Convert inches to millimeters (in to mm)
To convert inches to millimeters, multiply the value in inches by 25.4. Machinists, woodworkers and 3D-printing hobbyists use it to turn imperial plans, fractional drill sizes and pipe or screen dimensions into millimeters. This inches to mm converter works both ways: type inches or millimeters into either box above and the other box, the worked formula and the chart below update as you type.
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. Inches measure TV, laptop and phone screens worldwide, as well as height, waist sizes, wheel rims and timber in the US. One inch is 25.4 millimeters.
The millimeter (mm), spelled millimetre in British English, is one thousandth of a meter — the smallest division on a standard metric ruler. Going the other way, one millimeter equals 0.0393701 inches. 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.
How to convert inches to mm with this tool
- Type a number into the Inches (in) box — it starts at 0.25 in as an example.
- Read the answer in the Millimeters (mm) box and in the result panel, which also shows the formula with your number.
- To go the other way, type into the millimeters box, or press the swap button to open the MM to Inches converter with your value carried over.
- Choose how many decimal places you want, or tap a quick value such as 0.125 in, 0.25 in, 0.5 in.
- Press Copy result to paste the conversion into a message, spreadsheet or note.
Inches to MM converter features
Converts both ways
Type in the inches box to get millimeters, or in the millimeters box to get inches. 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 0.25 in × 25.4 = 6.35 mm, so you can check it or copy the method.
Quick values and chart
Tap common values like 0.125 in, 0.25 in, 0.5 in, or read the inches-to-millimeters table further down this page.
Inches to MM formula
The formula is mm = in × 25.4. It is exact because one inch is exactly 25.4 millimeters by international definition.
The reverse formula is in = mm ÷ 25.4 — or use the MM to Inches converter. Three worked examples:
- A ¼-inch bolt: 0.25 in × 25.4 = 6.35 mm
- A ½-inch socket: 0.5 in × 25.4 = 12.7 mm
- A 3.5-inch drive bay: 3.5 in × 25.4 = 88.9 mm
Inches to MM quick reference
The values people look up most often, worked out with the exact factor:
- 1 in = 25.4 mm
- 1 mm = 0.0393701 in
- 0.125 in = 3.175 mm
- 0.25 in = 6.35 mm
- 0.5 in = 12.7 mm
- 2 in = 50.8 mm
Quick tip: For a mental estimate, multiply by 100 and divide by 4 (that is, × 25), which is about 1.6% low: 2 in → 50 mm, against the exact 50.8 mm.
Related conversions: Inches to CM, Inches to Feet and the full length converter.
Inches to MM conversion table
Values in millimeters are rounded to 4 decimal places (more for very small numbers). For any value not listed, type it into the converter above.
| in | mm | in | mm |
|---|---|---|---|
| 0.0625 | 1.5875 | 1.25 | 31.75 |
| 0.125 | 3.175 | 1.5 | 38.1 |
| 0.1875 | 4.7625 | 1.75 | 44.45 |
| 0.25 | 6.35 | 2 | 50.8 |
| 0.3125 | 7.9375 | 2.5 | 63.5 |
| 0.375 | 9.525 | 3 | 76.2 |
| 0.4375 | 11.1125 | 4 | 101.6 |
| 0.5 | 12.7 | 5 | 127 |
| 0.625 | 15.875 | 6 | 152.4 |
| 0.75 | 19.05 | 8 | 203.2 |
| 0.875 | 22.225 | 10 | 254 |
| 1 | 25.4 | 12 | 304.8 |
Inches to MM FAQ
How many millimeters is 1 in?
1 inch is 25.4 millimeters: 1 in × 25.4 = 25.4 mm.
How do I convert inches to mm?
Multiply the value in inches by 25.4. For example, 0.125 in × 25.4 = 3.175 mm. To convert back, divide the value in millimeters by 25.4 — or use the MM to Inches converter.
How many inches are in one millimeter?
One millimeter is 0.03937 inches, because 1 mm ÷ 25.4 = 0.03937 in.
What is 0.125 inches in millimeters?
How do I convert fractions of an inch to mm?
Turn the fraction into a decimal first, then multiply by 25.4. For 3/8 in: 3 ÷ 8 = 0.375, and 0.375 in = 9.525 mm. For 5/16 in: 0.3125 in = 7.9375 mm.