Unified inch · UNC
1/2-13 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
7/16
0.4375" · 63% thread
Or 11 mm for 67%
Form tap
15/32
0.4688" · 60% thread
Or 11.8 mm for 68%
Pick your material for its % of thread, tapping RPM and the rigid-tap code.
Thread dimensions
- Major Ø
- 0.5000" (12.700 mm)
- Threads per inch
- 13 (pitch 0.0769")
- Pitch Ø (basic)
- 0.4500" (11.431 mm)
- Minor Ø (internal, basic)
- 0.4167" (10.585 mm)
- Thread depth (internal)
- 0.0416" (1.058 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 |
|---|---|---|---|
| 10.6 mm | 0.4173 | 10.60 | 83% |
| 10.7 mm | 0.4213 | 10.70 | 79% |
| 27/64 | 0.4219 | 10.72 | 78% |
| 10.8 mm | 0.4252 | 10.80 | 75% |
| 10.9 mm | 0.4291 | 10.90 | 71% |
| 11 mmsuggested | 0.4331 | 11.00 | 67% |
| 11.1 mm | 0.4370 | 11.10 | 63% |
| 7/16 | 0.4375 | 11.11 | 63% |
| 11.2 mm | 0.4409 | 11.20 | 59% |
| 11.3 mm | 0.4449 | 11.30 | 55% |
| 11.4 mm | 0.4488 | 11.40 | 51% |
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 |
|---|---|---|---|
| 11.6 mm | 0.4567 | 11.60 | 83% |
| 11.7 mm | 0.4606 | 11.70 | 75% |
| 11.8 mmsuggested | 0.4646 | 11.80 | 68% |
| 11.9 mm | 0.4685 | 11.90 | 60% |
| 15/32 | 0.4688 | 11.91 | 60% |
| 12 mm | 0.4724 | 12.00 | 53% |
3-wire measurement
- Best wire size
- 0.0444" (1.128 mm)
- Over wires, at basic pitch Ø
- 0.5167" (13.123 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