How to use it
- Enter the length in inches (decimals, e.g. 0.375).
- Read the result in millimeters.
- Use the fraction chart for 1/16, 1/8, 3/8 and similar sizes.
- Check the pipe table for real outside diameters.
The formula
mm = inches × 25.4 and inches = mm ÷ 25.4.
The inch is defined as exactly 25.4 mm, so there's no rounding in the factor. Example: a 3/8-inch bolt is 0.375 × 25.4 = 9.525 mm.
Many tape measures and rulers sold in the US show inches on one edge and centimeters on the other, but plans, spec sheets and product listings often use only one system. When a drawing gives a size such as 1 3/8 in, add the whole number and the fraction first (1.375 in) and then convert: 34.925 mm. To go from millimeters to the nearest fraction, see the FAQ below. If the size you need is in feet as well, convert the feet to inches first (1 ft = 12 in = 304.8 mm).
Fractional inch chart
| Inches | Decimal | Millimeters |
|---|---|---|
| 1/16 | 0.0625 | 1.588 |
| 1/8 | 0.125 | 3.175 |
| 3/16 | 0.1875 | 4.763 |
| 1/4 | 0.25 | 6.35 |
| 5/16 | 0.3125 | 7.938 |
| 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 |
To convert a fraction yourself, divide the top number by the bottom one and multiply by 25.4.
Pipe sizes are nominal
A "1/2-inch" pipe is not 1/2 inch across. Pipe sizes (NPS in the US, DN in metric countries) are trade names. A 1/2-inch steel pipe has an outside diameter of about 0.84 in (21.3 mm), and a 1-inch pipe is about 1.32 in (33.7 mm). The table in the tool lists the real outside diameters for common threaded steel pipe sizes. Copper tubing and PVC follow their own sizing standards, so always measure the old part or check the product's spec sheet before you buy fittings.
Metric vs. SAE tools
Wrench and socket sets come in inch (SAE) and metric sizes, and some are close but not interchangeable:
| Metric | Nearest inch size |
|---|---|
| 10 mm | 3/8 in (9.525 mm) |
| 13 mm | 1/2 in (12.7 mm) |
| 16 mm | 5/8 in (15.875 mm) |
| 19 mm | 3/4 in (19.05 mm) |
A near match can round off a bolt head under torque, so use the correct system when it matters.
Precision tips
Choose up to 8 decimal places. Woodworking rarely needs more than a millimeter; machining and 3D printing often work to 0.01 mm. Calipers often show both units, but a converter helps when a drawing is in one system and your tools are in the other. The tool also shows the result in centimeters, meters and feet. Everything is calculated in your browser. For whole inches and centimeters with a TV size calculator, see inches to cm.
Frequently asked questions
How many mm is 1 inch?
Exactly 25.4 millimeters.
How many inches is 10 mm?
10 mm is 0.3937 inches, just over 3/8 inch.
What is 1/4 inch in mm?
6.35 mm.
Why isn't a 1/2-inch pipe 12.7 mm wide?
Pipe sizes are nominal names. A 1/2-inch steel pipe is about 21.3 mm (0.84 in) on the outside.
How do I convert mm to fractional inches?
Divide by 25.4 to get decimal inches, then multiply the decimal part by 16 to get sixteenths. 20 mm = 0.787 in ≈ 12.6/16, so about 13/16 in.
Last updated: November 12, 2026