Tapping Drill Size Calculator
The tap drill (pilot hole) determines how much thread you cut. Drill too small and the tap binds or snaps; drill too large and the thread is weak. This calculator uses the standard 60-degree ISO and Unified thread profile to give the theoretical hole diameter for any major diameter, pitch, and target thread engagement. Enter metric values directly, or for imperial threads enter the pitch as 1 divided by the threads per inch. The widely-used default is 75 percent engagement, which keeps almost all the strength while cutting tapping torque sharply.
Tap drill formula
Single thread height (full) = 0.54127 * pitch
Tap drill = major diameter - (engagement / 100) * pitch * 1.0825
Engaged depth = (engagement / 100) * 0.54127 * pitch
Clearance from major = major diameter - tap drill
The factor 1.0825 is 2 times 0.54127, the basic height of an ISO/Unified 60-degree thread expressed as a fraction of pitch, applied to both sides of the hole diameter. For imperial threads, set pitch = 1 / threads per inch.
Choosing a drill
- 75 percent engagement is the general-purpose default for steel and aluminum.
- Lower engagement (50 to 65 percent) eases tapping in hard or deep holes.
- Round up to the nearest available drill size at or above the computed value.
- For metric M10x1.5 the formula gives about 8.38 mm, matching the standard 8.5 mm tap drill.
- Use cutting fluid and back the tap out periodically to clear chips.
Tapping drill size: frequently asked questions
How do I calculate tap drill size?
For a target thread engagement, tap drill diameter = major diameter - (thread percent / 100) x pitch x 1.0825 for metric (60-degree) threads. For UNC/UNF threads using threads per inch, convert pitch = 1 / TPI first. The constant 1.0825 comes from the ISO/Unified 60-degree thread geometry where full thread height equals 0.54127 times pitch and the hole spans twice that.
What thread engagement percentage should I use?
A 75 percent thread engagement is the common general-purpose target: it gives most of the strength of a full thread while greatly reducing tapping torque and tap breakage. Engagements of 50 to 65 percent are used in hard or thick materials to ease tapping; the calculator lets you set any value.
Why not just cut a 100 percent thread?
Going from 75 to 100 percent thread engagement adds only a few percent of holding strength but dramatically increases tapping torque and the chance of breaking the tap. Standard practice and machinery handbooks therefore recommend roughly 75 percent for general work.
Does this work for both metric and imperial threads?
Yes. For metric, enter the major diameter in millimeters and the pitch in millimeters (for example 1.5 mm for M10x1.5). For imperial, enter the major diameter in inches and the pitch as 1 divided by threads per inch (for example 1/20 = 0.05 for 1/4-20). The 60-degree geometry constant is the same.
Is the calculated drill an exact standard drill size?
It is the theoretical hole diameter. You then choose the nearest available drill (number, letter, fractional, or metric) at or just above that value. Picking a slightly larger drill lowers thread engagement a little and tapping torque a lot, which is usually acceptable for general work.
Official sources
- International Organization for Standardization: ISO, ISO metric screw thread geometry.
- American Society of Mechanical Engineers: ASME, Unified inch screw thread standards.
Reviewed by the CalculatorHub team, edited by James Graham, 17 June 2026. See our methodology.