About the Inches to MM
This inches to mm converter accepts a length in decimal inches (2.5) or in fractional inches exactly as it appears on a tape measure, drawing or product label (5/16, 1 3/8, 1-1/4") and converts it to millimeters, centimeters and meters. It also shows the decimal inch value and the nearest sixteenth, so you can sanity-check what you typed.
It is built for machinists and makers matching imperial drawings to metric tooling, cyclists and mechanics choosing bolts, drill bits and sockets, 3D-printing users entering dimensions into slicers, and anyone comparing a US product size with a metric one. The chart below lists every sixteenth of an inch plus whole inches up to a foot.
The conversion is exact: since the 1959 international agreement one inch is defined as exactly 25.4 millimeters, so any rounding you see is only from the number of decimals displayed. Negative values (such as a −1/16" tolerance) are converted with their sign kept.
With the default inputs, the millimeters is 31.75 mm. Change any value above to recalculate instantly.
How to use the inches to mm
- 1Type the length in inches — as a decimal like 2.5 or a fraction like 1 3/8.
- 2Read the millimeter result.
- 3Check the decimal inches line to confirm the fraction was read correctly.
- 4Use the chart for quick lookups of common fractional sizes.
Formula and method
One inch is exactly 25.4 millimeters, so the length in inches is multiplied by 25.4. When you enter a fraction, it is first turned into a decimal: the numerator is divided by the denominator and any whole inches are added, so 1 3/8 becomes 1 + 3 ÷ 8 = 1.375 in, which is 34.925 mm.
Centimeters divide millimeters by 10 and meters divide by 1,000. The nearest-sixteenth line rounds the decimal inches to the closest 1/16" for tape-measure reading.
- in
- Inch = 25.4 mm exactly
- mm
- Millimeter = 1/1000 m
Worked examples
1 1/4 inch pipe or bolt
1 1/4 = 1.25 in. Multiplying by 25.4 gives 31.75 mm, or 3.175 cm.
3/8 inch drill bit
3 ÷ 8 = 0.375 in, and 0.375 × 25.4 = 9.525 mm — so a 9.5 mm metric bit is the closest substitute.
2.5 inches
2.5 × 25.4 = 63.5 mm, which is 0.0635 m. On a tape measure it is 2 1/2".
One foot (12 inches)
12 × 25.4 = 304.8 mm. That is why a foot is often rounded to 300 mm in metric building plans.
Frequently asked questions
How many mm are in an inch?+
One inch is exactly 25.4 millimeters. Half an inch is 12.7 mm, a quarter inch is 6.35 mm and an eighth of an inch is 3.175 mm.
How do I convert fractional inches to mm?+
Divide the top number by the bottom number, add any whole inches, then multiply by 25.4. For 5/16": 5 ÷ 16 = 0.3125, and 0.3125 × 25.4 = 7.9375 mm.
What is 1/4 inch in mm?+
1/4 inch is 6.35 mm. The nearest metric sizes are 6 mm (slightly smaller) and 6.5 mm (slightly larger), which matters when choosing bolts or drill bits.
Is 25 mm the same as 1 inch?+
Close but not the same: 25 mm is 0.984 inch, about 0.4 mm short. For rough estimates people use 25 mm per inch, but precise work should use 25.4.
How do I convert mm back to inches?+
Divide millimeters by 25.4. For example 10 mm ÷ 25.4 = 0.3937 inches, which is just over 3/8". Use the mm to inches converter for the full reverse conversion.