5/16-18 Tap Drill Size & Thread Specs
Everything for a 5/16-18 UNC thread on one page: tap drill (F), clearance holes, wrench and hex key sizes, and the nearest metric thread.
SP-TD-01 · 5/16-18
5/16-18 specs
SizePlate
5/16-18 UNC · tap with
F drill
.2570″
6.53 mm · 77% thread
- Metric drill
- 6.5 mm · .2559
- Clearance
- P / Q
- Wrench
- 1/2″
- Cap screw key
- 1/4″
Thread
- Nominal size
- 5/16″ · 7.94 mm
- Major diameter
- .3125″
- Threads per inch
- 18
- Pitch
- .0556″ · 1.411 mm
- Series
- UNC (coarse)
- UNF version
- 5/16-24
Drilling
- Tap drill (75%)
- F · .2570
- Metric substitute
- 6.5 mm · .2559
- Tap drill (≈50%)
- J · .2770
- Close fit
- P · .3230
- Free fit
- Q · .3320
Tools
- Hex bolt wrench
- 1/2″
- Hex nut wrench
- 1/2″
- Socket head cap key
- 1/4″
- Button head key
- 3/16″
- Flat head key
- 3/16″
Nearest metric
- Thread
- M8 × 1.25
- Major diameter
- 8 mm
- Pitch
- 1.25 mm
- Tap drill
- 6.8 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 |
|---|---|---|---|
| 6.4 mm | .2520 | 6.4 | 84% |
| 6.5 mm | .2559 | 6.5 | 78% |
| F | .2570 | 6.53 | 77% |
| 6.6 mm | .2598 | 6.6 | 73% |
| G | .2610 | 6.63 | 71% |
| 6.7 mm | .2638 | 6.7 | 68% |
| 17/64 | .2656 | 6.75 | 65% |
| H | .2660 | 6.76 | 64% |
| 6.8 mm | .2677 | 6.8 | 62% |
| 6.9 mm | .2717 | 6.9 | 57% |
| I | .2720 | 6.91 | 56% |
| 7 mm | .2756 | 7 | 51% |
| J | .2770 | 7.04 | 49% |
| 7.1 mm | .2795 | 7.1 | 46% |
Neighboring sizes
Questions
What does 5/16-18 mean?
A 5/16″ nominal diameter with 18 threads per inch. It’s the coarse (UNC) thread for that size; the fine version is 5/16-24.
Can I use a 6.5 mm drill instead of F?
Usually. 6.5 mm is .2559″ against .2570″ for F, 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.