5/16" UNC tap drill size
The tap drill for 5/16-18 UNC is F (6.53 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% | 6.838 mm 0.2692" | 6.8 mm (62%) | letter I 0.2720" (56%) |
| 65% | 6.746 mm 0.2656" | 6.7 mm (68%) | 17/64" 0.2656" (65%) |
| 70% | 6.654 mm 0.2620" | 6.7 mm (68%) | letter G 0.2610" (71%) |
| 75% | 6.563 mm 0.2584" | 6.6 mm (73%) | letter F 0.2570" (77%) |
| 80% | 6.471 mm 0.2548" | 6.5 mm (78%) | letter F 0.2570" (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 6.7 mm drill gives 68%. For mild steel, brass and cast iron the F drill is fine.
- For a blind hole with 1.5 × D thread engagement (11.9 mm), drill about 17.6 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 5/16" UNC
| Fit | Drill |
|---|---|
| Close | 21/64" (8.33 mm) |
| Medium (normal) | 11/32" (8.73 mm) |
| Free (loose) | 23/64" (9.13 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.
5/16" UNC thread data
| Designation | 5/16-18 UNC |
|---|---|
| Standard | ASME B1.1 Unified basic profile (same 60° basic form as ISO metric) |
| Pitch | 18 TPI (1.411 mm pitch) |
| Thread angle | 60° |
| Major diameter | 0.313" (7.938 mm) |
| Pitch (effective) diameter | 0.276" (7.021 mm) |
| Minor diameter, external | 0.244" (6.206 mm) |
| Minor diameter, internal | 0.252" (6.41 mm) |
| Thread depth (internal) | 0.764 mm |
Basic sizes. Real screws are slightly under and real nuts slightly over these values because of tolerance classes.
Quick calculator
Nearest alternatives
If you don’t have the right drill, these give:
- 6.5 mm drill → about 78% thread
- 6.6 mm drill → about 73% thread
- 6.7 mm drill → about 68% thread
Threads close in size (not interchangeable – the pitch and/or angle differ):
- 5/16" UNF – 7.94 mm major, 24 TPI (1.058 mm pitch)
- 5/16" BSW – 7.94 mm major, 18 TPI (1.411 mm pitch)
- 5/16" BSF – 7.94 mm major, 22 TPI (1.155 mm pitch)
- M8 – 8 mm major, 1.25 mm pitch (20.3 TPI equivalent)
- M8 × 1 – 8 mm major, 1 mm pitch (25.4 TPI equivalent)
Questions
What size drill for a 5/16" UNC tap?
The tap drill for 5/16-18 UNC is F (6.53 mm). That’s the standard chart size and gives about 77% of full thread.
What size clearance hole for 5/16" UNC?
For a 5/16" UNC screw or bolt, drill 21/64" for a close fit, 11/32" for a medium (normal) fit or 23/64" for a free fit. These come from our stated rule, not a standards table.
What is the pitch of 5/16" UNC?
5/16-18 UNC has a 18 TPI (1.411 mm pitch), a 60° thread angle and a 7.938 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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