1/4-20 Tap Drill Size & Thread Specs
Everything for a 1/4-20 UNC thread on one page: tap drill (#7), clearance holes, wrench and hex key sizes, and the nearest metric thread.
SP-TD-01 · 1/4-20
1/4-20 specs
SizePlate
1/4-20 UNC · tap with
#7 drill
.2010″
5.11 mm · 75% thread
- Metric drill
- 5.1 mm · .2008
- 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
- 20
- Pitch
- .0500″ · 1.270 mm
- Series
- UNC (coarse)
- UNF version
- 1/4-28
Drilling
- Tap drill (75%)
- #7 · .2010
- Metric substitute
- 5.1 mm · .2008
- Tap drill (≈50%)
- 7/32 · .2188
- 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 |
|---|---|---|---|
| #9 | .1960 | 4.98 | 83% |
| 5 mm | .1969 | 5 | 82% |
| #8 | .1990 | 5.05 | 79% |
| 5.1 mm | .2008 | 5.1 | 76% |
| #7 | .2010 | 5.11 | 75% |
| 13/64 | .2031 | 5.16 | 72% |
| #6 | .2040 | 5.18 | 71% |
| 5.2 mm | .2047 | 5.2 | 70% |
| #5 | .2055 | 5.22 | 69% |
| 5.3 mm | .2087 | 5.3 | 64% |
| #4 | .2090 | 5.31 | 63% |
| 5.4 mm | .2126 | 5.4 | 58% |
| #3 | .2130 | 5.41 | 57% |
| 5.5 mm | .2165 | 5.5 | 52% |
| 7/32 | .2188 | 5.56 | 48% |
| 5.6 mm | .2205 | 5.6 | 45% |
| #2 | .2210 | 5.61 | 45% |
Neighboring sizes
Questions
What does 1/4-20 mean?
A 1/4″ nominal diameter with 20 threads per inch. It’s the coarse (UNC) thread for that size; the fine version is 1/4-28.
Can I use a 5.1 mm drill instead of #7?
Usually. 5.1 mm is .2008″ against .2010″ for #7, 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.