M18 tap drill size and thread dimensions
M18 has a major diameter of 18.000 mm and a pitch of 2.5 mm. Every other dimension below follows from those two numbers and the 60-degree thread form.
- Clearance hole
- 19.440
- Pitch diameter
- 16.3762
- Minor diameter of the nut thread
- 15.2937
- Minor diameter of the bolt thread
- 14.9328
- Tensile stress area
- 192.473 mm2
- Breaking load at that tensile strength
- 153 978
- The old "diameter minus pitch" drill
- 15.500
- Engagement that rule actually gives
- 92.4 %
- Threads engaged over that length
- 10.80
- Equivalent threads per inch
- 10.160
Show the working
- Thread engagement = 75
- Clearance hole allowance = 8
- Length of engagement = 27
- Material tensile strength = 800
- major = 18 = 18
- p = 2.5 = 2.5
- h = 2.5 * 0.8660254037844386 = 2.165 06
- Tap drill = 18 - 75/100 * 1.082532 * 2.5 = 15.9703
- The old "diameter minus pitch" drill = 18 - 2.5 = 15.5
- Engagement that rule actually gives = 100 / 1.082532 = 92.376
- Pitch diameter = 18 - 0.649519 * 2.5 = 16.3762
- Minor diameter of the nut thread = 18 - 1.082532 * 2.5 = 15.2937
- Minor diameter of the bolt thread = 18 - 1.226869 * 2.5 = 14.9328
- Tensile stress area = pi/4 * (18 - 0.938194 * 2.5)^2 = 192.473
- Clearance hole = 18 * (1 + 8/100) = 19.44
- Breaking load at that tensile strength = 192.473 * 800 = 153 978
- shear_area = pi * 15.2937 * 27 * (0.5 + 0.57735 * (16.3762 - 15.2937)/2.5) = 972.941
- Threads engaged over that length = 2.5 > 0 ? 27 / 2.5 : 0 = 10.8
- Equivalent threads per inch = 2.5 > 0 ? 25.4 / 2.5 : 0 = 10.16
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.
M18 at common engagements
| Engagement | Tap drill (mm) | Tap drill (in) |
|---|---|---|
| 65% | 16.241 | 0.6394 |
| 70% | 16.106 | 0.6341 |
| 75% | 15.970 | 0.6288 |
| 80% | 15.835 | 0.6234 |
| 85% | 15.700 | 0.6181 |
| 90% | 15.564 | 0.6128 |
Pick the nearest drill above the figure, never below it.
Frequently asked
What drill size do I need for M18?
15.97 mm (0.6288 in) for 75% thread engagement. The traditional rule gives 15.50 mm, which is a tighter and harder-tapping hole.
What is the pitch of M18?
2.5 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.