M4 tap drill size and thread dimensions
M4 has a major diameter of 4.000 mm and a pitch of 0.7 mm. Every other dimension below follows from those two numbers and the 60-degree thread form.
- Clearance hole
- 4.320
- Pitch diameter
- 3.5453
- Minor diameter of the nut thread
- 3.2422
- Minor diameter of the bolt thread
- 3.1412
- Tensile stress area
- 8.779 mm2
- Breaking load at that tensile strength
- 7023
- The old "diameter minus pitch" drill
- 3.300
- Engagement that rule actually gives
- 92.4 %
- Threads engaged over that length
- 8.57
- Equivalent threads per inch
- 36.286
Show the working
- Thread engagement = 75
- Clearance hole allowance = 8
- Length of engagement = 6
- Material tensile strength = 800
- major = 4 = 4
- p = 0.7 = 0.7
- h = 0.7 * 0.8660254037844386 = 0.606 218
- Tap drill = 4 - 75/100 * 1.082532 * 0.7 = 3.431 67
- The old "diameter minus pitch" drill = 4 - 0.7 = 3.3
- Engagement that rule actually gives = 100 / 1.082532 = 92.376
- Pitch diameter = 4 - 0.649519 * 0.7 = 3.545 34
- Minor diameter of the nut thread = 4 - 1.082532 * 0.7 = 3.242 23
- Minor diameter of the bolt thread = 4 - 1.226869 * 0.7 = 3.141 19
- Tensile stress area = pi/4 * (4 - 0.938194 * 0.7)^2 = 8.778 72
- Clearance hole = 4 * (1 + 8/100) = 4.32
- Breaking load at that tensile strength = 8.778 72 * 800 = 7022.98
- shear_area = pi * 3.242 23 * 6 * (0.5 + 0.57735 * (3.545 34 - 3.242 23)/0.7) = 45.8359
- Threads engaged over that length = 0.7 > 0 ? 6 / 0.7 : 0 = 8.571 43
- Equivalent threads per inch = 0.7 > 0 ? 25.4 / 0.7 : 0 = 36.2857
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.
M4 at common engagements
| Engagement | Tap drill (mm) | Tap drill (in) |
|---|---|---|
| 65% | 3.507 | 0.1381 |
| 70% | 3.470 | 0.1366 |
| 75% | 3.432 | 0.1351 |
| 80% | 3.394 | 0.1336 |
| 85% | 3.356 | 0.1321 |
| 90% | 3.318 | 0.1306 |
Pick the nearest drill above the figure, never below it.
Frequently asked
What drill size do I need for M4?
3.43 mm (0.1351 in) for 75% thread engagement. The traditional rule gives 3.30 mm, which is a tighter and harder-tapping hole.
What is the pitch of M4?
0.7 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.