Engineering Reference

Tap Drill Size Calculator

Compute the tap drill for a thread from its major diameter and pitch, and see how close the standard drills are.

Data verified 2026-09-29 · based on n/a — standard engineering relationships, no single governing revision

Quick Answer

Tap drill is chosen for about 75% thread engagement: pilot = major diameter − pitch, then rounded to the nearest standard drill. For a 1/4-20 that gives 0.2000 in, which lands on a #7 drill at 0.2010 in.

Tap Drill Calculator

The Formulas Used

pilot = major diameter − pitch  where pitch = 1 / TPI
Then select the nearest standard drill from the number, letter or fractional series.

Choosing the Actual Drill

The computed pilot diameter almost never matches a standard drill exactly, so you round to the nearest one. The achieved engagement then differs slightly from 75% — the table on the tap drill chart shows the drill actually chosen for every thread size, alongside the exact pilot diameter.

Where two drills are equidistant, take the larger. That keeps engagement just under 75% rather than over it, which matters because tapping torque rises sharply above 75% and a broken tap in a finished part is expensive.

When to Deviate

In hard materials, or in deep holes where chip clearance is poor, go one drill size larger than the calculation suggests. You lose some engagement but greatly reduce the risk of breaking the tap.

Frequently Asked Questions

What is the tap drill size for 1/4-20?
A #7 drill, 0.2010 in. The pilot is 0.2500 − 0.0500 = 0.2000 in, and #7 is the nearest standard drill.
What is the formula for tap drill size?
Pilot diameter = major diameter − pitch, where pitch is 1 divided by the threads per inch. Then round to the nearest standard drill.
Why 75% thread engagement?
It is the standard compromise: enough thread to carry the load, but not so much that tapping torque becomes dangerous. Above 75% the tap removes more material per revolution and breaks far more readily.
Should I go bigger or smaller than the calculated drill?
Bigger, when in doubt. A slightly shallow thread is far less costly than a broken tap, which usually scraps the part.
How do I find the tap drill for a metric thread?
Use pilot = nominal diameter − pitch in millimetres, then round to the nearest metric drill. For M10x1.5 that is 10 − 1.5 = 8.5 mm.

Related

Value Sources

Each data column on this page is tied to the source it came from. The numbers in square brackets correspond to the table headers above.

#SourceTypeRevision / method
[1]ASME B1.1 — Unified Inch Screw ThreadsstandardASME B1.1-2019 — source
[2]ASTM A615 — Deformed steel bars for concrete reinforcementstandardASTM A615/A615M-20 — source
[3]ASTM E140 — Hardness Conversion TablesstandardASTM E140-12b — source
[4]Values computed in your browserderivedEvaluated locally from the formulas shown on the page. No data leaves the device.
[5]ISO 4287 — Surface texture: Profile methodstandardISO 4287:1997 — source
[6]ISO 68-1 — Basic profilestandardISO 68-1:2023 — source
[7]NFPA 70 NEC Table 310.16standardNEC 2023 (NFPA 70-2023) — source

Data Sources

StandardRevisionWhat it covers on this page
Formulas as shown on this pagen/a — standard engineering relationships, no single governing revisionevery value this calculator produces

Cross-checked against:

Derived values — the following values on this page are calculated, not taken directly from the standard:

ValueHow it is derived
All outputsComputed in the browser from the formulas above. No data leaves the device.

Outputs are computed from the formulas shown. Verify against the governing standard for design or acceptance work.

Accuracy and use. The values on this page are compiled from the published standards and cross-checked sources listed above. Where values are derived, the derivation is stated. No warranty, express or implied, is made as to the accuracy or completeness of this information, and no liability is accepted for any loss or damage arising from its use. Engineering reference data is provided for guidance in preliminary work — before a value is used for design, fabrication or acceptance testing, verify it against the current revision of the governing standard and against your own inspection. The user assumes all risk and responsibility in connection with the use of this information.

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