Convert millimeters to inches (mm to in)
To convert millimeters to inches, divide the value in millimeters by 25.4 — the same as multiplying by about 0.0393701. It is useful for drill bits, screws and bolts, rainfall totals, jewelry and phone thickness quoted in millimeters when you think in inches. This mm to inches converter works both ways: type millimeters or inches into either box above and the other box, the worked formula and the chart below update as you type.
The millimeter (mm), spelled millimetre in British English, is one thousandth of a meter — the smallest division on a standard metric ruler. Engineers, machinists and product designers work almost entirely in millimeters, and the unit is used for rainfall totals, screw and drill sizes, jewelry, phone thickness and camera lenses. One millimeter is 0.0393701 inches.
The inch (in or ″) is a unit of the imperial and US customary systems. Its name comes from the Latin uncia, “one twelfth”, and since the 1959 international yard and pound agreement it has been exactly 25.4 millimeters. Going the other way, one inch equals 25.4 millimeters. Inches measure TV, laptop and phone screens worldwide, as well as height, waist sizes, wheel rims and timber in the US.
How to convert mm to inches with this tool
- Type a number into the Millimeters (mm) box — it starts at 10 mm as an example.
- Read the answer in the Inches (in) box and in the result panel, which also shows the formula with your number.
- To go the other way, type into the inches box, or press the swap button to open the Inches to MM converter with your value carried over.
- Choose how many decimal places you want, or tap a quick value such as 5 mm, 10 mm, 25.4 mm.
- Press Copy result to paste the conversion into a message, spreadsheet or note.
MM to Inches converter features
Converts both ways
Type in the millimeters box to get inches, or in the inches box to get millimeters. The other box follows instantly.
Exact factor
Calculations use 1 in = 25.4 mm, and rounding is applied only to the answer you see — choose Auto or 0–10 decimal places.
Formula with your number
The working is shown under the result, for example 10 mm ÷ 25.4 = 0.3937 in, so you can check it or copy the method.
Quick values and chart
Tap common values like 5 mm, 10 mm, 25.4 mm, or read the millimeters-to-inches table further down this page.
MM to Inches formula
The formula is in = mm ÷ 25.4. It is exact because one inch is exactly 25.4 millimeters by international definition.
The reverse formula is mm = in × 25.4 — or use the Inches to MM converter. Three worked examples:
- A 10 mm socket or wrench: 10 mm ÷ 25.4 = 0.3937 in
- A 6.35 mm (¼-inch) headphone plug: 6.35 mm ÷ 25.4 = 0.25 in
- A 50 mm camera lens: 50 mm ÷ 25.4 = 1.9685 in
MM to Inches quick reference
The values people look up most often, worked out with the exact factor:
- 1 mm = 0.0393701 in
- 1 in = 25.4 mm
- 5 mm = 0.1969 in
- 10 mm = 0.3937 in
- 25.4 mm = 1 in
- 50 mm = 1.9685 in
Quick tip: Dividing by 25 instead of 25.4 gives a fast estimate that is only about 1.6% high: 50 mm ÷ 25 = 2 in, versus the exact 1.9685 in.
Related conversions: CM to Inches, Feet to Inches and the full length converter.
MM to Inches conversion table
Values in inches are rounded to 4 decimal places (more for very small numbers). For any value not listed, type it into the converter above.
| mm | in | mm | in |
|---|---|---|---|
| 0.5 | 0.01969 | 25.4 | 1 |
| 1 | 0.03937 | 30 | 1.1811 |
| 2 | 0.07874 | 35 | 1.378 |
| 3 | 0.1181 | 40 | 1.5748 |
| 4 | 0.1575 | 45 | 1.7717 |
| 5 | 0.1969 | 50 | 1.9685 |
| 6 | 0.2362 | 60 | 2.3622 |
| 7 | 0.2756 | 70 | 2.7559 |
| 8 | 0.315 | 75 | 2.9528 |
| 9 | 0.3543 | 80 | 3.1496 |
| 10 | 0.3937 | 90 | 3.5433 |
| 12 | 0.4724 | 100 | 3.937 |
| 15 | 0.5906 | 150 | 5.9055 |
| 16 | 0.6299 | 200 | 7.874 |
| 18 | 0.7087 | 300 | 11.811 |
| 20 | 0.7874 | 500 | 19.685 |
| 22 | 0.8661 | 1,000 | 39.3701 |
| 25 | 0.9843 |
MM to Inches FAQ
How many inches is 5 mm?
5 millimeters is 0.1969 inches: 5 mm ÷ 25.4 = 0.1969 in.
How do I convert mm to inches?
Divide the value in millimeters by 25.4 — the same as multiplying by about 0.0393701. For example, 100 mm ÷ 25.4 = 3.937 in. To convert back, multiply the value in inches by 25.4 — or use the Inches to MM converter.
How many millimeters are in one inch?
One inch is 25.4 millimeters, because 1 in × 25.4 = 25.4 mm.
What is 100 millimeters in inches?
What fraction of an inch is 1 mm?
One millimeter is 0.03937 in — a little under 1/25 of an inch (1/25 in = 1.016 mm). It sits between 1/32 in (0.79375 mm) and 1/16 in (1.5875 mm), the smallest marks on most imperial rulers.