Metric Tap Drill Size Calculator
Find the tap drill size for a metric thread, the percentage of thread you get with the drill you have, the clearance hole for a bolt, and the thread diameters.
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How this calculator works
The tap drill is the hole drilled before tapping. The rule for a metric thread is the major diameter minus the pitch, so an M5 thread with a 0.8 mm pitch needs a 4.2 mm drill. That leaves about 77 percent of a full thread, which is the usual choice for steel.
A smaller drill leaves more thread but needs more torque to tap and risks breaking the tap. A larger drill makes tapping easier but leaves less thread to hold. Enter the drill you have and the calculator shows the percentage of thread it leaves.
The calculator also gives the pitch diameter, the minor diameters of the tapped hole and the bolt, the tensile stress area, and the ISO 273 clearance holes for a bolt that passes through the other part.
Formulas used
- d_{tap} = D - P
- \text{engagement} = \dfrac{D - d}{1.299\,P}\times100
- A_t = \dfrac{\pi}{4}(D - 0.9382\,P)^2
Worked example
With the default numbers in the calculator above:
- Thread size and pitch: M5.00 × 0.80 mm
- Tap drill size (major diameter minus pitch): 4.20 mm
- Nearest 0.1 mm drill: 4.20 mm
- Thread engagement with that drill: 76.98 %
- Pitch diameter of the thread: 4.48 mm
- Minor diameter of the tapped hole: 4.13 mm
- Minor diameter of the bolt: 4.02 mm
- Tensile stress area of the bolt: 14.18 mm²
- Clearance hole for a bolt through the other part: 5.30 mm close, 5.50 mm medium, 5.80 mm coarse
Change the numbers in the calculator to see the full working, every step, for your own case.
A second worked example
A different case, with these numbers: Thread size M8.
- Thread size and pitch: M8.00 × 1.25 mm
- Tap drill size (major diameter minus pitch): 6.75 mm
- Nearest 0.1 mm drill: 6.80 mm
- Thread engagement with that drill: 76.98 %
- Pitch diameter of the thread: 7.19 mm
- Minor diameter of the tapped hole: 6.65 mm
- Minor diameter of the bolt: 6.47 mm
- Tensile stress area of the bolt: 36.61 mm²
- Clearance hole for a bolt through the other part: 8.40 mm close, 9.00 mm medium, 10.00 mm coarse
Common questions
What drill size for an M5 thread?
A 4.2 mm drill for the standard 0.8 mm coarse pitch.
What drill for aluminium?
The same rule works. Many people use a drill 0.05 to 0.1 mm larger to ease tapping. Use a cutting fluid made for aluminium.
Is more thread always better?
No. Above about 75 percent the extra strength is small and the tap works much harder, so it is more likely to break.
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Got a question about this? Ask an engineering question and get a plain-language answer with references.
Further reading: ISO 261 and ISO 724 (metric thread sizes and the basic profile) and ISO 273 (clearance holes for bolts). Tap makers' data sheets.
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