M12 tap drill size and thread dimensions
M12 has a major diameter of 12.000 mm and a pitch of 1.75 mm. Every other dimension below follows from those two numbers and the 60-degree thread form.
- Clearance hole
- 12.960
- Pitch diameter
- 10.8633
- Minor diameter of the nut thread
- 10.1056
- Minor diameter of the bolt thread
- 9.8530
- Tensile stress area
- 84.267 mm2
- Breaking load at that tensile strength
- 67 413
- The old "diameter minus pitch" drill
- 10.250
- Engagement that rule actually gives
- 92.4 %
- Threads engaged over that length
- 10.29
- Equivalent threads per inch
- 14.514
Show the working
- Thread engagement = 75
- Clearance hole allowance = 8
- Length of engagement = 18
- Material tensile strength = 800
- major = 12 = 12
- p = 1.75 = 1.75
- h = 1.75 * 0.8660254037844386 = 1.515 54
- Tap drill = 12 - 75/100 * 1.082532 * 1.75 = 10.5792
- The old "diameter minus pitch" drill = 12 - 1.75 = 10.25
- Engagement that rule actually gives = 100 / 1.082532 = 92.376
- Pitch diameter = 12 - 0.649519 * 1.75 = 10.8633
- Minor diameter of the nut thread = 12 - 1.082532 * 1.75 = 10.1056
- Minor diameter of the bolt thread = 12 - 1.226869 * 1.75 = 9.852 98
- Tensile stress area = pi/4 * (12 - 0.938194 * 1.75)^2 = 84.2665
- Clearance hole = 12 * (1 + 8/100) = 12.96
- Breaking load at that tensile strength = 84.2665 * 800 = 67 413.2
- shear_area = pi * 10.1056 * 18 * (0.5 + 0.57735 * (10.8633 - 10.1056)/1.75) = 428.592
- Threads engaged over that length = 1.75 > 0 ? 18 / 1.75 : 0 = 10.2857
- Equivalent threads per inch = 1.75 > 0 ? 25.4 / 1.75 : 0 = 14.5143
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.
M12 at common engagements
| Engagement | Tap drill (mm) | Tap drill (in) |
|---|---|---|
| 65% | 10.769 | 0.4240 |
| 70% | 10.674 | 0.4202 |
| 75% | 10.579 | 0.4165 |
| 80% | 10.484 | 0.4128 |
| 85% | 10.390 | 0.4090 |
| 90% | 10.295 | 0.4053 |
Pick the nearest drill above the figure, never below it.
Frequently asked
What drill size do I need for M12?
10.58 mm (0.4165 in) for 75% thread engagement. The traditional rule gives 10.25 mm, which is a tighter and harder-tapping hole.
What is the pitch of M12?
1.75 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.