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Math & Conversion
Measurement Converter
Instantly convert standard woodworking fractions to decimal inches and exact metric millimeters.Instantly convert metric millimeters into decimal inches and standard fractional cut sizes.
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The Math Behind Conversions
Converting measurements by hand relies on two standard constants.
| Calculation | Formula | Notes |
|---|---|---|
| Fraction to Decimal | Numerator ÷ Denominator | For example, 3/8 becomes 3 ÷ 8 = 0.375. Add this to any whole number (e.g. 5 + 0.375 = 5.375). |
| Inches to Millimeters | Inches × 25.4 | There are exactly 25.4 millimeters in one inch. For example, 1/4" (0.25") × 25.4 = 6.35 mm. |
| Millimeters to Inches | Millimeters ÷ 25.4 | Divide by 25.4 to convert back to decimal inches, then multiply by 16 (or 32) to find the nearest fractional mark. |
Quick Reference Chart
| Fraction | Decimal (in) | Metric (mm) |
|---|---|---|
| 1/16" | 0.0625" | 1.6 mm |
| 1/8" | 0.125" | 3.2 mm |
| 3/16" | 0.1875" | 4.8 mm |
| 1/4" | 0.25" | 6.4 mm |
| 5/16" | 0.3125" | 7.9 mm |
| 3/8" | 0.375" | 9.5 mm |
| 7/16" | 0.4375" | 11.1 mm |
| 1/2" | 0.5" | 12.7 mm |
| 9/16" | 0.5625" | 14.3 mm |
| 5/8" | 0.625" | 15.9 mm |
| 11/16" | 0.6875" | 17.5 mm |
| 3/4" | 0.75" | 19.1 mm |
| 13/16" | 0.8125" | 20.6 mm |
| 7/8" | 0.875" | 22.2 mm |
| 15/16" | 0.9375" | 23.8 mm |
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Frequently Asked Questions
Why do woodworkers use fractions instead of decimals?
The imperial system is deeply ingrained in North American construction. Tape measures are marked in physical 1/16" increments, making fractions the fastest way to communicate and mark lines on wood without doing decimal conversions in your head.
How exact is the metric conversion?
The conversion factor of 1 inch = 25.4 millimeters is exact by international definition. Calculating 1/2" as 12.7 mm is mathematically perfect.
What is the difference between 1/16" and 1/32" precision?
Most framing and general carpentry is done to the nearest 1/16", which is the smallest mark on a standard tape measure. Fine woodworking, cabinetry, and machining often require precision down to the nearest 1/32" or even 1/64" to ensure joints fit perfectly without visible gaps. When converting from metric to imperial, 1/32" will give you a tighter tolerance.
Can I enter a mixed measurement?
Yes, this converter understands mixed numbers like "5 3/8" or "10-1/2". You can also type decimals like "5.375" directly. It is designed to act exactly like a carpenter's brain.
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