1/4-20 UNC tap drill size and thread dimensions
1/4-20 UNC has a major diameter of 6.350 mm and a pitch of 20 threads per inch (1.2700 mm). Every other dimension below follows from those two numbers and the 60-degree thread form.
- Clearance hole
- 6.858
- Pitch diameter
- 5.5251
- Minor diameter of the nut thread
- 4.9752
- Minor diameter of the bolt thread
- 4.7919
- Tensile stress area
- 20.899 mm2
- Breaking load at that tensile strength
- 16 720
- The old "diameter minus pitch" drill
- 5.080
- Engagement that rule actually gives
- 92.4 %
- Threads engaged over that length
- 7.48
- Equivalent threads per inch
- 20.000
Show the working
- Thread engagement = 75
- Clearance hole allowance = 8
- Length of engagement = 9.5
- Material tensile strength = 800
- major = 6.35 = 6.35
- p = 1.27 = 1.27
- h = 1.27 * 0.8660254037844386 = 1.099 85
- Tap drill = 6.35 - 75/100 * 1.082532 * 1.27 = 5.318 89
- The old "diameter minus pitch" drill = 6.35 - 1.27 = 5.08
- Engagement that rule actually gives = 100 / 1.082532 = 92.376
- Pitch diameter = 6.35 - 0.649519 * 1.27 = 5.525 11
- Minor diameter of the nut thread = 6.35 - 1.082532 * 1.27 = 4.975 18
- Minor diameter of the bolt thread = 6.35 - 1.226869 * 1.27 = 4.791 88
- Tensile stress area = pi/4 * (6.35 - 0.938194 * 1.27)^2 = 20.8995
- Clearance hole = 6.35 * (1 + 8/100) = 6.858
- Breaking load at that tensile strength = 20.8995 * 800 = 16 719.6
- shear_area = pi * 4.975 18 * 9.5 * (0.5 + 0.57735 * (5.525 11 - 4.975 18)/1.27) = 111.364
- Threads engaged over that length = 1.27 > 0 ? 9.5 / 1.27 : 0 = 7.480 31
- Equivalent threads per inch = 1.27 > 0 ? 25.4 / 1.27 : 0 = 20
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.
1/4-20 UNC at common engagements
| Engagement | Tap drill (mm) | Tap drill (in) |
|---|---|---|
| 65% | 5.456 | 0.2148 |
| 70% | 5.388 | 0.2121 |
| 75% | 5.319 | 0.2094 |
| 80% | 5.250 | 0.2067 |
| 85% | 5.181 | 0.2040 |
| 90% | 5.113 | 0.2013 |
Pick the nearest drill above the figure, never below it.
Frequently asked
What drill size do I need for 1/4-20 UNC?
5.32 mm (0.2094 in) for 75% thread engagement. The traditional rule gives 5.08 mm, which is a tighter and harder-tapping hole.
What is the pitch of 1/4-20 UNC?
20 threads per inch (1.2700 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.