Tap Drill Size & Thread Pitch Lookup

Find thread dimensions and the right tap drill size for metric and unified inch threads.

Many shops drill for roughly 65 to 75 percent.
Theoretical tap drill
Standard drill just under
Standard drill just over

Nominal thread dimensions

Dimensionmminch
Major diameter, d
Pitch, P
Threads per inch
Pitch diameter, d2
Minor diameter, internal thread, D1
Minor diameter, external thread, d3
Thread height, internal
Thread height, external
Crest flat width
Root flat width

How it works

The calculator uses the 60 degree thread profile shared by ISO metric and Unified inch threads. Every dimension follows from the major diameter and the pitch.

Pitch diameter: d2 = d − 0.649519 × P
Internal minor diameter: D1 = d − 1.082532 × P
External minor diameter: d3 = d − 1.226869 × P
Crest and root flats: P / 8 and P / 4
Tap drill: drill = d − 0.01299 × (percent thread) × P

Here d is the major diameter and P is the pitch. For inch threads, P = 1 / TPI. The percent thread is how much of the full thread depth the tap will cut, so a smaller hole gives a higher percentage.

Worked example

For an M10 × 1.5 coarse thread, the pitch diameter is 10 − 0.649519 × 1.5 = 9.026 mm, and the internal minor diameter is 10 − 1.082532 × 1.5 = 8.376 mm. For a 75% thread, the tap drill is 10 − 0.01299 × 75 × 1.5 = 8.54 mm. The nearest standard drill under that is 8.5 mm, which leaves about 77% thread. The next size up, 8.6 mm, leaves about 72%.

Things to know

Choosing a percentage. Many shops drill for roughly 65 to 75 percent. Going higher adds little strength but raises tapping torque, which is a common cause of broken taps. Tougher materials generally call for the lower end.

Nominal values only. The dimensions shown are basic profile values before allowance and tolerance. Real threads are also limited by their tolerance class, such as 6g and 6H for metric or 2A and 2B for inch, so use the standard’s tables when you need exact limits.

Cutting taps only. These drill sizes are for cutting taps. Forming taps displace material instead of cutting it and need a different, larger drill.

Drill sets vary. The standard drill lists here are general ones: metric steps for metric threads, and number, letter, and fractional drills for inch threads. Your set may not include every size.

Common questions

Why does a tap drill chart sometimes list a different drill?

Charts usually round to a common drill size and often assume a fixed percentage. Check the percent thread shown above for the drill you plan to use, since the chart may have been built around a different value.

Can I enter a thread that isn’t in the list?

Yes. Choose Custom size at the bottom of the size list, then enter the major diameter and the pitch. For inch threads, enter the threads per inch instead of the pitch.

Does this cover pipe threads?

No. This calculator covers straight metric and Unified threads. Pipe threads such as NPT and BSP are tapered or follow different rules and need their own tables.

Sources

  • Oberg, E., et al., Machinery’s Handbook, Industrial Press (tap drill sizes and percentage of thread; the drill is D − (percent × pitch ÷ 76.98)).
  • ASME B1.1, Unified Inch Screw Threads (UN and UNR thread form); ASME B1.13M and ISO 261 / ISO 724 (metric screw threads).
  • ISO 965-1, ISO general purpose metric screw threads: tolerances.

Last reviewed: October 8, 2026