1" UNC tap drill size
The tap drill for 1-8 UNC is 7/8" (22.22 mm). That’s the standard chart size and gives about 77% 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% | 22.925 mm 0.9026" | 23 mm (58%) | 29/32" 0.9062" (58%) |
| 65% | 22.719 mm 0.8945" | 22.75 mm (64%) | 57/64" 0.8906" (67%) |
| 70% | 22.513 mm 0.8863" | 22.5 mm (70%) | 57/64" 0.8906" (67%) |
| 75% | 22.307 mm 0.8782" | 22.25 mm (76%) | 7/8" 0.8750" (77%) |
| 80% | 22.1 mm 0.8701" | 22 mm (82%) | 7/8" 0.8750" (77%) |
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 22.75 mm drill gives 64%. For mild steel, brass and cast iron the 7/8" drill is fine.
- At this size many people bore the hole and screwcut the thread on the lathe instead of using a hand tap. See the thread calculator for depths and the infeed.
- For a blind hole with 1.5 × D thread engagement (38.1 mm), drill about 50.8 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 1" UNC
| Fit | Drill |
|---|---|
| Close | 1-3/64" (26.59 mm) |
| Medium (normal) | 1-3/32" (27.78 mm) |
| Free (loose) | 1-5/32" (29.37 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.
1" UNC thread data
| Designation | 1-8 UNC |
|---|---|
| Standard | ASME B1.1 Unified basic profile (same 60° basic form as ISO metric) |
| Pitch | 8 TPI (3.175 mm pitch) |
| Thread angle | 60° |
| Major diameter | 1.000" (25.4 mm) |
| Pitch (effective) diameter | 0.919" (23.338 mm) |
| Minor diameter, external | 0.847" (21.505 mm) |
| Minor diameter, internal | 0.865" (21.963 mm) |
| Thread depth (internal) | 1.719 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:
- 22 mm drill → about 82% thread
- 22.25 mm drill → about 76% thread
- 22.5 mm drill → about 70% thread
Threads close in size (not interchangeable – the pitch and/or angle differ):
- 1" UNF – 25.4 mm major, 12 TPI (2.117 mm pitch)
- 1" BSW – 25.4 mm major, 8 TPI (3.175 mm pitch)
- 1" BSF – 25.4 mm major, 10 TPI (2.54 mm pitch)
- G3/4 (3/4" BSP) – 26.44 mm major, 14 TPI (1.814 mm pitch)
- M24 – 24 mm major, 3 mm pitch (8.5 TPI equivalent)
Questions
What size drill for a 1" UNC tap?
The tap drill for 1-8 UNC is 7/8" (22.22 mm). That’s the standard chart size and gives about 77% of full thread.
What size clearance hole for 1" UNC?
For a 1" UNC screw or bolt, drill 1-3/64" for a close fit, 1-3/32" for a medium (normal) fit or 1-5/32" for a free fit. These come from our stated rule, not a standards table.
What is the pitch of 1" UNC?
1-8 UNC has a 8 TPI (3.175 mm pitch), a 60° thread angle and a 25.4 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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