M12x1.25 tap drill size and thread dimensions
M12x1.25 has a major diameter of 12.000 mm and a pitch of 1.25 mm. Every other dimension below follows from those two numbers and the 60-degree thread form.
- Clearance hole
- 12.960
- Pitch diameter
- 11.1881
- Minor diameter of the nut thread
- 10.6468
- Minor diameter of the bolt thread
- 10.4664
- Tensile stress area
- 92.072 mm2
- Breaking load at that tensile strength
- 73 657
- The old "diameter minus pitch" drill
- 10.750
- Engagement that rule actually gives
- 92.4 %
- Threads engaged over that length
- 14.40
- Equivalent threads per inch
- 20.320
Show the working
- Thread engagement = 75
- Clearance hole allowance = 8
- Length of engagement = 18
- Material tensile strength = 800
- major = 12 = 12
- p = 1.25 = 1.25
- h = 1.25 * 0.8660254037844386 = 1.082 53
- Tap drill = 12 - 75/100 * 1.082532 * 1.25 = 10.9851
- The old "diameter minus pitch" drill = 12 - 1.25 = 10.75
- Engagement that rule actually gives = 100 / 1.082532 = 92.376
- Pitch diameter = 12 - 0.649519 * 1.25 = 11.1881
- Minor diameter of the nut thread = 12 - 1.082532 * 1.25 = 10.6468
- Minor diameter of the bolt thread = 12 - 1.226869 * 1.25 = 10.4664
- Tensile stress area = pi/4 * (12 - 0.938194 * 1.25)^2 = 92.0718
- Clearance hole = 12 * (1 + 8/100) = 12.96
- Breaking load at that tensile strength = 92.0718 * 800 = 73 657.5
- shear_area = pi * 10.6468 * 18 * (0.5 + 0.57735 * (11.1881 - 10.6468)/1.25) = 451.548
- Threads engaged over that length = 1.25 > 0 ? 18 / 1.25 : 0 = 14.4
- Equivalent threads per inch = 1.25 > 0 ? 25.4 / 1.25 : 0 = 20.32
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.
M12x1.25 at common engagements
| Engagement | Tap drill (mm) | Tap drill (in) |
|---|---|---|
| 65% | 11.120 | 0.4378 |
| 70% | 11.053 | 0.4351 |
| 75% | 10.985 | 0.4325 |
| 80% | 10.917 | 0.4298 |
| 85% | 10.850 | 0.4272 |
| 90% | 10.782 | 0.4245 |
Pick the nearest drill above the figure, never below it.
Frequently asked
What drill size do I need for M12x1.25?
10.99 mm (0.4325 in) for 75% thread engagement. The traditional rule gives 10.75 mm, which is a tighter and harder-tapping hole.
What is the pitch of M12x1.25?
1.25 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.