M1 tap drill size and thread dimensions
M1 has a major diameter of 1.000 mm and a pitch of 0.25 mm. Every other dimension below follows from those two numbers and the 60-degree thread form.
- Clearance hole
- 1.080
- Pitch diameter
- 0.8376
- Minor diameter of the nut thread
- 0.7294
- Minor diameter of the bolt thread
- 0.6933
- Tensile stress area
- 0.460 mm2
- Breaking load at that tensile strength
- 368
- The old "diameter minus pitch" drill
- 0.750
- Engagement that rule actually gives
- 92.4 %
- Threads engaged over that length
- 6.00
- Equivalent threads per inch
- 101.600
Show the working
- Thread engagement = 75
- Clearance hole allowance = 8
- Length of engagement = 1.5
- Material tensile strength = 800
- major = 1 = 1
- p = 0.25 = 0.25
- h = 0.25 * 0.8660254037844386 = 0.216 506
- Tap drill = 1 - 75/100 * 1.082532 * 0.25 = 0.797 025
- The old "diameter minus pitch" drill = 1 - 0.25 = 0.75
- Engagement that rule actually gives = 100 / 1.082532 = 92.376
- Pitch diameter = 1 - 0.649519 * 0.25 = 0.837 62
- Minor diameter of the nut thread = 1 - 1.082532 * 0.25 = 0.729 367
- Minor diameter of the bolt thread = 1 - 1.226869 * 0.25 = 0.693 283
- Tensile stress area = pi/4 * (1 - 0.938194 * 0.25)^2 = 0.460 177
- Clearance hole = 1 * (1 + 8/100) = 1.08
- Breaking load at that tensile strength = 0.460 177 * 800 = 368.142
- shear_area = pi * 0.729 367 * 1.5 * (0.5 + 0.57735 * (0.837 62 - 0.729 367)/0.25) = 2.5778
- Threads engaged over that length = 0.25 > 0 ? 1.5 / 0.25 : 0 = 6
- Equivalent threads per inch = 0.25 > 0 ? 25.4 / 0.25 : 0 = 101.6
Values are shown to six significant figures here; the arithmetic itself runs on exact fractions and is only rounded where a number is printed.
Scenarios & local history
Compare a saved scenario
Recent calculations
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.
M1 at common engagements
| Engagement | Tap drill (mm) | Tap drill (in) |
|---|---|---|
| 65% | 0.824 | 0.0324 |
| 70% | 0.811 | 0.0319 |
| 75% | 0.797 | 0.0314 |
| 80% | 0.783 | 0.0308 |
| 85% | 0.770 | 0.0303 |
| 90% | 0.756 | 0.0298 |
Pick the nearest drill above the figure, never below it.
Frequently asked
What drill size do I need for M1?
0.80 mm (0.0314 in) for 75% thread engagement. The traditional rule gives 0.75 mm, which is a tighter and harder-tapping hole.
What is the pitch of M1?
0.25 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.