1/2-13 UNC tap drill size and thread dimensions
1/2-13 UNC has a major diameter of 12.700 mm and a pitch of 13 threads per inch (1.9538 mm). Every other dimension below follows from those two numbers and the 60-degree thread form.
- Clearance hole
- 13.716
- Pitch diameter
- 11.4309
- Minor diameter of the nut thread
- 10.5849
- Minor diameter of the bolt thread
- 10.3029
- Tensile stress area
- 92.748 mm2
- Breaking load at that tensile strength
- 74 198
- The old "diameter minus pitch" drill
- 10.746
- Engagement that rule actually gives
- 92.4 %
- Threads engaged over that length
- 9.78
- Equivalent threads per inch
- 13.000
Show the working
- Thread engagement = 75
- Clearance hole allowance = 8
- Length of engagement = 19.1
- Material tensile strength = 800
- major = 12.7 = 12.7
- p = 1.9538461538461538 = 1.953 85
- h = 1.953 85 * 0.8660254037844386 = 1.692 08
- Tap drill = 12.7 - 75/100 * 1.082532 * 1.953 85 = 11.1137
- The old "diameter minus pitch" drill = 12.7 - 1.953 85 = 10.7462
- Engagement that rule actually gives = 100 / 1.082532 = 92.376
- Pitch diameter = 12.7 - 0.649519 * 1.953 85 = 11.4309
- Minor diameter of the nut thread = 12.7 - 1.082532 * 1.953 85 = 10.5849
- Minor diameter of the bolt thread = 12.7 - 1.226869 * 1.953 85 = 10.3029
- Tensile stress area = pi/4 * (12.7 - 0.938194 * 1.953 85)^2 = 92.7475
- Clearance hole = 12.7 * (1 + 8/100) = 13.716
- Breaking load at that tensile strength = 92.7475 * 800 = 74 198
- shear_area = pi * 10.5849 * 19.1 * (0.5 + 0.57735 * (11.4309 - 10.5849)/1.953 85) = 476.356
- Threads engaged over that length = 1.953 85 > 0 ? 19.1 / 1.953 85 : 0 = 9.775 59
- Equivalent threads per inch = 1.953 85 > 0 ? 25.4 / 1.953 85 : 0 = 13
Values are shown to six significant figures here; the arithmetic itself runs on exact fractions and is only rounded where a number is printed.
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The formula
- tap drill = D - (engagement / 100) x 1.082532 x P
- pitch diameter = D - 0.649519 P
- internal minor diameter = D - 1.082532 P
- tensile stress area = (pi/4)(D - 0.938194 P)^2
The familiar "drill equals diameter minus pitch" rule actually produces about 92% thread engagement, not 75%. Past roughly 75% the extra holding power is negligible and the tapping torque rises sharply, which is why 75% or even 65% is the shop standard.
The tensile stress area formula is the ISO 898-1 definition, based on the mean of the pitch and minor diameters.
Cut a clearance hole rather than a close fit unless the joint is dowelled: thread clearance is what lets parts line up.
1/2-13 UNC at common engagements
| Engagement | Tap drill (mm) | Tap drill (in) |
|---|---|---|
| 65% | 11.325 | 0.4459 |
| 70% | 11.219 | 0.4417 |
| 75% | 11.114 | 0.4375 |
| 80% | 11.008 | 0.4334 |
| 85% | 10.902 | 0.4292 |
| 90% | 10.796 | 0.4251 |
Pick the nearest drill above the figure, never below it.
Frequently asked
What drill size do I need for 1/2-13 UNC?
11.11 mm (0.4375 in) for 75% thread engagement. The traditional rule gives 10.75 mm, which is a tighter and harder-tapping hole.
What is the pitch of 1/2-13 UNC?
13 threads per inch (1.9538 mm).
Where the numbers come from
- Thread geometry follows the ISO 68-1 basic profile shared by ISO metric and Unified inch threads; the stress-area definition is from ISO 898-1.