How to Convert Decimal Inches to Fractions
Rulers and tape measures in the US use fractional inches — halves, quarters, eighths, sixteenths — rather than decimals. Converting between the two is a core skill in woodworking, machining, and construction.
The Conversion Method
- Choose your denominator based on the precision you need (8, 16, 32, or 64).
- Multiply the decimal by the denominator. Example: 0.3125 × 16 = 5.
- Use the result as the numerator: 5/16".
- Simplify if possible. If both numerator and denominator are even, divide both by 2.
Common Inch Fractions to Decimal
| Fraction | Decimal | mm |
|---|---|---|
| 1/16" | 0.0625 | 1.588 |
| 1/8" | 0.125 | 3.175 |
| 3/16" | 0.1875 | 4.763 |
| 1/4" | 0.25 | 6.35 |
| 5/16" | 0.3125 | 7.938 |
| 3/8" | 0.375 | 9.525 |
| 7/16" | 0.4375 | 11.113 |
| 1/2" | 0.5 | 12.7 |
| 9/16" | 0.5625 | 14.288 |
| 5/8" | 0.625 | 15.875 |
| 11/16" | 0.6875 | 17.463 |
| 3/4" | 0.75 | 19.05 |
| 13/16" | 0.8125 | 20.638 |
| 7/8" | 0.875 | 22.225 |
| 15/16" | 0.9375 | 23.813 |
| 1" | 1.0 | 25.4 |
Reading a Ruler
Standard US rulers divide each inch into 16 equal parts. The marks decrease in length as the fraction gets smaller:
- Longest mark (besides the inch) = 1/2"
- Next longest = 1/4" and 3/4"
- Shorter = eighths (1/8, 3/8, 5/8, 7/8)
- Shortest = sixteenths
Precision measuring tools like calipers may mark to 1/32" or 1/64".
Inches to Metric Conversion
Since 1959, the international inch is defined as exactly 25.4 millimeters. This makes conversions exact:
- Inches to mm: multiply by 25.4
- Inches to cm: multiply by 2.54
- Inches to meters: multiply by 0.0254
- Inches to feet: divide by 12
When to Use Which Precision
- 1/8": Rough carpentry, framing, drywall
- 1/16": Finish carpentry, cabinetmaking, most woodworking
- 1/32": Fine woodworking, metal fabrication
- 1/64": Precision machining, engineering tolerances
- 1/128": Ultra-precision work, scientific instruments