Fractional inches are the language of tape measures, wrenches, and drill bits, but decimal inches are what calculators, CNC machines, and CAD software understand. This converter takes any fractional inch measurement and gives you the exact decimal equivalent, plus bonus metric conversions for when you need to cross that bridge too.
How to Calculate Fractional Inches to Decimal (Step by Step)
Step One: Divide the numerator by the denominator
The fraction is simply a division problem. Take your numerator and divide by the denominator.
Fraction decimal = Numerator / Denominator
Fraction decimal = 3 / 8 = 0.375
Step Two: Add the whole inches
Combine the whole number with the fractional decimal to get the total measurement.
Decimal inches = Whole inches + Fraction decimal
Decimal inches = 3 + 0.375 = 3.375000 in
Step Three: Convert to metric (bonus)
Multiply the decimal inches by 25.4 for millimeters or 2.54 for centimeters.
Millimeters = 3.375 x 25.4 = 85.725 mm
Centimeters = 3.375 x 2.54 = 8.573 cm
What Your Results Mean
The Decimal Inches result is the primary output you need for any calculation, CAD entry, or further conversion. The Millimeters and Centimeters results save you a separate conversion step when working with metric specifications or international suppliers.
Common fraction-to-decimal reference
| Fraction | Decimal | mm |
|---|---|---|
| 1/32 | 0.03125 | 0.794 |
| 1/16 | 0.0625 | 1.588 |
| 3/32 | 0.09375 | 2.381 |
| 1/8 | 0.125 | 3.175 |
| 5/32 | 0.15625 | 3.969 |
| 3/16 | 0.1875 | 4.763 |
| 7/32 | 0.21875 | 5.556 |
| 1/4 | 0.25 | 6.35 |
| 9/32 | 0.28125 | 7.144 |
| 5/16 | 0.3125 | 7.938 |
| 11/32 | 0.34375 | 8.731 |
| 3/8 | 0.375 | 9.525 |
| 13/32 | 0.40625 | 10.319 |
| 7/16 | 0.4375 | 11.113 |
| 15/32 | 0.46875 | 11.906 |
| 1/2 | 0.5 | 12.7 |
| 17/32 | 0.53125 | 13.494 |
| 9/16 | 0.5625 | 14.288 |
| 19/32 | 0.59375 | 15.081 |
| 5/8 | 0.625 | 15.875 |
| 21/32 | 0.65625 | 16.669 |
| 11/16 | 0.6875 | 17.463 |
| 23/32 | 0.71875 | 18.256 |
| 3/4 | 0.75 | 19.05 |
| 25/32 | 0.78125 | 19.844 |
| 13/16 | 0.8125 | 20.638 |
| 27/32 | 0.84375 | 21.431 |
| 7/8 | 0.875 | 22.225 |
| 29/32 | 0.90625 | 23.019 |
| 15/16 | 0.9375 | 23.813 |
| 31/32 | 0.96875 | 24.606 |
| 1 | 1.0 | 25.4 |
Why this matters in practice
When programming a CNC router, you enter dimensions in decimal inches (or mm). A cabinet side that’s “23 and 5/16 inches” becomes 23.3125 in your G-code. Getting this conversion wrong by even one sixteenth (0.0625”) means parts won’t fit, so precision matters.
To go the other direction and convert a decimal back to a tape-measure-friendly fraction, use the inches to fraction calculator. If you need the final value in metric only, the inches to mm converter and inches to cm converter work directly from decimal inch inputs.