Convert INCHES to MM, Free
Type any INCHES value and see the MM result instantly. No button press needed.
How to convert INCHES to MM
One inch is defined as exactly 25.4 millimeters. To convert inches to millimeters, multiply by 25.4. This is a fixed, exact relationship established by international agreement in 1959. Whether you are reading US technical drawings, sizing fasteners, specifying material thicknesses, or converting display screen dimensions for a product listing, the calculation is always the same: multiply by 25.4. The conversion runs instantly in your browser.
This conversion is especially common in manufacturing and engineering where drawings may mix metric and imperial dimensions. A 3/8-inch bolt has a shank diameter of 9.525 mm. A half-inch plywood sheet is 12.7 mm thick. Pipe sizes in the US are specified in nominal inch sizes that do not correspond directly to actual dimensions, which adds a layer of complexity beyond simple unit conversion. For piping, always check the actual outside diameter separately.
Enter the inch measurement
Type the length in inches. Decimals and fractions are both accepted (e.g., 2.375 or 2-3/8).
Get the millimeter result
The result appears instantly: inches x 25.4. The conversion is exact with no rounding in the formula.
Set display precision
Choose how many decimal places to show. For machining, 3 decimal places (0.001 mm) is typical; for woodworking, 1-2 is usually enough.
Convert in bulk if needed
Enter multiple inch values on separate lines to convert a list of measurements simultaneously.
Frequently asked questions
How many millimeters is one inch?
Exactly 25.4 mm. This is the international definition, not a rounded approximation.
How do I convert fractions of an inch to mm?
Convert the fraction to a decimal first, then multiply by 25.4. For example, 3/4 inch = 0.75 x 25.4 = 19.05 mm.
What is 1/2 inch in mm?
0.5 x 25.4 = 12.7 mm exactly.
How do I convert mm back to inches?
Divide by 25.4. For example, 50 mm / 25.4 = 1.9685 inches.
Why do screen sizes use inches even in metric countries?
Display screen dimensions are marketed in inches globally as an industry convention established by American manufacturers. The physical measurement is in inches regardless of the country where it is sold.