M3 tap drill size and thread dimensions
M3 has a major diameter of 3.000 mm and a pitch of 0.5 mm. Every other dimension below follows from those two numbers and the 60-degree thread form.
- Clearance hole
- 3.240
- Pitch diameter
- 2.6752
- Minor diameter of the nut thread
- 2.4587
- Minor diameter of the bolt thread
- 2.3866
- Tensile stress area
- 5.031 mm2
- Breaking load at that tensile strength
- 4025
- The old "diameter minus pitch" drill
- 2.500
- Engagement that rule actually gives
- 92.4 %
- Threads engaged over that length
- 9.00
- Equivalent threads per inch
- 50.800
Show the working
- Thread engagement = 75
- Clearance hole allowance = 8
- Length of engagement = 4.5
- Material tensile strength = 800
- major = 3 = 3
- p = 0.5 = 0.5
- h = 0.5 * 0.8660254037844386 = 0.433 013
- Tap drill = 3 - 75/100 * 1.082532 * 0.5 = 2.594 05
- The old "diameter minus pitch" drill = 3 - 0.5 = 2.5
- Engagement that rule actually gives = 100 / 1.082532 = 92.376
- Pitch diameter = 3 - 0.649519 * 0.5 = 2.675 24
- Minor diameter of the nut thread = 3 - 1.082532 * 0.5 = 2.458 73
- Minor diameter of the bolt thread = 3 - 1.226869 * 0.5 = 2.386 57
- Tensile stress area = pi/4 * (3 - 0.938194 * 0.5)^2 = 5.030 84
- Clearance hole = 3 * (1 + 8/100) = 3.24
- Breaking load at that tensile strength = 5.030 84 * 800 = 4024.68
- shear_area = pi * 2.458 73 * 4.5 * (0.5 + 0.57735 * (2.675 24 - 2.458 73)/0.5) = 26.0697
- Threads engaged over that length = 0.5 > 0 ? 4.5 / 0.5 : 0 = 9
- Equivalent threads per inch = 0.5 > 0 ? 25.4 / 0.5 : 0 = 50.8
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.
M3 at common engagements
| Engagement | Tap drill (mm) | Tap drill (in) |
|---|---|---|
| 65% | 2.648 | 0.1043 |
| 70% | 2.621 | 0.1032 |
| 75% | 2.594 | 0.1021 |
| 80% | 2.567 | 0.1011 |
| 85% | 2.540 | 0.1000 |
| 90% | 2.513 | 0.0989 |
Pick the nearest drill above the figure, never below it.
Frequently asked
What drill size do I need for M3?
2.59 mm (0.1021 in) for 75% thread engagement. The traditional rule gives 2.50 mm, which is a tighter and harder-tapping hole.
What is the pitch of M3?
0.5 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.