1/4" UNC tap drill size
The tap drill for 1/4-20 UNC is #7 (5.11 mm). That’s the standard chart size and gives about 75% 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% | 5.36 mm 0.2110" | 5.4 mm (58%) | #3 0.2130" (57%) |
| 65% | 5.278 mm 0.2078" | 5.3 mm (64%) | #4 0.2090" (63%) |
| 70% | 5.195 mm 0.2045" | 5.2 mm (70%) | #6 0.2040" (71%) |
| 75% | 5.113 mm 0.2013" | 5.1 mm (76%) | #7 0.2010" (75%) |
| 80% | 5.03 mm 0.1980" | 5 mm (82%) | #8 0.1990" (79%) |
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 5.3 mm drill gives 64%. For mild steel, brass and cast iron the #7 drill is fine.
- For a blind hole with 1.5 × D thread engagement (9.5 mm), drill about 14.6 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 1/4" UNC
| Fit | Drill |
|---|---|
| Close | 17/64" (6.75 mm) |
| Medium (normal) | 9/32" (7.14 mm) |
| Free (loose) | 19/64" (7.54 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.
1/4" UNC thread data
| Designation | 1/4-20 UNC |
|---|---|
| Standard | ASME B1.1 Unified basic profile (same 60° basic form as ISO metric) |
| Pitch | 20 TPI (1.27 mm pitch) |
| Thread angle | 60° |
| Major diameter | 0.250" (6.35 mm) |
| Pitch (effective) diameter | 0.218" (5.525 mm) |
| Minor diameter, external | 0.189" (4.792 mm) |
| Minor diameter, internal | 0.196" (4.975 mm) |
| Thread depth (internal) | 0.687 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:
- 5 mm drill → about 82% thread
- 5.1 mm drill → about 76% thread
- 5.2 mm drill → about 70% thread
Threads close in size (not interchangeable – the pitch and/or angle differ):
- 1/4" UNF – 6.35 mm major, 28 TPI (0.907 mm pitch)
- 1/4" BSW – 6.35 mm major, 20 TPI (1.27 mm pitch)
- 1/4" BSF – 6.35 mm major, 26 TPI (0.977 mm pitch)
- M6 – 6 mm major, 1 mm pitch (25.4 TPI equivalent)
- 0BA – 6 mm major, 1 mm pitch (25.4 TPI equivalent)
Questions
What size drill for a 1/4" UNC tap?
The tap drill for 1/4-20 UNC is #7 (5.11 mm). That’s the standard chart size and gives about 75% of full thread.
What size clearance hole for 1/4" UNC?
For a 1/4" UNC screw or bolt, drill 17/64" for a close fit, 9/32" for a medium (normal) fit or 19/64" for a free fit. These come from our stated rule, not a standards table.
What is the pitch of 1/4" UNC?
1/4-20 UNC has a 20 TPI (1.27 mm pitch), a 60° thread angle and a 6.35 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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