Thread Calculator
Tap drill, pitch, clearance hole and thread engagement for metric threads. 40 sizes from M1.6 to M36, coarse and fine.
Thread depth follows the rule of thumb per material group, softer materials need more thread. Drill depth includes three pitches of allowance for the tap lead and chip room.
Metric ISO thread, complete table
Coarse threads per ISO 261 plus the common fine threads. Tap drills per DIN 336 rounded to available drill sizes, clearance holes per DIN EN 20273.
| Thread | P (mm) | Tap drill | d2 | d3 | Clearance c / m / f |
|---|---|---|---|---|---|
| M1.6 | 0.35 | Ø1.25 | 1.373 | 1.171 | 1.7 / 1.8 / 2.0 |
| M2 | 0.4 | Ø1.6 | 1.740 | 1.509 | 2.2 / 2.4 / 2.6 |
| M2.5 | 0.45 | Ø2.05 | 2.208 | 1.948 | 2.7 / 2.9 / 3.1 |
| M3 | 0.5 | Ø2.5 | 2.675 | 2.387 | 3.2 / 3.4 / 3.6 |
| M3.5 | 0.6 | Ø2.9 | 3.110 | 2.764 | 3.7 / 3.9 / 4.2 |
| M4 | 0.7 | Ø3.3 | 3.545 | 3.141 | 4.3 / 4.5 / 4.8 |
| M5 | 0.8 | Ø4.2 | 4.480 | 4.018 | 5.3 / 5.5 / 5.8 |
| M6 | 1 | Ø5.0 | 5.350 | 4.773 | 6.4 / 6.6 / 7.0 |
| M7 | 1 | Ø6.0 | 6.350 | 5.773 | 7.4 / 7.6 / 8.0 |
| M8 | 1.25 | Ø6.8 | 7.188 | 6.466 | 8.4 / 9.0 / 10.0 |
| M10 | 1.5 | Ø8.5 | 9.026 | 8.160 | 10.5 / 11.0 / 12.0 |
| M12 | 1.75 | Ø10.2 | 10.863 | 9.853 | 13.0 / 13.5 / 14.5 |
| M14 | 2 | Ø12.0 | 12.701 | 11.546 | 15.0 / 15.5 / 16.5 |
| M16 | 2 | Ø14.0 | 14.701 | 13.546 | 17.0 / 17.5 / 18.5 |
| M18 | 2.5 | Ø15.5 | 16.376 | 14.933 | 19.0 / 20.0 / 21.0 |
| M20 | 2.5 | Ø17.5 | 18.376 | 16.933 | 21.0 / 22.0 / 24.0 |
| M22 | 2.5 | Ø19.5 | 20.376 | 18.933 | 23.0 / 24.0 / 26.0 |
| M24 | 3 | Ø21.0 | 22.051 | 20.319 | 25.0 / 26.0 / 28.0 |
| M27 | 3 | Ø24.0 | 25.051 | 23.319 | 28.0 / 30.0 / 32.0 |
| M30 | 3.5 | Ø26.5 | 27.727 | 25.706 | 31.0 / 33.0 / 35.0 |
| M33 | 3.5 | Ø29.5 | 30.727 | 28.706 | 34.0 / 36.0 / 38.0 |
| M36 | 4 | Ø32.0 | 33.402 | 31.092 | 37.0 / 39.0 / 42.0 |
| M6x0.75 | 0.75 | Ø5.2 | 5.513 | 5.080 | 6.4 / 6.6 / 7.0 |
| M8x1 | 1 | Ø7.0 | 7.350 | 6.773 | 8.4 / 9.0 / 10.0 |
| M8x0.75 | 0.75 | Ø7.2 | 7.513 | 7.080 | 8.4 / 9.0 / 10.0 |
| M10x1.25 | 1.25 | Ø8.8 | 9.188 | 8.466 | 10.5 / 11.0 / 12.0 |
| M10x1 | 1 | Ø9.0 | 9.350 | 8.773 | 10.5 / 11.0 / 12.0 |
| M12x1.5 | 1.5 | Ø10.5 | 11.026 | 10.160 | 13.0 / 13.5 / 14.5 |
| M12x1.25 | 1.25 | Ø10.8 | 11.188 | 10.466 | 13.0 / 13.5 / 14.5 |
| M12x1 | 1 | Ø11.0 | 11.350 | 10.773 | 13.0 / 13.5 / 14.5 |
| M14x1.5 | 1.5 | Ø12.5 | 13.026 | 12.160 | 15.0 / 15.5 / 16.5 |
| M16x1.5 | 1.5 | Ø14.5 | 15.026 | 14.160 | 17.0 / 17.5 / 18.5 |
| M18x1.5 | 1.5 | Ø16.5 | 17.026 | 16.160 | 19.0 / 20.0 / 21.0 |
| M20x1.5 | 1.5 | Ø18.5 | 19.026 | 18.160 | 21.0 / 22.0 / 24.0 |
| M22x1.5 | 1.5 | Ø20.5 | 21.026 | 20.160 | 23.0 / 24.0 / 26.0 |
| M24x2 | 2 | Ø22.0 | 22.701 | 21.546 | 25.0 / 26.0 / 28.0 |
| M24x1.5 | 1.5 | Ø22.5 | 23.026 | 22.160 | 25.0 / 26.0 / 28.0 |
| M27x2 | 2 | Ø25.0 | 25.701 | 24.546 | 28.0 / 30.0 / 32.0 |
| M30x2 | 2 | Ø28.0 | 28.701 | 27.546 | 31.0 / 33.0 / 35.0 |
| M30x1.5 | 1.5 | Ø28.5 | 29.026 | 28.160 | 31.0 / 33.0 / 35.0 |
Why the D minus P rule gives 92 percent
Tap drill = D minus P is the rule every machinist carries in their head. For M8 that is 8 minus 1.25, so 6.75 mm, and the 6.8 sits in the drill rack. What the rule does not say: it does not hit the minor diameter of the nut, it sits above it.
The theoretical full profile calls for a hole at D1, which is 6.647 mm for M8. At 6.8 mm you are left with 88.7 percent engagement. Across all sizes the rule lands near 92 percent, because the pitch P and the 1.0825 times P of the profile are almost the same size.
In practice: the last few percent of engagement cost disproportionate cutting force but add almost no holding power. From roughly 65 percent the thread carries the full bolt load, because the bolt fails first. In stainless and titanium it therefore pays to drill one or two tenths larger.
From the shop floor: When a tap breaks repeatedly it is rarely the tap. Check the tap drill diameter first, and for blind holes the chip room. Three pitches of drill depth beyond the thread is the minimum, in tough materials make it four.
How threads are made
Internal and external threads, single point turned. Non standard pitches and multi start threads included.
Driven tools on the lathe so the part comes off finished in one setup. M2 to M20 as a rule.
For large threads, hard materials and blind holes with little run out. One tool covers every diameter of the same pitch.
Sliding head lathes produce slender turned parts with the thread in the same setup. For special threads or non standard pitches send the drawing and we place it with a vetted partner shop that can cut the thread.
Frequently asked questions
Which drill for M8?
6.8 mm. It follows from the rule tap drill equals nominal diameter minus pitch, so 8 minus 1.25 equals 6.75 mm, rounded up to the available 6.8 drill. That leaves 88.7 percent thread engagement.
How do you calculate the tap drill diameter?
Tap drill equals nominal diameter minus pitch. For M6 that is 6 minus 1.0 equals 5.0 mm, for M10 it is 10 minus 1.5 equals 8.5 mm. The same rule applies to fine threads with their smaller pitch, so M10x1 needs 9.0 mm.
What is the difference between coarse and fine thread?
A fine thread has a smaller pitch at the same nominal diameter. It transmits higher loads, resists loosening under vibration and needs less engagement length, but it is more sensitive to dirt and damage.
How deep does a blind hole thread need to be?
As a rule of thumb one times the diameter in steel, 1.5 times in aluminium and twice in plastic. Add at least three pitches of extra drill depth for the tap lead and chips.
What clearance hole belongs to M6?
Per DIN EN 20273 that is 6.4 mm close, 6.6 mm medium and 7.0 mm free. Medium is the normal case, close only for guided joints, free when positional tolerances have to be absorbed.
Are special threads possible?
Yes. In thread turning the pitch can be chosen freely, including multi start and non standard. Trapezoidal and round threads as well. Send the drawing and we get back to you within 24 hours with the right partner shop.
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Part with a thread
Send a drawing or STEP file. We review the enquiry and pass it on to a vetted partner shop. We will get back to you within 24 hours.
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