Before tapping a hole you drill it to the thread’s minor diameter, more or less. Charts list one drill per tap; this calculator computes the drill for the percentage of thread you want (75% is standard) from the Machinery’s Handbook formula and then finds the closest number, letter, fractional and metric drills, reporting the thread engagement each one gives. It covers #0 to 1 in Unified threads and M1.6 to M24 metric, coarse and fine.
Tap drill size calculator
How it works
- Inch threads: drill = major diameter − (0.01299 × % thread ÷ threads per inch)
- Metric threads: drill = major diameter − (0.01299 × % thread × pitch in mm)
- Engagement of a chosen drill: % = (major − drill) ÷ (0.01299 × pitch)
The constant 0.01299 comes from the geometry of a 60° thread. A 1/4-20 tap at 75% wants a 0.201 in hole, which is why every chart says #7. For metric threads the familiar rule “drill = diameter − pitch” gives about 77% thread, the same as the standard chart.
Worked example
A 1/4-20 UNC thread at 75% engagement:
- Thread: 1/4-20 UNC
- Thread engagement: 75%
The calculator gives:
- Closest standard drill: #7 (0.201 in, 5.11 mm) = 75% thread
- Theoretical drill for 75%: 0.2013 in (5.11 mm)
- Nearest number or letter drill: #7 (0.201 in, 5.11 mm) = 75% thread
- Nearest fractional drill: 13/64" (0.2031 in, 5.16 mm) = 72% thread
- Nearest metric drill: 5.1 mm (0.2008 in, 5.1 mm) = 76% thread
- Thread: 1/4-20 UNC: major 0.25 in, 20 threads per inch
The theoretical drill is 0.2013 in and the closest standard drill is #7 at 0.201 in, giving 75% thread. The nearest fractional drill, 13/64 in (0.2031), gives 72%, and 5.1 mm gives 76%: all three will make a usable thread in aluminium or mild steel.
Common tap drill sizes at 75% thread
| Thread | Tap drill | Thread % |
|---|---|---|
| #4-40 UNC | #43 (0.089 in) | 71% |
| #6-32 UNC | #36 (0.1065 in) | 78% |
| #8-32 UNC | #29 (0.136 in) | 69% |
| #10-24 UNC | #25 (0.1495 in) | 75% |
| #10-32 UNF | #21 (0.159 in) | 76% |
| 1/4-20 UNC | #7 (0.201 in) | 75% |
| 1/4-28 UNF | #3 (0.213 in) | 80% |
| 5/16-18 UNC | F (0.257 in) | 77% |
| 3/8-16 UNC | 5/16" (0.3125 in) | 77% |
| 7/16-14 UNC | U (0.368 in) | 75% |
| 1/2-13 UNC | 27/64" (0.4219 in) | 78% |
| 5/8-11 UNC | 17/32" (0.5313 in) | 79% |
| 3/4-10 UNC | 21/32" (0.6563 in) | 72% |
| M3 x 0.5 | 2.5 mm (0.0984 in) | 77% |
| M4 x 0.7 | 3.3 mm (0.1299 in) | 77% |
| M5 x 0.8 | 4.2 mm (0.1654 in) | 77% |
| M6 x 1 | 5 mm (0.1969 in) | 77% |
| M8 x 1.25 (coarse) | 6.8 mm (0.2677 in) | 74% |
| M10 x 1.5 (coarse) | 8.5 mm (0.3346 in) | 77% |
| M12 x 1.75 (coarse) | 10.3 mm (0.4055 in) | 75% |
Notes and limits
- 75% thread is the usual target: the strength gain above it is small and tapping torque rises steeply. Use 50 to 60% in tough steels and stainless, up to 85% in plastics and soft aluminium.
- A tap drill makes the hole for the tap. The clearance hole for the bolt itself is larger: ASME B18.2.8 lists close and free fits.
- Drill diameters are the nominal sizes in ASME B94.11M; real holes drill slightly oversize, so the engagement is usually a little under the figure shown.
- Roll-form (fluteless) taps need a larger hole than cutting taps: use the tap maker’s chart, not this one.
- Thread engagement is not the same as the depth of the hole. Drill deeper than the tap will reach so chips have somewhere to go.
Frequently asked questions
What drill bit do I use for a 1/4-20 tap?
A #7 drill (0.201 in) for the standard 75% thread. 13/64 in is a close fractional substitute at about 72%.
What size hole for an M6 tap?
5.0 mm, which gives about 77% thread. That follows the rule drill = diameter − pitch (6 − 1).
What size drill for a 3/8-16 tap?
5/16 in (0.3125), giving 77% thread. The letter O drill (0.316) gives about 73%.
What size drill for a 10-24 tap?
#25 (0.1495 in) at 75%. For 10-32, use #21 (0.159 in).
Does percentage of thread matter?
Yes. Under about 50% the thread strips easily; over about 85% the tap works very hard and small taps break. 75% is the compromise most charts assume.
Sources
- ASME B1.1, Unified Inch Screw Threads (UN and UNR Thread Form): major diameters and threads per inch
- ASME B94.11M, Twist Drills, Straight Shank and Taper Shank, Combined Drills and Countersinks (number, letter and fractional drill diameters)
- ASME B18.2.8, Clearance Holes for Bolts, Screws and Studs
Last reviewed: 26 September 2026. The calculation is covered by automated tests with known answers taken from the sources above. Found an error? Please let us know.
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