Drill Size Decimal Chart
This chart cross-references fractional drill sizes with their closest number or letter drill equivalent. 1/4" matches letter E exactly (0.2500"), while 1/16" is closest to #52 (0.0635"), just 0.001" larger.
Fractional, Number and Letter Drills
US drill bits come in three overlapping sizing systems, and knowing how they line up against each other is essential for finding a substitute or reading an older drawing.
Fractional sizes (1/16", 1/4", 1/2") are the most familiar, but they jump in coarse 1/64" steps. Number drills (#80 through #1) and letter drills (A through Z) fill in the space between fractional sizes with much finer increments, which is exactly why tap drill charts and close-tolerance hole specs often call out a number or letter size instead of a fraction.
Fractional Sizes vs Their Closest Match
Covering 1/64" through 7/16" — the range where number and letter drills sit close enough to be a useful cross-reference.
| Fraction | Decimal | Nearest match | Match decimal | Difference |
|---|---|---|---|---|
| 1/64" | 0.0156" | #78 | 0.016" | +0.0004" |
| 1/32" | 0.0313" | #68 | 0.031" | -0.0003" |
| 3/64" | 0.0469" | #56 | 0.0465" | -0.0004" |
| 1/16" | 0.0625" | #52 | 0.0635" | +0.001" |
| 5/64" | 0.0781" | #47 | 0.0785" | +0.0004" |
| 3/32" | 0.0938" | #42 | 0.0935" | -0.0003" |
| 7/64" | 0.1094" | #35 | 0.11" | +0.0006" |
| 1/8" | 0.125" | #30 | 0.1285" | +0.0035" |
| 9/64" | 0.1406" | #28 | 0.1405" | -0.0001" |
| 5/32" | 0.1563" | #22 | 0.157" | +0.0008" |
| 11/64" | 0.1719" | #17 | 0.173" | +0.0011" |
| 3/16" | 0.1875" | #12 | 0.189" | +0.0015" |
| 13/64" | 0.2031" | #6 | 0.204" | +0.0009" |
| 7/32" | 0.2188" | #2 | 0.221" | +0.0023" |
| 15/64" | 0.2344" | A | 0.234" | -0.0004" |
| 1/4" | 0.25" | E | 0.25" | +0" |
| 17/64" | 0.2656" | H | 0.266" | +0.0004" |
| 9/32" | 0.2813" | K | 0.281" | -0.0002" |
| 19/64" | 0.2969" | M | 0.295" | -0.0019" |
| 5/16" | 0.3125" | O | 0.316" | +0.0035" |
| 21/64" | 0.3281" | Q | 0.332" | +0.0039" |
| 11/32" | 0.3438" | S | 0.348" | +0.0042" |
| 23/64" | 0.3594" | T | 0.358" | -0.0014" |
| 3/8" | 0.375" | V | 0.377" | +0.002" |
| 25/64" | 0.3906" | W | 0.386" | -0.0046" |
| 13/32" | 0.4063" | Y | 0.404" | -0.0022" |
| 27/64" | 0.4219" | Z | 0.413" | -0.0089" |
| 7/16" | 0.4375" | Z | 0.413" | -0.0245" |
Frequently Asked Questions
What number drill is closest to 1/16 inch?
#52 (0.0635") is the closest number drill to 1/16" (0.0625"), just 0.001" larger — a common near-equivalent cited in machinist references.
What letter drill equals 1/4 inch exactly?
Letter E equals exactly 0.2500", the same decimal value as 1/4". It's one of the few points where a fractional and lettered size coincide precisely.
Why does this chart stop before 1/2 inch?
Letter drills only run from A (0.234") to Z (0.413"). Beyond about 7/16" (0.4375"), no letter or number drill sits close enough to be a meaningful cross-reference, so fractional sizing takes over from that point up.
Why do number and letter drills exist alongside fractional sizes at all?
Number and letter drills fill in the gaps between fractional sizes with much finer steps, which matters for tap drill sizing and close-tolerance holes where a sixty-fourth-inch jump is too coarse. The full range is covered on the <a href="/number-drill/">number drill size chart</a> and <a href="/letter-drill/">letter drill size chart</a>.
Is the nearest drill always a safe substitute?
Not automatically — the "difference" column shows the exact gap, and for a close-tolerance hole even a thousandth of an inch can matter. Use the nearest match as a starting point, then confirm against the actual tolerance the job requires.
Reference Charts
Every other chart in this cluster, one click away.