What is the Fraction to MM Conversion Chart Calculator?
This calculator converts a fractional inch measurement into its equivalent in millimeters, centimeters, and decimal inches. Fractional inches are common on US and imperial rulers, drill bit sets, wrenches, and woodworking plans, while millimeters dominate engineering drawings and metric tooling. This tool bridges the two systems instantly so you never have to guess.
How to use it
Enter the numerator (the top number of the fraction) and the denominator (the bottom number). For example, for 3/8 inch, type 3 in the numerator and 8 in the denominator. The calculator returns the decimal inch, the millimeter value, and the centimeter value. It works for any fraction, including ones like 5/16, 7/64, or even whole numbers if you set the denominator to 1.
The formula explained
One inch is defined as exactly 25.4 millimeters. To convert, first turn the fraction into a decimal by dividing the numerator by the denominator, then multiply that decimal by 25.4: $$\text{mm} = \frac{\text{numerator}}{\text{denominator}} \times 25.4$$. Centimeters are simply the millimeter value divided by 10.
Worked example
Convert 3/8 inch to millimeters. First, \(3 \div 8 = 0.375\) inches (the decimal value). Then $$0.375 \times 25.4 = 9.525 \text{ mm}.$$ In centimeters that is 0.9525 cm. So 3/8 inch equals 9.525 mm.
FAQ
Is one inch exactly 25.4 mm? Yes. Since 1959 the international inch has been defined as exactly 25.4 millimeters, so the conversion is exact, not an approximation.
What about common fractions like 1/2 and 1/4? 1/2 inch = 12.7 mm, 1/4 inch = 6.35 mm, 1/8 inch = 3.175 mm, and 1/16 inch = 1.5875 mm.
Can I enter improper fractions? Yes. For example, 9/8 gives 1.125 inches = 28.575 mm. You can also use a denominator of 1 to convert a whole inch value.
