Tapping Drill & Clearance Hole
Determine the tapping drill (core hole) size for metric ISO coarse threads (DIN 13) and the matching clearance hole per DIN EN ISO 273. Pick the thread size – the calculator returns the pitch, the tapping drill diameter for about 100 percent thread engagement (d_core = D − P), the clearance holes in the fine, medium and coarse series and a guideline value for the thread engagement length. As a printable workshop chart from M1.6 to M64.
Tapping drill & clearance hole calculator
Values for metric ISO coarse threads per DIN 13 / ISO 262; clearance holes per DIN EN ISO 273. The engagement lengths are guideline values and do not replace a strength verification of the bolted joint.
Results
Calculating …
Workshop chart (coarse thread)
| Thread | P [mm] | Tapping drill [mm] | fine [mm] | medium [mm] | coarse [mm] |
|---|---|---|---|---|---|
| M1.6 | 0.35 | 1.25 | 1.7 | 1.8 | 2 |
| M2 | 0.4 | 1.6 | 2.2 | 2.4 | 2.6 |
| M2.5 | 0.45 | 2.05 | 2.7 | 2.9 | 3.1 |
| M3 | 0.5 | 2.5 | 3.2 | 3.4 | 3.6 |
| M4 | 0.7 | 3.3 | 4.3 | 4.5 | 4.8 |
| M5 | 0.8 | 4.2 | 5.3 | 5.5 | 5.8 |
| M6 | 1 | 5 | 6.4 | 6.6 | 7 |
| M8 | 1.25 | 6.8 | 8.4 | 9 | 10 |
| M10 | 1.5 | 8.5 | 10.5 | 11 | 12 |
| M12 | 1.75 | 10.2 | 13 | 13.5 | 14.5 |
| M14 | 2 | 12 | 15 | 15.5 | 16.5 |
| M16 | 2 | 14 | 17 | 17.5 | 18.5 |
| M18 | 2.5 | 15.5 | 19 | 20 | 21 |
| M20 | 2.5 | 17.5 | 21 | 22 | 24 |
| M22 | 2.5 | 19.5 | 23 | 24 | 26 |
| M24 | 3 | 21 | 25 | 26 | 28 |
| M27 | 3 | 24 | 28 | 30 | 32 |
| M30 | 3.5 | 26.5 | 31 | 33 | 35 |
| M33 | 3.5 | 29.5 | 34 | 36 | 38 |
| M36 | 4 | 32 | 37 | 39 | 42 |
| M39 | 4 | 35 | 40 | 42 | 45 |
| M42 | 4.5 | 37.5 | 43 | 45 | 48 |
| M45 | 4.5 | 40.5 | 46 | 48 | 52 |
| M48 | 5 | 43 | 50 | 52 | 56 |
| M52 | 5 | 47 | 54 | 56 | 62 |
| M56 | 5.5 | 50.5 | 58 | 62 | 66 |
| M60 | 5.5 | 54.5 | 62 | 66 | 70 |
| M64 | 6 | 58 | 66 | 70 | 74 |
Formulas and fundamentals
The tapping drill (core hole) diameter for a metric ISO coarse thread (DIN 13 / ISO 262) follows to good approximation from the nominal diameter and the pitch: d_core = D − P. For M8 with pitch P = 1.25 mm this is 6.75 mm by calculation; in practice the nearest standard drill of 6.8 mm is used. This approximation corresponds to roughly 100 percent theoretical thread engagement. A slightly larger core hole (lower engagement, e.g. 70 to 80 percent) markedly reduces the cutting force and the risk of breaking the tap, while the strength of the joint drops only a little – for tough materials and hand tapping this is often the better choice.
The clearance hole for the bolt shank is independent of the thread pitch and depends only on the nominal diameter. DIN EN ISO 273 (formerly DIN EN 20273) defines three series: fine for fitted bolts and tight positional tolerance, medium as the standard in general machine design and coarse for loose assembly tolerances or welded structures. For M8 these are 8.4 mm (fine), 9.0 mm (medium) and 10.0 mm (coarse). The values apply likewise to a fine thread of the same nominal diameter, since only the shank diameter matters.
For load-bearing joints the thread engagement length is also decisive so that the female thread does not fail before the bolt. As a guideline a minimum engagement of about one to one-and-a-half times the nominal diameter applies in steel, and rather one-and-a-half to two times in cast iron and aluminium. These rules of thumb do not replace a strength verification; the governing factors are the material pairing, the bolt property class and the preload.
Worked example
For an M10 coarse thread in a steel plate the pitch is P = 1.5 mm. The tapping drill (core hole) diameter follows from d_core = D − P = 10 − 1.5 = 8.5 mm – here the formula matches the standard drill exactly. So the hole is pre-drilled at 8.5 mm before tapping.
If an M10 bolt is to pass freely through the same part, the clearance hole follows DIN EN ISO 273: 10.5 mm (fine), 11.0 mm (medium) or 12.0 mm (coarse). In general machine design the medium series with 11.0 mm is chosen.
For a blind-hole thread in steel a usable engagement length of about 10 to 15 mm (approx. 1 to 1.5·D) is sufficient as a guideline. The drilled hole itself must be deeper than the usable thread length to allow for the thread runout and chip space.
Frequently asked questions
How do you calculate the tapping drill size of a thread?
For metric coarse threads the approximation d_core = D − P applies: nominal diameter minus pitch. For M8 (P = 1.25) this is 6.75 mm; in practice the standard drill of 6.8 mm is used. This formula corresponds to about 100 percent engagement; the tabulated values are rounded to common drill sizes.
What is the difference between the fine, medium and coarse clearance hole series?
DIN EN ISO 273 defines three series: fine for tight positional tolerances and fitted bolts, medium as the standard in machine design and coarse for loose assembly or welded assemblies. Larger clearance holes ease assembly but reduce the bearing area under the bolt head and the permissible positional deviation.
Does a fine thread need a different core hole?
Yes. Because a fine thread has a smaller pitch, its core hole is larger than that of a coarse thread of the same nominal diameter. Here too d_core = D − P applies with the respective fine-thread pitch. Example M12x1.5: d_core = 12 − 1.5 = 10.5 mm versus 10.2 mm for the M12 coarse thread (P = 1.75). The clearance hole stays the same.
Why is a slightly larger core hole often drilled?
A core hole for a full 100 percent engagement greatly increases the cutting force and the risk of breaking the tap, but adds little strength. In practice a slightly larger hole for 70 to 80 percent engagement is often drilled, especially for tough materials and hand tapping. The strength of the joint drops only marginally.
How deep must a blind-hole thread be engaged?
As a guideline a minimum engagement of about 1 to 1.5·D applies in steel and 1.5 to 2·D in cast iron or aluminium, so that the female thread does not fail before the bolt. The drilled hole is made deeper still to allow for the thread runout and chip space. For a reliable verification the material, property class and preload must be considered.