Tap Drill Chart
A tap drill is smaller than a thread's nominal size, leaving just enough material for the tap to cut clean threads. A 1/4-20 UNC thread uses a #7 drill (0.201"), noticeably smaller than the bolt's 0.250" major diameter.
Leaving Room for the Tap to Cut
Drilling a hole the exact diameter of a bolt would leave nothing for a tap to cut into — the pilot hole has to be smaller, so the tap's cutting edges have material to bite and form threads from.
The standard target is roughly 75% thread engagement: enough thread contact for a strong joint, without making the hole so tight that the tap binds or breaks. That 75% figure isn't arbitrary — full 100% thread engagement barely increases joint strength over 75%, while it makes the tap significantly harder to turn and far more likely to snap off inside the hole.
Estimating a Tap Drill Size
A simple approximation gets close to the standard tap drill size for roughly 75% thread engagement.
D is the thread's major diameter and TPI is its threads per inch. This approximates the standard chart value closely, though the exact published sizes in the table below are what to actually use.
- DThe thread's nominal major diameter, such as 0.375" for 3/8".
- TPIThreads per inch, such as 16 for 3/8-16 UNC.
- DrillThe estimated tap drill diameter: D − (1 ÷ TPI).
Worked estimate — 3/8-16 UNC
- 1 ÷ 16 = 0.0625Invert the threads per inch
- 0.375 − 0.0625 = 0.3125Subtract from the major diameter
Estimated drill 0.3125" — matches the standard 5/16" tap drill exactly
Standard UNC & UNF Tap Drill Sizes
| Thread | Tap drill | Decimal | MM |
|---|---|---|---|
| #4-40 UNC | #43 | 0.089" | 2.261 mm |
| #6-32 UNC | #36 | 0.1065" | 2.705 mm |
| #8-32 UNC | #29 | 0.136" | 3.454 mm |
| #10-24 UNC | #25 | 0.1495" | 3.797 mm |
| #10-32 UNF | #21 | 0.159" | 4.039 mm |
| 1/4-20 UNC | #7 | 0.201" | 5.105 mm |
| 1/4-28 UNF | #3 | 0.213" | 5.41 mm |
| 5/16-18 UNC | F | 0.257" | 6.528 mm |
| 5/16-24 UNF | I | 0.272" | 6.909 mm |
| 3/8-16 UNC | 5/16" | 0.3125" | 7.938 mm |
| 3/8-24 UNF | Q | 0.332" | 8.433 mm |
| 7/16-14 UNC | U | 0.368" | 9.347 mm |
| 7/16-20 UNF | 25/64" | 0.3906" | 9.921 mm |
| 1/2-13 UNC | 27/64" | 0.4219" | 10.716 mm |
| 1/2-20 UNF | 29/64" | 0.4531" | 11.509 mm |
| 9/16-12 UNC | 31/64" | 0.4844" | 12.304 mm |
| 5/8-11 UNC | 17/32" | 0.5313" | 13.495 mm |
| 3/4-10 UNC | 21/32" | 0.6563" | 16.67 mm |
Letter and number tap drills are covered in full on the letter drill size chart and number drill size chart.
Frequently Asked Questions
What size drill do you use for a 1/4-20 tap?
A #7 drill (0.201"). Drilling this size before tapping gives roughly 75% thread engagement — enough for a strong thread without making the tap difficult to turn.
What does the "20" mean in 1/4-20?
It's the threads per inch (TPI) — a 1/4-20 bolt has 20 threads in every inch of its length, the coarse (UNC) thread count standard for a 1/4" diameter.
What's the difference between a tap drill for UNC vs UNF?
A UNF (fine) thread has more threads per inch than UNC (coarse) at the same diameter, so its tap drill is slightly larger — closer to the full major diameter — than the UNC tap drill for the same nominal size. For 1/2", UNC uses a 27/64" tap drill while UNF uses the larger 29/64".
What happens if I use the wrong tap drill size?
Too small a tap drill leaves too little clearance, forcing the tap and risking breakage inside the hole. Too large a tap drill leaves too little thread engagement, producing a weak thread that can strip under load.
What is the tap drill size for 3/8-16?
5/16" (0.3125") is the standard tap drill for a 3/8-16 UNC thread.
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