5/8-18 Tap Drill Size & Thread Specs
Everything for a 5/8-18 UNF thread on one page: tap drill (37/64), clearance holes, wrench and hex key sizes, and the nearest metric thread.
SP-TD-01 · 5/8-18
5/8-18 specs
SizePlate
5/8-18 UNF · tap with
37/64 drill
.5781″
14.68 mm · 65% thread
- Metric drill
- 14.5 mm · .5709
- Clearance
- 41/64 / 21/32
- Wrench
- 15/16″
- Cap screw key
- 1/2″
Thread
- Nominal size
- 5/8″ · 15.88 mm
- Major diameter
- .6250″
- Threads per inch
- 18
- Pitch
- .0556″ · 1.411 mm
- Series
- UNF (fine)
- UNC version
- 5/8-11
Drilling
- Tap drill (75%)
- 37/64 · .5781
- Metric substitute
- 14.5 mm · .5709
- Tap drill (≈50%)
- 19/32 · .5938
- Close fit
- 41/64 · .6406
- Free fit
- 21/32 · .6563
Tools
- Hex bolt wrench
- 15/16″
- Hex nut wrench
- 15/16″
- Socket head cap key
- 1/2″
- Button head key
- 3/8″
- Flat head key
- 3/8″
Nearest metric
- Thread
- M16 × 2.0
- Major diameter
- 16 mm
- Pitch
- 2 mm
- Tap drill
- 14 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 |
|---|---|---|---|
| 14.5 mm | .5709 | 14.5 | 75% |
| 37/64 | .5781 | 14.68 | 65% |
| 15 mm | .5906 | 15 | 48% |
Neighboring sizes
Questions
What does 5/8-18 mean?
A 5/8″ nominal diameter with 18 threads per inch. It’s the fine (UNF) thread for that size; the coarse version is 5/8-11.
Can I use a 14.5 mm drill instead of 37/64?
Usually. 14.5 mm is .5709″ against .5781″ for 37/64, 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.