3/8-16 Tap Drill Size & Thread Specs
Everything for a 3/8-16 UNC thread on one page: tap drill (5/16), clearance holes, wrench and hex key sizes, and the nearest metric thread.
SP-TD-01 · 3/8-16
3/8-16 specs
SizePlate
3/8-16 UNC · tap with
5/16 drill
.3125″
7.94 mm · 77% thread
- Metric drill
- 7.9 mm · .3110
- Clearance
- W / X
- Wrench
- 9/16″
- Cap screw key
- 5/16″
Thread
- Nominal size
- 3/8″ · 9.52 mm
- Major diameter
- .3750″
- Threads per inch
- 16
- Pitch
- .0625″ · 1.587 mm
- Series
- UNC (coarse)
- UNF version
- 3/8-24
Drilling
- Tap drill (75%)
- 5/16 · .3125
- Metric substitute
- 7.9 mm · .3110
- Tap drill (≈50%)
- Q · .3320
- Close fit
- W · .3860
- Free fit
- X · .3970
Tools
- Hex bolt wrench
- 9/16″
- Hex nut wrench
- 9/16″
- Socket head cap key
- 5/16″
- Button head key
- 7/32″
- Flat head key
- 7/32″
Nearest metric
- Thread
- M10 × 1.5
- Major diameter
- 10 mm
- Pitch
- 1.5 mm
- Tap drill
- 8.5 mm
- Interchangeable
- No
Not interchangeable means the threads look close but won't run together: forcing one into the other strips both.
Drill size by thread percentage
Larger drill, less thread, easier tapping. 75% is the usual target.
| Drill | Decimal | mm | Thread |
|---|---|---|---|
| 7.8 mm | .3071 | 7.8 | 84% |
| 7.9 mm | .3110 | 7.9 | 79% |
| 5/16 | .3125 | 7.94 | 77% |
| 8 mm | .3150 | 8 | 74% |
| O | .3160 | 8.03 | 73% |
| 8.1 mm | .3189 | 8.1 | 69% |
| 8.2 mm | .3228 | 8.2 | 64% |
| P | .3230 | 8.2 | 64% |
| 8.3 mm | .3268 | 8.3 | 59% |
| 21/64 | .3281 | 8.33 | 58% |
| 8.4 mm | .3307 | 8.4 | 55% |
| Q | .3320 | 8.43 | 53% |
| 8.5 mm | .3346 | 8.5 | 50% |
| 8.6 mm | .3386 | 8.6 | 45% |
Neighboring sizes
Questions
What does 3/8-16 mean?
A 3/8″ nominal diameter with 16 threads per inch. It’s the coarse (UNC) thread for that size; the fine version is 3/8-24.
Can I use a 7.9 mm drill instead of 5/16?
Usually. 7.9 mm is .3110″ against .3125″ for 5/16, which changes the thread engagement by a few percent at most. The thread-percentage table above shows exactly how much thread each nearby drill leaves.