Drill Bit Size Converter
Turn fractional inches to decimal inches and millimeters.
What it does
Drill bits print as fractions like 1/4 or 3/8, while calipers read decimals and metric prints millimeters. Converting needs division plus the exact inch at 25.4 millimeters.
The tool above takes numerator and denominator and returns decimal inches, millimeters, and reduced fraction. The defaults 1 over 4 match the worked example, so the page opens showing 0.25 in. Pick new parts to update all three results.
Decimal is numerator divided by denominator. Millimeters multiply by 25.4 exactly. Reduced divides both parts by their greatest common divisor, so 2/4 becomes 1/2.
Use decimals for digital calipers, millimeters for metric prints, and reduced fractions for bit indexes. Number and letter gauges need separate tables beyond fractions.
This page shows the method, one full hand checked example at 1/4, a reference table, and practice checks that run through the tool.
How to use
- Type numerator 1 to 128.
- Pick denominator 2, 4, 8, 16, 32, or 64.
- Read Decimal inches as the main result.
- Read Millimeters for metric prints.
- Read Reduced fraction for the index name.
- Mic bits with calipers to confirm.
Formula
- numerator
- Top of the fraction.
- denominator
- Bottom, power of 2 here.
- 25.4
- Millimeters per inch, exact.
Fractional bits only here. Number and letter gauges use separate lookup tables.
Worked example
Example: 1/4 in
Convert quarter inch bit to decimal and metric.
- Write 1 over 4.
- Decimal: 1 / 4 = 0.25 in.
- Metric: 0.25 x 25.4 = 6.35 mm.
- GCD of 1 and 4 is 1.
- Reduced stays 1/4.
- Check: 6.35 / 25.4 = 0.25 in.
Reference table
| Fraction | Decimal | mm |
|---|---|---|
| 1/4 | 0.25 in | 6.35 mm |
| 1/2 | 0.5 in | 12.7 mm |
| 3/8 | 0.375 in | 9.525 mm |
| 5/16 | 0.3125 in | 7.938 mm |
Limits
- Fractions only to 64ths here.
- No number or letter gauges.
- Nominal sizes. Wear changes cutting diameter.
- Four decimals suit shops. Grinding needs more.
Common mistakes
- Reading 3/8 as 0.38 instead of 0.375.
- Mixing 25.4 with 2.54 for inches.
- Skipping reduction, so 2/4 stays unreduced.
- Using fractional table for number gauges.
- Trusting printed size on worn bits.
Try it yourself
Quick checks. Answers sit tucked under each question.
1/2 in?
Decimal 0.5 in, 12.7 mm, reduced 1/2.
3/8 in?
Decimal 0.375 in, 9.525 mm, reduced 3/8.
Questions
1/4 in decimal?
0.25 in.
mm?
Decimal times 25.4. Here 6.35 mm.
Reduced?
Divide by GCD. Here 1/4.
Number bits?
Not here. Separate gauge tables.
Why 25.4?
Exact millimeters per inch.
Wear?
Mic bits. Printed size drifts with use.