Unified inch · UNC
1/4-20 UNC tap drill size
For general work, OSG suggests 65–70% of full thread. Harder materials and deep holes want less; the calculator sets it from your material.
Cut tap
#5
0.2055" · 69% thread
Form tap
#1
0.2280" · 65% thread
Pick your material for its % of thread, tapping RPM and the rigid-tap code.
Thread dimensions
- Major Ø
- 0.2500" (6.350 mm)
- Threads per inch
- 20 (pitch 0.0500")
- Pitch Ø (basic)
- 0.2175" (5.525 mm)
- Minor Ø (internal, basic)
- 0.1959" (4.975 mm)
- Thread depth (internal)
- 0.0271" (0.687 mm)
Basic sizes of the 60° form. Tolerance classes (2B/3B) set the limits around them.
Cut tap drills
For a cutting (fluted or spiral) tap. Every standard drill from 50% to 85% thread; suggested rows are in the 65–70% band.
| Drill | Ø in | Ø mm | Thread |
|---|---|---|---|
| #9 | 0.1960 | 4.98 | 83% |
| 5 mm | 0.1969 | 5.00 | 82% |
| #8 | 0.1990 | 5.05 | 79% |
| 5.1 mm | 0.2008 | 5.10 | 76% |
| #7 | 0.2010 | 5.11 | 75% |
| 13/64 | 0.2031 | 5.16 | 72% |
| #6 | 0.2040 | 5.18 | 71% |
| 5.2 mmsuggested | 0.2047 | 5.20 | 70% |
| #5suggested | 0.2055 | 5.22 | 69% |
| 5.3 mm | 0.2087 | 5.30 | 64% |
| #4 | 0.2090 | 5.31 | 63% |
| 5.4 mm | 0.2126 | 5.40 | 58% |
| #3 | 0.2130 | 5.41 | 57% |
| 5.5 mm | 0.2165 | 5.50 | 52% |
Form tap drills
A form (roll) tap displaces metal instead of cutting it, so it needs a bigger hole. Every standard drill from 50% to 85% thread; suggested rows are in the 65–70% band.
| Drill | Ø in | Ø mm | Thread |
|---|---|---|---|
| 5.7 mm | 0.2244 | 5.70 | 75% |
| #1suggested | 0.2280 | 5.79 | 65% |
| 5.8 mm | 0.2283 | 5.80 | 64% |
| 5.9 mm | 0.2323 | 5.90 | 52% |
3-wire measurement
- Best wire size
- 0.0289" (0.733 mm)
- Over wires, at basic pitch Ø
- 0.2608" (6.625 mm)
For the external thread (the screw). The best wire touches the flanks at the pitch diameter.
Sources
- ASME B1.1 — Unified Inch Screw Threads (UN and UNR thread form)
- OSG — Tap drill size and pitch diameter limit chart (formula, suggested % of full thread)
- Cutting Tool Engineering — "Threads will roll": form tap hole size = D − 0.0068 × % / TPI
- ASME B94.11M — Twist drills (fractional, number, letter and metric sizes)
- NIST (NBS) Letter Circular 23 and Pratt & Whitney — three-wire thread measurement, best wire size