Letter Drill Bit Sizes (A to Z)
Letter drill sizes run from A (0.234") to Z (0.413"), picking up where number drills leave off. Letter E equals exactly 1/4" (0.250"), one of the few points where a lettered and fractional size coincide.
Where Letter Sizes Fit
Letter drills are a 26-size sequence running A through Z, covering roughly 0.234" to 0.413" — the range immediately above where the number drill sequence stops at #1 (0.228").
Their sizes are a standardized, assigned sequence rather than the output of a formula — much like number drills, each letter's decimal size is fixed by the same ANSI/ASME B94.11M standard used industry-wide, so a letter G drill from any manufacturer measures the same 0.261". They're most commonly seen on tap drill charts, where a UNC or UNF thread's ideal pilot hole often lands between two fractional sizes and needs a letter drill to hit precisely.
Converting a Letter Size to Millimetres
Once you have a letter's decimal inch value, converting it to millimetres uses the same exact constant as everywhere else on this site.
MM equals the letter drill's decimal inch value multiplied by 25.4.
- INThe letter drill's standardized decimal inch size, looked up from the table below.
- MMThe millimetre equivalent: IN × 25.4.
Worked calculation — letter Q
- Q = 0.332"Look up the standardized size
- 0.332 × 25.4 = 8.4328Convert to millimetres
Letter Q = 0.332" = 8.433 mm
All 26 Letter Drill Sizes
| Letter | Decimal | MM | Letter | Decimal | MM |
|---|---|---|---|---|---|
| A | 0.234" | 5.944 | N | 0.302" | 7.671 |
| B | 0.238" | 6.045 | O | 0.316" | 8.026 |
| C | 0.242" | 6.147 | P | 0.323" | 8.204 |
| D | 0.246" | 6.248 | Q | 0.332" | 8.433 |
| E | 0.25" | 6.35 | R | 0.339" | 8.611 |
| F | 0.257" | 6.528 | S | 0.348" | 8.839 |
| G | 0.261" | 6.629 | T | 0.358" | 9.093 |
| H | 0.266" | 6.756 | U | 0.368" | 9.347 |
| I | 0.272" | 6.909 | V | 0.377" | 9.576 |
| J | 0.277" | 7.036 | W | 0.386" | 9.804 |
| K | 0.281" | 7.137 | X | 0.397" | 10.084 |
| L | 0.29" | 7.366 | Y | 0.404" | 10.262 |
| M | 0.295" | 7.493 | Z | 0.413" | 10.49 |
Frequently Asked Questions
What is the smallest letter drill size?
Letter A is the smallest, at 0.234" (5.944 mm).
What is the largest letter drill size?
Letter Z is the largest, at 0.413" (10.490 mm).
Why do letter drills start at A instead of continuing number drills?
Letter drills were introduced as a separate series specifically to cover sizes just above where number drills — which stop at #1, 0.228" — leave off, rather than continuing the number sequence, which was already an established standard in its own right.
What is letter drill Q in decimal inches?
Letter Q measures 0.332" (8.433 mm).
What tap sizes commonly use a letter drill?
Larger UNC and UNF threads, such as 7/16-14 and 1/2-13, commonly specify a letter drill for their tap drill size — see the tap drill size chart in the machinist tools section for the complete list.
Drill Size References
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