Tool Belt CalcMeasure
Drill Size Chart
Fractional drill sizes in decimal and millimetres.
| Fraction | Decimal (in) | Metric (mm) |
|---|---|---|
| 1/32 | 0.0313 | 0.79 |
| 1/16 | 0.0625 | 1.59 |
| 3/32 | 0.0938 | 2.38 |
| 1/8 | 0.1250 | 3.17 |
| 5/32 | 0.1563 | 3.97 |
| 3/16 | 0.1875 | 4.76 |
| 7/32 | 0.2188 | 5.56 |
| 1/4 | 0.2500 | 6.35 |
| 9/32 | 0.2813 | 7.14 |
| 5/16 | 0.3125 | 7.94 |
| 11/32 | 0.3438 | 8.73 |
| 3/8 | 0.3750 | 9.52 |
| 13/32 | 0.4063 | 10.32 |
| 7/16 | 0.4375 | 11.11 |
| 15/32 | 0.4688 | 11.91 |
| 1/2 | 0.5000 | 12.70 |
| 17/32 | 0.5313 | 13.49 |
| 9/16 | 0.5625 | 14.29 |
| 19/32 | 0.5938 | 15.08 |
| 5/8 | 0.6250 | 15.88 |
| 21/32 | 0.6563 | 16.67 |
| 11/16 | 0.6875 | 17.46 |
| 23/32 | 0.7188 | 18.26 |
| 3/4 | 0.7500 | 19.05 |
| 25/32 | 0.7813 | 19.84 |
| 13/16 | 0.8125 | 20.64 |
| 27/32 | 0.8438 | 21.43 |
| 7/8 | 0.8750 | 22.22 |
| 29/32 | 0.9063 | 23.02 |
| 15/16 | 0.9375 | 23.81 |
| 31/32 | 0.9688 | 24.61 |
| 1 | 1.0000 | 25.40 |
Thirty-seconds from 1/16 to 1 inch. Fractional drills step in sixty-fourths, so any size between these is simply the odd sixty-fourth in between — a 15/64 bit sits midway between 7/32 and 1/4.
Fractional drills step in sixty-fourths, so the whole set is arithmetic rather than a list to memorise. Every thirty-second from 1/16 to a full inch, with decimals and millimetres.
Why use this tool?
Complete set
Every thirty-second from 1/16 to 1 in, generated rather than sampled.
Decimal and metric
For comparing against a caliper reading or a metric specification.
Reduced fractions
Shown the way they are marked on the bit rather than as raw sixty-fourths.
Odd sizes derivable
Any size between two rows is the odd sixty-fourth in between.
How this drill size chart works
Fractional drill sizes are simply fractions of an inch in sixty-fourth increments, so the decimal equivalent is the numerator divided by the denominator and the metric is that times 25.4.
The fractions are reduced to lowest terms because that is how bits are marked — a bit stamped 1/4 is sixteen sixty-fourths, and nobody labels it that way.
Sizes between the listed rows are the odd sixty-fourths. A 15/64 bit falls midway between 7/32 and 1/4, which is worth knowing when a hole needs to be a whisker larger than a common size.
How to use it
Step 1: Find the fraction
As marked on the bit shank.
Step 2: Read the decimal
For comparing against a caliper or a specified hole size.
Step 3: Use metric if needed
For matching against metric fasteners or drawings.
Step 4: Go up, not down, for taps
The tap drill calculator gives the exact figure; pick the nearest bit above it.
Example usage
- A quarter inch
- 1/4 in is 0.25 decimal and 6.35 mm exactly.
- Close metric matches
- 3/8 in is 9.53 mm, close to a 9.5 mm bit but not identical. In precise work that 0.03 mm can matter.
- Between sizes
- Need something between 7/32 (0.2188) and 1/4 (0.25)? That is 15/64, or 0.2344.
Frequently asked questions
What is 3/8 inch in millimetres?
9.525 mm. Close to a 9.5 mm metric bit but not identical, which occasionally matters for a press fit.
What sizes do fractional drills come in?
Sixty-fourth increments, though most sets carry thirty-seconds or sixteenths. This lists every thirty-second from 1/16 to 1 inch.
How do I convert a drill size to decimal?
Divide the numerator by the denominator. A 5/16 bit is 5 ÷ 16, or 0.3125 inches.
What drill do I use for a tap?
Use the tap drill calculator for the exact figure, then pick the nearest bit above it. A slightly large hole loses a little thread engagement; a small one breaks taps.
Are number and letter drills the same system?
No — they are separate series filling the gaps between fractional sizes, running roughly from 0.228 down for numbers and 0.234 up for letters.
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