3/8-16 UNC tap drill size and thread dimensions
3/8-16 UNC has a major diameter of 9.525 mm and a pitch of 16 threads per inch (1.5875 mm). Every other dimension below follows from those two numbers and the 60-degree thread form.
- Clearance hole
- 10.287
- Pitch diameter
- 8.4939
- Minor diameter of the nut thread
- 7.8065
- Minor diameter of the bolt thread
- 7.5773
- Tensile stress area
- 50.714 mm2
- Breaking load at that tensile strength
- 40 571
- The old "diameter minus pitch" drill
- 7.937
- Engagement that rule actually gives
- 92.4 %
- Threads engaged over that length
- 9.01
- Equivalent threads per inch
- 16.000
Show the working
- Thread engagement = 75
- Clearance hole allowance = 8
- Length of engagement = 14.3
- Material tensile strength = 800
- major = 9.524999999999999 = 9.525
- p = 1.5875 = 1.5875
- h = 1.5875 * 0.8660254037844386 = 1.374 82
- Tap drill = 9.525 - 75/100 * 1.082532 * 1.5875 = 8.236 11
- The old "diameter minus pitch" drill = 9.525 - 1.5875 = 7.9375
- Engagement that rule actually gives = 100 / 1.082532 = 92.376
- Pitch diameter = 9.525 - 0.649519 * 1.5875 = 8.493 89
- Minor diameter of the nut thread = 9.525 - 1.082532 * 1.5875 = 7.806 48
- Minor diameter of the bolt thread = 9.525 - 1.226869 * 1.5875 = 7.577 35
- Tensile stress area = pi/4 * (9.525 - 0.938194 * 1.5875)^2 = 50.7141
- Clearance hole = 9.525 * (1 + 8/100) = 10.287
- Breaking load at that tensile strength = 50.7141 * 800 = 40 571.2
- shear_area = pi * 7.806 48 * 14.3 * (0.5 + 0.57735 * (8.493 89 - 7.806 48)/1.5875) = 263.028
- Threads engaged over that length = 1.5875 > 0 ? 14.3 / 1.5875 : 0 = 9.007 87
- Equivalent threads per inch = 1.5875 > 0 ? 25.4 / 1.5875 : 0 = 16
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.
3/8-16 UNC at common engagements
| Engagement | Tap drill (mm) | Tap drill (in) |
|---|---|---|
| 65% | 8.408 | 0.3310 |
| 70% | 8.322 | 0.3276 |
| 75% | 8.236 | 0.3243 |
| 80% | 8.150 | 0.3209 |
| 85% | 8.064 | 0.3175 |
| 90% | 7.978 | 0.3141 |
Pick the nearest drill above the figure, never below it.
Frequently asked
What drill size do I need for 3/8-16 UNC?
8.24 mm (0.3243 in) for 75% thread engagement. The traditional rule gives 7.94 mm, which is a tighter and harder-tapping hole.
What is the pitch of 3/8-16 UNC?
16 threads per inch (1.5875 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.