What this calculator does
This tool converts a fraction of an inch — such as 3/8", 11/16", or 1‑1/2" — into its decimal-inch value and into metric millimeters and centimeters. It is handy for woodworking, machining, sewing, 3D printing and any project where you need to translate fractional imperial measurements into precise decimal or metric figures.
How to use it
Enter the numerator (top number) and denominator (bottom number) of your fraction. If your measurement also has a whole-inch part, type it in the "Whole inches" field — otherwise leave it as 0. The calculator returns the decimal-inch equivalent, the millimeter value, and the centimeter value.
The formula explained
A fraction is simply a division: the fractional part equals the numerator a divided by the denominator b. Adding any whole inches w gives the total in decimal inches: $$\text{in} = w + \frac{a}{b}$$ Because one inch is defined as exactly 25.4 millimeters, the metric value is $$\text{mm} = \text{in} \times 25.4$$ and centimeters are simply \(\text{mm} \div 10\).
Worked example
Convert 3/8 of an inch. The fraction $$3 \div 8 = 0.375$$ so the decimal value is 0.375 inches. In metric, $$0.375 \times 25.4 = 9.525 \text{ mm}$$ which is 0.9525 cm. For a measurement of 1‑1/2", enter whole = 1, numerator = 1, denominator = 2: that gives 1.5 in, or 38.1 mm.
FAQ
Why is one inch exactly 25.4 mm? Since 1959 the international inch has been defined as precisely 25.4 millimeters, so the conversion is exact, not rounded.
Can I enter mixed numbers? Yes — put the whole number in the "Whole inches" field and the fraction in the numerator/denominator fields.
What if I enter 0 as the denominator? Division by zero is undefined, so the fraction is treated as zero to avoid an error; make sure the denominator is a real value like 2, 4, 8, 16, or 32.
