Fractional Drill Bit Sizes
Fractional drill bits run from 1/64" to 1" (and beyond) in steps of one sixty-fourth of an inch. A 1/4" bit drills a 0.25" hole, and a 3/8" bit drills a 9.525 mm hole — the system most drill indexes are built around.
The Sizing System Every Shop Starts With
Fractional sizing is the oldest and most common drill bit system in US shops, stepping in the same halving pattern — halves, quarters, eighths, sixteenths — as a tape measure.
A basic fractional index usually runs 1/16" to 3/8" or 1/2" in 1/16" steps, which is enough for the overwhelming majority of drilling: pilot holes, hardware clearance, dowel holes, and general fabrication. A more complete set steps down to 1/32" or 1/64" for finer control, but even then, fractional sizing has a built-in limitation — it can only land on 64 distinct positions between 0" and 1", spaced roughly 0.0156" apart.
That gap is exactly what number and letter drill sizes exist to close, covered on their own pages, along with guidance on choosing between all three systems.
How Fractional Sizes Are Numbered
Every fractional size is simply a count of sixty-fourths, reduced to its simplest form.
D, the decimal size, equals the position n (from 1 to 64) divided by 64, then the fraction n/64 is reduced to lowest terms for the name.
- nThe position in the sequence, from 1 to 64.
- DThe decimal size: n ÷ 64.
Worked calculation — n = 24
- 24 ÷ 64 = 0.375Decimal value
- GCD(24, 64) = 8Reduce the fraction
- 24/8 = 3, 64/8 = 8Simplified fraction
Position 24 = 3/8" = 0.375" = 9.525 mm
All 64 Fractional Drill Sizes
| Fraction | Decimal | MM | Fraction | Decimal | MM |
|---|---|---|---|---|---|
| 1/64" | 0.015625" | 0.3969 | 33/64" | 0.515625" | 13.0969 |
| 1/32" | 0.03125" | 0.7937 | 17/32" | 0.53125" | 13.4937 |
| 3/64" | 0.046875" | 1.1906 | 35/64" | 0.546875" | 13.8906 |
| 1/16" | 0.0625" | 1.5875 | 9/16" | 0.5625" | 14.2875 |
| 5/64" | 0.078125" | 1.9844 | 37/64" | 0.578125" | 14.6844 |
| 3/32" | 0.09375" | 2.3812 | 19/32" | 0.59375" | 15.0812 |
| 7/64" | 0.109375" | 2.7781 | 39/64" | 0.609375" | 15.4781 |
| 1/8" | 0.125" | 3.175 | 5/8" | 0.625" | 15.875 |
| 9/64" | 0.140625" | 3.5719 | 41/64" | 0.640625" | 16.2719 |
| 5/32" | 0.15625" | 3.9688 | 21/32" | 0.65625" | 16.6687 |
| 11/64" | 0.171875" | 4.3656 | 43/64" | 0.671875" | 17.0656 |
| 3/16" | 0.1875" | 4.7625 | 11/16" | 0.6875" | 17.4625 |
| 13/64" | 0.203125" | 5.1594 | 45/64" | 0.703125" | 17.8594 |
| 7/32" | 0.21875" | 5.5562 | 23/32" | 0.71875" | 18.2562 |
| 15/64" | 0.234375" | 5.9531 | 47/64" | 0.734375" | 18.6531 |
| 1/4" | 0.25" | 6.35 | 3/4" | 0.75" | 19.05 |
| 17/64" | 0.265625" | 6.7469 | 49/64" | 0.765625" | 19.4469 |
| 9/32" | 0.28125" | 7.1437 | 25/32" | 0.78125" | 19.8438 |
| 19/64" | 0.296875" | 7.5406 | 51/64" | 0.796875" | 20.2406 |
| 5/16" | 0.3125" | 7.9375 | 13/16" | 0.8125" | 20.6375 |
| 21/64" | 0.328125" | 8.3344 | 53/64" | 0.828125" | 21.0344 |
| 11/32" | 0.34375" | 8.7312 | 27/32" | 0.84375" | 21.4312 |
| 23/64" | 0.359375" | 9.1281 | 55/64" | 0.859375" | 21.8281 |
| 3/8" | 0.375" | 9.525 | 7/8" | 0.875" | 22.225 |
| 25/64" | 0.390625" | 9.9219 | 57/64" | 0.890625" | 22.6219 |
| 13/32" | 0.40625" | 10.3187 | 29/32" | 0.90625" | 23.0187 |
| 27/64" | 0.421875" | 10.7156 | 59/64" | 0.921875" | 23.4156 |
| 7/16" | 0.4375" | 11.1125 | 15/16" | 0.9375" | 23.8125 |
| 29/64" | 0.453125" | 11.5094 | 61/64" | 0.953125" | 24.2094 |
| 15/32" | 0.46875" | 11.9063 | 31/32" | 0.96875" | 24.6062 |
| 31/64" | 0.484375" | 12.3031 | 63/64" | 0.984375" | 25.0031 |
| 1/2" | 0.5" | 12.7 | 1" | 1" | 25.4 |
Frequently Asked Questions
What is the smallest fractional drill bit size?
1/64" (0.015625") is the finest standard fractional drill bit size.
How many fractional drill sizes are there from 1/64 to 1 inch?
There are 64 distinct sixty-fourths from 1/64" to 1", even though most of them reduce to a simpler fraction — 32/64 becomes 1/2", for instance — there are exactly 64 unique positions in the sequence.
When is a fractional drill bit good enough?
Whenever the hole doesn't need to land within a tolerance tighter than about 1/64" (0.0156"), which covers most woodworking, general fabrication and hobbyist projects.
What is 3/8 inch drill bit in mm?
A 3/8" drill bit measures 9.525 mm, found by multiplying 0.375 by 25.4.
Do fractional drill bits go larger than 1 inch?
Yes — fractional bits continue past 1" in the same 1/64" steps (or coarser 1/16" or 1/8" steps for larger sizes) for bigger holes. Number and letter systems don't extend that far, since they were designed specifically to fill the gaps below an inch.
Drill Size References
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