1/4-28 Tap Drill Size & Thread Specs
Everything for a 1/4-28 UNF thread on one page: tap drill (#3), clearance holes, wrench and hex key sizes, and the nearest metric thread.
SP-TD-01 · 1/4-28
1/4-28 specs
SizePlate
1/4-28 UNF · tap with
#3 drill
.2130″
5.41 mm · 80% thread
- Metric drill
- 5.4 mm · .2126
- Clearance
- F / H
- Wrench
- 7/16″
- Cap screw key
- 3/16″
Thread
- Nominal size
- 1/4″ · 6.35 mm
- Major diameter
- .2500″
- Threads per inch
- 28
- Pitch
- .0357″ · 0.907 mm
- Series
- UNF (fine)
- UNC version
- 1/4-20
Drilling
- Tap drill (75%)
- #3 · .2130
- Metric substitute
- 5.4 mm · .2126
- Tap drill (≈50%)
- #1 · .2280
- Close fit
- F · .2570
- Free fit
- H · .2660
Tools
- Hex bolt wrench
- 7/16″
- Hex nut wrench
- 7/16″
- Socket head cap key
- 3/16″
- Button head key
- 5/32″
- Flat head key
- 5/32″
Nearest metric
- Thread
- M6 × 1.0
- Major diameter
- 6 mm
- Pitch
- 1 mm
- Tap drill
- 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 |
|---|---|---|---|
| 5.4 mm | .2126 | 5.4 | 81% |
| #3 | .2130 | 5.41 | 80% |
| 5.5 mm | .2165 | 5.5 | 72% |
| 7/32 | .2188 | 5.56 | 67% |
| 5.6 mm | .2205 | 5.6 | 64% |
| #2 | .2210 | 5.61 | 63% |
| 5.7 mm | .2244 | 5.7 | 55% |
| #1 | .2280 | 5.79 | 47% |
| 5.8 mm | .2283 | 5.8 | 47% |
Neighboring sizes
Questions
What does 1/4-28 mean?
A 1/4″ nominal diameter with 28 threads per inch. It’s the fine (UNF) thread for that size; the coarse version is 1/4-20.
Can I use a 5.4 mm drill instead of #3?
Usually. 5.4 mm is .2126″ against .2130″ for #3, 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.