Tap Drill Size Calculator
Tap Drill Finder
Select a thread size and engagement percentage. Returns the calculated tap drill diameter and the two nearest standard drill sizes with their actual thread engagement.
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Select a thread size
Tap Drill Quick Reference
Tap drills at 75% thread
Clearance holes per ASME B18.2.8
The Math Behind the Drill
This gets you to ~77% thread engagement — close enough for the shop floor without a calculator. It works for both inch and metric threads.
Both UN and ISO threads use a 60° included angle. The tap drill sets how deep the internal thread goes — a bigger hole means shallower threads, less engagement, and less tapping torque. The shaded zone is what's actually carrying load: the flank contact area between the pitch diameter and the drill diameter.
P = 1 / TPI
Pitch in inches from threads per inch. Metric threads specify pitch directly.d₁ = D − 1.0825 × P
Minor diameter — the hole you'd need for 100% thread (theoretical max, never used).D_drill = D − 0.9743 × P
At 75% engagement. The 0.9743 factor is why "D − P" works as a mental shortcut.%T = (d₂ − D_drill) / (d₂ − d₁) × 100
Back-calculate actual engagement for any drill size. This is how you verify a substitute drill.Going from 75% to 100% engagement gains 5% more strength but roughly doubles the tapping torque. Above 83%, you're mostly just breaking taps. Below 60%, the bolt fails before the thread strips in steel — but in soft materials like aluminum, 50% is standard practice.
Soft metals — aluminum, brass
The bolt is always stronger than the thread material. At 50% engagement in aluminum,
the bolt's tensile cross-section is still the weakest link — it will snap before
the thread strips. Going to 75% doubles the tapping torque for zero practical
strength gain. You're just making the tap work harder for nothing.Work-hardening — stainless, titanium
These materials get harder as you deform them. Deeper threads mean more material removal,
more heat, and a harder work-hardened layer that resists the tap. Reducing engagement to
60–70% dramatically lowers tapping torque and heat buildup. The strength loss is negligible —
the bolt still fails first in tension.Tough alloys — 4140, 4340, Inconel
High strength means high cutting forces. Every additional percent of engagement
increases tapping torque exponentially, not linearly. At 65%, you retain >90%
of thread strength while cutting tap breakage dramatically. Above 35 HRC,
even 65% may break HSS taps — switch to carbide or thread milling.Plastics — nylon, Delrin, PPS
Plastic threads fail by cracking the boss, not by stripping. Over-engagement
creates deeper grooves that act as stress concentrators, making cracks more likely.
50% engagement keeps the thread shallow enough to carry load without initiating
fracture. Roll-form taps displace material instead of cutting, which is even better.The common assumption is "more thread = stronger." In practice, the failure mode determines the right engagement. If the bolt breaks before the thread strips — which it does in most soft materials at 50% — adding more thread is wasted effort and added risk.
Thread data per ASME B1.1 (UN inch) and ISO 261 (metric). Standard drill sizes per ANSI/ASME B94.11M. Strength estimates based on published thread-stripping research — actual values depend on material, class of fit, and length of engagement.
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