12-24 UNC tap drill size and thread dimensions
12-24 UNC has a major diameter of 5.486 mm and a pitch of 24 threads per inch (1.0583 mm). Every other dimension below follows from those two numbers and the 60-degree thread form.
- Clearance hole
- 5.925
- Pitch diameter
- 4.7990
- Minor diameter of the nut thread
- 4.3407
- Minor diameter of the bolt thread
- 4.1880
- Tensile stress area
- 15.858 mm2
- Breaking load at that tensile strength
- 12 687
- The old "diameter minus pitch" drill
- 4.428
- Engagement that rule actually gives
- 92.4 %
- Threads engaged over that length
- 7.75
- Equivalent threads per inch
- 24.000
Show the working
- Thread engagement = 75
- Clearance hole allowance = 8
- Length of engagement = 8.2
- Material tensile strength = 800
- major = 5.4864 = 5.4864
- p = 1.0583333333333333 = 1.058 33
- h = 1.058 33 * 0.8660254037844386 = 0.916 544
- Tap drill = 5.4864 - 75/100 * 1.082532 * 1.058 33 = 4.627 14
- The old "diameter minus pitch" drill = 5.4864 - 1.058 33 = 4.428 07
- Engagement that rule actually gives = 100 / 1.082532 = 92.376
- Pitch diameter = 5.4864 - 0.649519 * 1.058 33 = 4.798 99
- Minor diameter of the nut thread = 5.4864 - 1.082532 * 1.058 33 = 4.340 72
- Minor diameter of the bolt thread = 5.4864 - 1.226869 * 1.058 33 = 4.187 96
- Tensile stress area = pi/4 * (5.4864 - 0.938194 * 1.058 33)^2 = 15.8582
- Clearance hole = 5.4864 * (1 + 8/100) = 5.925 31
- Breaking load at that tensile strength = 15.8582 * 800 = 12 686.6
- shear_area = pi * 4.340 72 * 8.2 * (0.5 + 0.57735 * (4.798 99 - 4.340 72)/1.058 33) = 83.8662
- Threads engaged over that length = 1.058 33 > 0 ? 8.2 / 1.058 33 : 0 = 7.748 03
- Equivalent threads per inch = 1.058 33 > 0 ? 25.4 / 1.058 33 : 0 = 24
Values are shown to six significant figures here; the arithmetic itself runs on exact fractions and is only rounded where a number is printed.
Scenarios & local history
Compare a saved scenario
Recent calculations
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.
12-24 UNC at common engagements
| Engagement | Tap drill (mm) | Tap drill (in) |
|---|---|---|
| 65% | 4.742 | 0.1867 |
| 70% | 4.684 | 0.1844 |
| 75% | 4.627 | 0.1822 |
| 80% | 4.570 | 0.1799 |
| 85% | 4.513 | 0.1777 |
| 90% | 4.455 | 0.1754 |
Pick the nearest drill above the figure, never below it.
Frequently asked
What drill size do I need for 12-24 UNC?
4.63 mm (0.1822 in) for 75% thread engagement. The traditional rule gives 4.43 mm, which is a tighter and harder-tapping hole.
What is the pitch of 12-24 UNC?
24 threads per inch (1.0583 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.