Inches to mm Converter
Convert inches to millimetres exactly. Enter inches and get millimetres and centimetres, plus the nearest 1/16 and 1/32 inch fraction. It also works backwards to turn millimetres into inches and a fraction, which is handy for tools, bolts and woodwork.
Inches to millimetres converter
How to use the inches to mm converter
- Enter the inches. Type the number of inches you want to convert. Decimals work, and so do fractions entered as decimals.
- Check the units. The converter starts as inches to millimetres. Use the From and To menus to change either unit, or press Swap to reverse them.
- Read the result. The answer appears at the top, with the conversion factor and the same value in other common units underneath.
- Copy or print. Copy the result to paste into a message, or print a clean summary to keep with your plans.
How to convert inches to millimetres
millimetres = inches × 25.4
inches = millimetres ÷ 25.4
The inch is defined as exactly 25.4 millimetres, so multiplying by 25.4 is exact. Going the other way, the result is a decimal that rarely lands on a fraction, so the calculator also gives the nearest 1/16 and 1/32 inch for reading a tape measure or choosing a drill bit.
Worked examples
Common fractions. 1/4 inch = 6.35 mm, 3/8 inch = 9.525 mm, 1/2 inch = 12.7 mm and 3/4 inch = 19.05 mm.
A 2×4 stud. Its actual size is 1½ × 3½ inches, or 38.1 × 88.9 mm.
Millimetres to inches. 10 mm is 0.3937 inches, which is just over 25/64 inch (about 3/8 inch).
A bolt. An M8 bolt is 8 mm, or 0.315 inches, close to 5/16 inch (7.94 mm).
Inch fractions in millimetres
| Inches | Decimal | Millimetres |
|---|---|---|
| 1/16 | 0.0625 | 1.5875 |
| 1/8 | 0.125 | 3.175 |
| 1/4 | 0.25 | 6.35 |
| 3/8 | 0.375 | 9.525 |
| 1/2 | 0.5 | 12.7 |
| 5/8 | 0.625 | 15.875 |
| 3/4 | 0.75 | 19.05 |
| 7/8 | 0.875 | 22.225 |
| 1 | 1 | 25.4 |
| 2 | 2 | 50.8 |
| 6 | 6 | 152.4 |
| 12 | 12 | 304.8 |
Using the fraction output
Metric tools and parts are specified in millimetres, while tape measures and many drawings use fractions of an inch. Enter millimetres with the units swapped, and the nearest 1/16 and 1/32 inch appear for a close imperial match. These are rounded, so for fits and tolerances keep working in millimetres. To enter a fraction such as 3 5/8, use the inches to decimal converter first, or the feet and inches to mm converter for mixed units.
Features
- Convert inches to millimetres and back, with a Swap button.
- Other common units shown at once, so you rarely need a second conversion.
- Exact definitions from NIST, with 8 significant digits.
- Copy result, reset and a printable report in five themes on A4 or Letter.
Related converters and calculators
Turn fractions into decimals with the inches to decimal converter, convert feet and inches with the feet and inches to mm converter, or see cm to inches and feet to inches.
Frequently asked questions
How many mm is 1 inch?
One inch is exactly 25.4 millimetres. Two inches is 50.8 mm and 12 inches is 304.8 mm.
How do I convert inches to mm?
Multiply the inches by 25.4. For a fraction, divide first: 3/8 inch is 0.375, and 0.375 × 25.4 = 9.525 mm.
How do I convert mm to inches?
Divide the millimetres by 25.4. For example 100 mm ÷ 25.4 = 3.937 inches, which is about 3 15/16 inches.
Is 25.4 exact?
Yes. Since the international yard and pound agreement of 1959, the inch has been exactly 25.4 mm in the US, UK and other countries.
What is 3/4 inch in mm?
Three quarters of an inch is 19.05 mm. That makes a ¾-inch spanner or socket very close to a 19 mm metric one.
Why is a 2×4 not 2 by 4 inches?
Lumber is named by its size before drying and planing. The finished dimensions are 1½ × 3½ inches, which is 38.1 × 88.9 mm.
Which fractions does the calculator show?
It rounds to the nearest 1/16 and 1/32 inch and reduces them, so 0.5 shows as 1/2. Use the decimal value if you need more precision.
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Sources and references
About this calculator
Written and maintained by Rashid Mehmood, founder of CalcTen. The formulas are unit-tested against hand-worked examples, and the content is reviewed whenever product sizes or guidance change. Last updated 14 November 2026. See our editorial policy.
Conversion factors come from NIST and other standards bodies, and results are rounded for display. For safety-critical work such as medicines, engineering or navigation, verify with an authoritative source. Read the disclaimer.