How to use the inch to millimeter calculator
- Choose the conversion direction.
- Enter the source measurement as a decimal or a fraction. Mixed numbers use a space, such as 1 3/8.
- Read the converted value. The fractional inch reference is rounded, while the conversion uses the exact 25.4 factor.
The formula
Millimeters = inches × 25.4; inches = millimeters ÷ 25.4
Worked example
¾ inch is exactly 19.05 mm. An 18 mm panel is approximately 0.708661 inches; it is not exactly ¾ inch thick.
Before you measure and cut
Converting a nominal product name does not reveal its actual thickness. Measure the stock before sizing joinery.
Avoid repeatedly rounding a measurement back and forth between systems. Keep the original precision until marking out.
Frequently asked questions
Is 25.4 an exact factor?
Yes. One inch is defined as exactly 25.4 millimeters.
Can I enter feet and inches together?
Convert feet to inches first. This field does not parse expressions such as 6 ft 2 in.