M24 tap drill size and thread dimensions
M24 has a major diameter of 24.000 mm and a pitch of 3.0 mm. Every other dimension below follows from those two numbers and the 60-degree thread form.
- Clearance hole
- 25.920
- Pitch diameter
- 22.0514
- Minor diameter of the nut thread
- 20.7524
- Minor diameter of the bolt thread
- 20.3194
- Tensile stress area
- 352.504 mm2
- Breaking load at that tensile strength
- 282 003
- The old "diameter minus pitch" drill
- 21.000
- Engagement that rule actually gives
- 92.4 %
- Threads engaged over that length
- 12.00
- Equivalent threads per inch
- 8.467
Show the working
- Thread engagement = 75
- Clearance hole allowance = 8
- Length of engagement = 36
- Material tensile strength = 800
- major = 24 = 24
- p = 3 = 3
- h = 3 * 0.8660254037844386 = 2.598 08
- Tap drill = 24 - 75/100 * 1.082532 * 3 = 21.5643
- The old "diameter minus pitch" drill = 24 - 3 = 21
- Engagement that rule actually gives = 100 / 1.082532 = 92.376
- Pitch diameter = 24 - 0.649519 * 3 = 22.0514
- Minor diameter of the nut thread = 24 - 1.082532 * 3 = 20.7524
- Minor diameter of the bolt thread = 24 - 1.226869 * 3 = 20.3194
- Tensile stress area = pi/4 * (24 - 0.938194 * 3)^2 = 352.504
- Clearance hole = 24 * (1 + 8/100) = 25.92
- Breaking load at that tensile strength = 352.504 * 800 = 282 003
- shear_area = pi * 20.7524 * 36 * (0.5 + 0.57735 * (22.0514 - 20.7524)/3) = 1760.28
- Threads engaged over that length = 3 > 0 ? 36 / 3 : 0 = 12
- Equivalent threads per inch = 3 > 0 ? 25.4 / 3 : 0 = 8.466 67
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.
M24 at common engagements
| Engagement | Tap drill (mm) | Tap drill (in) |
|---|---|---|
| 65% | 21.889 | 0.8618 |
| 70% | 21.727 | 0.8554 |
| 75% | 21.564 | 0.8490 |
| 80% | 21.402 | 0.8426 |
| 85% | 21.240 | 0.8362 |
| 90% | 21.077 | 0.8298 |
Pick the nearest drill above the figure, never below it.
Frequently asked
What drill size do I need for M24?
21.56 mm (0.8490 in) for 75% thread engagement. The traditional rule gives 21.00 mm, which is a tighter and harder-tapping hole.
What is the pitch of M24?
3.0 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.