3/4" UNC tap drill size
The tap drill for 3/4-10 UNC is 21/32" (16.67 mm). That’s the standard chart size and gives about 72% of full thread.
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Tap drill by thread percentage
75% is the usual workshop figure. Less thread is easier to tap; more adds little strength.
| Thread % | Ideal drill | Nearest metric drill | Nearest imperial drill |
|---|---|---|---|
| 60% | 17.07 mm 0.6721" | 17 mm (62%) | 43/64" 0.6719" (60%) |
| 65% | 16.905 mm 0.6656" | 17 mm (62%) | 43/64" 0.6719" (60%) |
| 70% | 16.74 mm 0.6591" | 16.75 mm (70%) | 21/32" 0.6563" (72%) |
| 75% | 16.575 mm 0.6526" | 16.5 mm (77%) | 21/32" 0.6563" (72%) |
| 80% | 16.41 mm 0.6461" | 16.5 mm (77%) | 41/64" 0.6406" (84%) |
About this size
UNC is the general-purpose American inch thread. It shares the 60° basic profile with ISO metric, so the same tap-drill rule applies.
- For stainless steel, duplex and other tough materials, go to about 65% thread: a 17 mm drill gives 62%. For mild steel, brass and cast iron the 21/32" drill is fine.
- For a blind hole with 1.5 × D thread engagement (28.6 mm), drill about 38.7 mm deep. That leaves room for the 3–4 tapered lead threads on a second or plug tap. In steel, 1 × D engagement is usually enough; in aluminium or cast iron allow 1.5–2 × D.
Clearance holes for 3/4" UNC
| Fit | Drill |
|---|---|
| Close | 25/32" (19.84 mm) |
| Medium (normal) | 13/16" (20.64 mm) |
| Free (loose) | 7/8" (22.22 mm) |
Worked out with our own rule, not taken from a standards table: close = the larger of D + 4% and D + 0.2 mm, medium = the larger of D + 8% and D + 0.4 mm, free = the larger of D + 15% and D + 0.75 mm, each rounded up to the next 1/64". Check against your drawing where the fit matters.
3/4" UNC thread data
| Designation | 3/4-10 UNC |
|---|---|
| Standard | ASME B1.1 Unified basic profile (same 60° basic form as ISO metric) |
| Pitch | 10 TPI (2.54 mm pitch) |
| Thread angle | 60° |
| Major diameter | 0.750" (19.05 mm) |
| Pitch (effective) diameter | 0.685" (17.4 mm) |
| Minor diameter, external | 0.627" (15.934 mm) |
| Minor diameter, internal | 0.642" (16.3 mm) |
| Thread depth (internal) | 1.375 mm |
Basic sizes. Real screws are slightly under and real nuts slightly over these values because of tolerance classes.
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Nearest alternatives
If you don’t have the right drill, these give:
- 16.25 mm drill → about 85% thread
- 16.5 mm drill → about 77% thread
- 16.75 mm drill → about 70% thread
Threads close in size (not interchangeable – the pitch and/or angle differ):
- 3/4" UNF – 19.05 mm major, 16 TPI (1.587 mm pitch)
- 3/4" BSW – 19.05 mm major, 10 TPI (2.54 mm pitch)
- 3/4" BSF – 19.05 mm major, 12 TPI (2.117 mm pitch)
- M20 – 20 mm major, 2.5 mm pitch (10.2 TPI equivalent)
- M20 × 1.5 – 20 mm major, 1.5 mm pitch (16.9 TPI equivalent)
Questions
What size drill for a 3/4" UNC tap?
The tap drill for 3/4-10 UNC is 21/32" (16.67 mm). That’s the standard chart size and gives about 72% of full thread.
What size clearance hole for 3/4" UNC?
For a 3/4" UNC screw or bolt, drill 25/32" for a close fit, 13/16" for a medium (normal) fit or 7/8" for a free fit. These come from our stated rule, not a standards table.
What is the pitch of 3/4" UNC?
3/4-10 UNC has a 10 TPI (2.54 mm pitch), a 60° thread angle and a 19.05 mm major diameter.
Sources: Basic sizes per ASME B1.1. Chart drill from the common UN 75% tapping chart where one exists. Number and letter drill sizes checked 25/10/2026. Values are for reference only – check your tap maker’s chart. See the disclaimer.
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