1/4-28 UNF tap drill size and thread dimensions
1/4-28 UNF has a major diameter of 6.350 mm and a pitch of 28 threads per inch (0.9071 mm). Every other dimension below follows from those two numbers and the 60-degree thread form.
- Clearance hole
- 6.858
- Pitch diameter
- 5.7608
- Minor diameter of the nut thread
- 5.3680
- Minor diameter of the bolt thread
- 5.2371
- Tensile stress area
- 23.749 mm2
- Breaking load at that tensile strength
- 18 999
- The old "diameter minus pitch" drill
- 5.443
- Engagement that rule actually gives
- 92.4 %
- Threads engaged over that length
- 10.47
- Equivalent threads per inch
- 28.000
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 = 0.9071428571428571 = 0.907 143
- h = 0.907 143 * 0.8660254037844386 = 0.785 609
- Tap drill = 6.35 - 75/100 * 1.082532 * 0.907 143 = 5.613 49
- The old "diameter minus pitch" drill = 6.35 - 0.907 143 = 5.442 86
- Engagement that rule actually gives = 100 / 1.082532 = 92.376
- Pitch diameter = 6.35 - 0.649519 * 0.907 143 = 5.760 79
- Minor diameter of the nut thread = 6.35 - 1.082532 * 0.907 143 = 5.367 99
- Minor diameter of the bolt thread = 6.35 - 1.226869 * 0.907 143 = 5.237 05
- Tensile stress area = pi/4 * (6.35 - 0.938194 * 0.907 143)^2 = 23.749
- Clearance hole = 6.35 * (1 + 8/100) = 6.858
- Breaking load at that tensile strength = 23.749 * 800 = 18 999.2
- shear_area = pi * 5.367 99 * 9.5 * (0.5 + 0.57735 * (5.760 79 - 5.367 99)/0.907 143) = 120.156
- Threads engaged over that length = 0.907 143 > 0 ? 9.5 / 0.907 143 : 0 = 10.4724
- Equivalent threads per inch = 0.907 143 > 0 ? 25.4 / 0.907 143 : 0 = 28
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.
1/4-28 UNF at common engagements
| Engagement | Tap drill (mm) | Tap drill (in) |
|---|---|---|
| 65% | 5.712 | 0.2249 |
| 70% | 5.663 | 0.2229 |
| 75% | 5.613 | 0.2210 |
| 80% | 5.564 | 0.2191 |
| 85% | 5.515 | 0.2171 |
| 90% | 5.466 | 0.2152 |
Pick the nearest drill above the figure, never below it.
Frequently asked
What drill size do I need for 1/4-28 UNF?
5.61 mm (0.2210 in) for 75% thread engagement. The traditional rule gives 5.44 mm, which is a tighter and harder-tapping hole.
What is the pitch of 1/4-28 UNF?
28 threads per inch (0.9071 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.