Inch to Decimal Calculator
Convert fractional inch measurements into decimal form for CAD, machining, and digital calipers. Enter a whole inch value, a numerator, and pick a denominator, and the calculator returns the exact decimal, thousandths, and millimeters.
Tape measures mark inches in fractions (1/2, 1/4, 1/8, 1/16, 1/32), but CNC controllers, CAD software, and digital calipers want decimal inches. The inch to decimal calculator handles this conversion exactly, with no rounding unless you choose a coarse denominator.
Formula
For a value written as W and n/d (whole inches + fraction), the decimal is:
decimal = W + (n ÷ d)
So 1-5/16" = 1 + (5 ÷ 16) = 1.3125". No extra steps.
Standard fraction to decimal reference
This chart covers the standard tape-measure and machinist fractions. Every value is exact (not rounded).
| Fraction | Decimal Inches | Thousandths | Millimeters |
|---|---|---|---|
| 1/64 | 0.015625 | 15.625 | 0.3969 |
| 1/32 | 0.03125 | 31.25 | 0.7938 |
| 1/16 | 0.0625 | 62.5 | 1.5875 |
| 3/32 | 0.09375 | 93.75 | 2.3813 |
| 1/8 | 0.125 | 125 | 3.175 |
| 5/32 | 0.15625 | 156.25 | 3.9688 |
| 3/16 | 0.1875 | 187.5 | 4.7625 |
| 7/32 | 0.21875 | 218.75 | 5.5563 |
| 1/4 | 0.25 | 250 | 6.35 |
| 5/16 | 0.3125 | 312.5 | 7.9375 |
| 3/8 | 0.375 | 375 | 9.525 |
| 7/16 | 0.4375 | 437.5 | 11.1125 |
| 1/2 | 0.5 | 500 | 12.7 |
| 9/16 | 0.5625 | 562.5 | 14.2875 |
| 5/8 | 0.625 | 625 | 15.875 |
| 11/16 | 0.6875 | 687.5 | 17.4625 |
| 3/4 | 0.75 | 750 | 19.05 |
| 13/16 | 0.8125 | 812.5 | 20.6375 |
| 7/8 | 0.875 | 875 | 22.225 |
| 15/16 | 0.9375 | 937.5 | 23.8125 |
| 1 | 1.0000 | 1000 | 25.4 |
Why decimal inches matter
Machinists and engineers work almost entirely in decimal because it lets them add, subtract, and multiply without converting between denominators. A bill of materials listing 0.250, 0.375, and 0.125 is immediately addable (total = 0.750). The same list in fractions (1/4, 3/8, 1/8) requires finding a common denominator first.
Thousandths ("thou" or "mil") are the standard shop unit for tolerance specs. A tolerance of ±0.005" means ±5 thousandths, or about ±0.127 mm. The calculator outputs thousandths directly so you can read values off like a caliper.
Converting back: decimal to fraction
If you already have a decimal and want the nearest fraction, this calculator pairs well with our inches to fraction tool, which takes a decimal like 0.392 and returns the closest 1/16 or 1/32 inch.