mm to inches
Formula: inches = millimeters ÷ 25.4 Substitute: inches = 50 ÷ 25.4 Divide: inches = 1.968503937007874 Displayed: 1.968504 in (6 decimals)
Fifty millimeters is the standard diameter of many metric pipe fittings and large fasteners. In imperial terms it is 1.968504 inches, which is close to but not the same as 2 inches. The difference of about 0.8 millimeters matters whenever a metric part has to fit an imperial socket or bore.
Example 5 — Convert 100 mm to inches
Formula: inches = millimeters ÷ 25.4 Substitute: inches = 100 ÷ 25.4 Divide: inches = 3.937007874015748 Displayed: 3.937008 in (6 decimals)
One hundred millimeters is exactly ten centimeters — a very common length in everyday metric measurements. In imperial units it is 3.937008 inches, often rounded informally to "just under 4 inches". The full value is useful whenever that hundred-millimeter dimension has to be checked against an imperial specification.
Millimeters to inches conversion chart
The table below lists the equivalent of common millimeter values in inches. Every number is generated from the same formula used by the interactive converter above.
| Millimeters (mm) | Inches (in) |
|---|---|
| 0.5 mm | 0.019685 in |
| 1 mm | 0.03937 in |
| 2 mm | 0.07874 in |
| 3 mm | 0.11811 in |
| 5 mm | 0.19685 in |
| 6 mm | 0.23622 in |
| 8 mm | 0.314961 in |
| 10 mm | 0.393701 in |
| 12 mm | 0.472441 in |
| 15 mm | 0.590551 in |
| 20 mm | 0.787402 in |
| 25 mm | 0.984252 in |
| 30 mm | 1.181102 in |
| 40 mm | 1.574803 in |
| 50 mm | 1.968504 in |
| 60 mm | 2.362205 in |
| 75 mm | 2.952756 in |
| 100 mm | 3.937008 in |
| 150 mm | 5.905512 in |
| 200 mm | 7.874016 in |
| 500 mm | 19.685039 in |
How to use this converter
-
Type a value in the Millimeters field.
Decimals such as
12.7or0.25are accepted, as are whole numbers. You can also paste a value from a drawing, a spreadsheet, or another page. - Watch the Inches field update instantly. The result refreshes on every keystroke with no delay and no loading indicator — the arithmetic runs locally in your browser.
- Adjust the display precision if you want a shorter figure. The precision selector below the converter changes how many decimal places are shown. Two or three decimals are usually enough for workshop and hobby use; four or six decimals suit engineering and manufacturing.
- Copy the result with the Copy button. The numeric value is placed on your clipboard and the action is confirmed visually and to screen readers.
- Use Swap to reverse the direction. The Swap button moves the current result into the input field and switches the conversion to inches to millimeters, so you can convert the other way without re-typing.
- Use Reset to start again. Reset clears the input, restores the empty state, and returns focus to the input field.
Practical uses for millimeters to inches conversion
Drill bits and fastener sizes
Drill bits are sold in both metric and imperial sizes, and the two series do not line up exactly. A 5-millimeter drill bit is 0.196850 inches, which is close to but not the same as a 13/64 inch bit (0.203125 inches). When a hole must match a fastener precisely — for a tapped thread, a press-fit dowel, or a rivet — always convert the metric size to inches and select the nearest imperial drill bit, rather than assuming the nominal sizes match.
3D printing and CNC machining
Many 3D printers accept G-code with either metric or imperial units, but the firmware must be told which. A model dimensioned at 20 millimeters is 0.787402 inches; entering 20.0 without converting would produce a part twenty times larger than intended. CNC machining has the same requirement, and tolerances of a tenth of a millimeter (about 0.004 inches) are routine, so the conversion must be exact rather than approximate.
Electronics and PCB design
Printed circuit board design mixes metric and imperial units constantly. Component pin spacings are traditionally defined in hundredths of an inch — a "0.1 inch header" has pins 2.54 millimeters apart — while the board outline and mounting holes are often specified in millimeters. Converting 2.54 millimeters to inches gives exactly 0.1, and this is why the two systems coexist in the same design tool without conflict.
Plumbing and pipe fittings
Pipes and fittings are notorious for mixing systems. A "half-inch" copper pipe in the United States has an outside diameter of about 12.7 millimeters, while a metric pipe with a 15-millimeter outside diameter is nearly but not exactly the same. The nominal sizes are labels, not measurements, so always convert the actual outside diameter from the manufacturer's specification rather than relying on the nominal name.
Jewellery, watchmaking, and precision craft
Fine jewellery and watchmaking work in fractions of a millimeter. A ring shank might be 1.5 millimeters thick, which is 0.059055 inches; a watch crystal might be 0.8 millimeters thick, which is 0.031496 inches. Because the numbers are so small, the decimal expansion after conversion is often significant, and rounding too aggressively at the wrong step can lose the value needed for the final measurement.
Imported tools and product specifications
Tools and equipment imported from metric countries routinely arrive with specifications in millimeters, while the workshop inventory is in inches. A wrench marked 13 millimeters is the equivalent of 0.511811 inches, which is close enough to a 1/2 inch spanner for some fasteners but not for others. Reading the actual millimetre size and converting it, rather than comparing nominal sizes, is the safest approach.
Frequently asked questions
How many inches is 1 mm?
One millimeter equals approximately 0.039370 inches.
The full value is 0.03937007874015748. This is a repeating decimal because 1 divided by 25.4 does not terminate. To verify: multiply 0.03937007874015748 by 25.4 and you get exactly 1 again.
What is the mm to inches formula?
Divide the millimeter value by 25.4.
For example, 50 millimeters divided by 25.4 equals 1.968504 inches. The number 25.4 is the exact number of millimeters in one international inch, fixed by the 1959 yard and pound agreement. Because it is a definition, it introduces no measurement uncertainty.
How many mm in an inch?
There are exactly 25.4 millimeters in one international inch.
This figure is not measured — it is defined, following from the 1959 agreement that fixed the inch at 2.54 centimeters and the metric relationship that defines one centimeter as ten millimeters. Any source that states 25 millimetres or "about 25" is out of date or rounding inappropriately.
How do I convert inches to mm?
Multiply the inch value by 25.4.
For example, 2 inches multiplied by 25.4 equals exactly 50.8 millimeters. Because multiplication and division are inverse operations, converting a value in one direction and then back in the other always returns the original number. You can also use the inches to centimeters converter and multiply the result by 10.
Is 25 mm the same as 1 inch?
No. One inch is exactly 25.4 millimetres, so 25 mm is slightly less than one inch.
Twenty-five millimetres equals 0.984252 inches, a shortfall of about 0.4 millimetres (0.0157 inches). For a single measurement the difference is small enough that many people ignore it, but across a long series of parts the shortfall accumulates and can cause a misfit. Always use the exact factor 25.4 when precision matters.
How do I read a mm value as a fraction of an inch?
Convert to inches first, then express the decimal as the nearest sixty-fourth of an inch.
For example, 10 millimetres is 0.393701 inches. To get the nearest fraction, multiply the decimal part by 64 and round: 0.393701 × 64 = 25.197, which rounds to 25. So 10 millimetres is about 25/64 inch. This method works for any denominator — use 32 for thirty-seconds, 16 for sixteenths, and so on.
Why is the result a long decimal?
Dividing by 25.4 usually produces a repeating decimal.
The value 25.4 is not a factor of any power of ten, so the result of dividing a whole number of millimetres by 25.4 does not terminate. M-Convert keeps the full precision internally and rounds only when the value is displayed. Changing the display precision changes what you see but never changes the underlying result.
Does this work for pipe and fastener sizes?
The arithmetic is correct for any millimetre value, but nominal pipe and fastener sizes are labels, not measurements.
A pipe sold as "half-inch" does not have an actual diameter of 0.5 inches, and a metric pipe sold as "15 mm" does not have an actual diameter of exactly 15 millimetres. Always work from the manufacturer's specification for the actual outside and inside diameters rather than relying on the nominal name. Once you have the real number, this converter will give you the correct equivalent in inches.