M2.5 tap drill size and thread dimensions
M2.5 has a major diameter of 2.500 mm and a pitch of 0.45 mm. Every other dimension below follows from those two numbers and the 60-degree thread form.
- Clearance hole
- 2.700
- Pitch diameter
- 2.2077
- Minor diameter of the nut thread
- 2.0129
- Minor diameter of the bolt thread
- 1.9479
- Tensile stress area
- 3.391 mm2
- Breaking load at that tensile strength
- 2713
- The old "diameter minus pitch" drill
- 2.050
- Engagement that rule actually gives
- 92.4 %
- Threads engaged over that length
- 8.44
- Equivalent threads per inch
- 56.444
Show the working
- Thread engagement = 75
- Clearance hole allowance = 8
- Length of engagement = 3.8
- Material tensile strength = 800
- major = 2.5 = 2.5
- p = 0.45 = 0.45
- h = 0.45 * 0.8660254037844386 = 0.389 711
- Tap drill = 2.5 - 75/100 * 1.082532 * 0.45 = 2.134 65
- The old "diameter minus pitch" drill = 2.5 - 0.45 = 2.05
- Engagement that rule actually gives = 100 / 1.082532 = 92.376
- Pitch diameter = 2.5 - 0.649519 * 0.45 = 2.207 72
- Minor diameter of the nut thread = 2.5 - 1.082532 * 0.45 = 2.012 86
- Minor diameter of the bolt thread = 2.5 - 1.226869 * 0.45 = 1.947 91
- Tensile stress area = pi/4 * (2.5 - 0.938194 * 0.45)^2 = 3.3908
- Clearance hole = 2.5 * (1 + 8/100) = 2.7
- Breaking load at that tensile strength = 3.3908 * 800 = 2712.64
- shear_area = pi * 2.012 86 * 3.8 * (0.5 + 0.57735 * (2.207 72 - 2.012 86)/0.45) = 18.0222
- Threads engaged over that length = 0.45 > 0 ? 3.8 / 0.45 : 0 = 8.444 44
- Equivalent threads per inch = 0.45 > 0 ? 25.4 / 0.45 : 0 = 56.4444
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.
M2.5 at common engagements
| Engagement | Tap drill (mm) | Tap drill (in) |
|---|---|---|
| 65% | 2.183 | 0.0860 |
| 70% | 2.159 | 0.0850 |
| 75% | 2.135 | 0.0840 |
| 80% | 2.110 | 0.0831 |
| 85% | 2.086 | 0.0821 |
| 90% | 2.062 | 0.0812 |
Pick the nearest drill above the figure, never below it.
Frequently asked
What drill size do I need for M2.5?
2.13 mm (0.0840 in) for 75% thread engagement. The traditional rule gives 2.05 mm, which is a tighter and harder-tapping hole.
What is the pitch of M2.5?
0.45 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.