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Tap Drill Size Calculator

Tap drill size from thread size and target engagement.

Your measurementsEvery assumption editable01

The nominal outside diameter — 0.25 for a 1/4 in thread.

75% is the usual target. Higher is not stronger in practice.

Tap drill sizeCalculated live02

Result

0.2013in drill

0.2013 in — 5.11 mm

Thread
0.25 in — 20 TPI
Full thread depth0.64952 ÷ TPI, per side
0.03248 in
Minor diameter at 100%
0.185 in
Engagement
75%
Tap drill size5.113 mm
0.2013 in

Computed from the unified thread form rather than read off a chart, so it works for any diameter and pitch including combinations no chart lists.

Seventy-five per cent engagement is the usual target, and going higher is a false economy. Past about that point the thread gains very little strength while tapping torque climbs steeply — which is how taps break in tough material.

Pick the nearest available drill above the calculated figure rather than below. A slightly larger hole loses a little engagement; a slightly smaller one dramatically increases the chance of snapping the tap.

Computed from the unified thread form rather than read off a chart, so it works for any diameter and pitch — including combinations no chart lists. Seventy-five per cent engagement is the usual target, and more is not better.

Why use this tool?What it does differently03

Why use this tool?

Any thread

Derived from the thread geometry, so unusual diameters and pitches work as well as common ones.

Explains why 75%

Past about that point strength barely rises while tapping torque climbs steeply — which is how taps snap.

Inches and millimetres

So you can pick from either drill set.

Rounding guidance

Go to the nearest drill above the figure, never below.

How this worksThe method04

How this tap drill size calculator works

For a unified thread, full thread depth per side is 0.64952 divided by threads per inch. The tap drill is the major diameter less twice that depth, scaled by the engagement percentage you want.

At a hundred per cent engagement the hole equals the thread's minor diameter, and the tap has to cut the full thread form. Backing off to seventy-five per cent removes a quarter of the material the tap must displace and dramatically reduces the torque needed.

That trade is heavily asymmetric. Going from seventy-five to a hundred per cent engagement adds only a few per cent of thread strength while raising tapping torque substantially — which is why the extra is rarely worth a broken tap.

How to use itStep by step05

How to use it

  1. Step 1: Enter the major diameter

    The nominal outside diameter — 0.25 for a quarter-inch thread.

  2. Step 2: Enter threads per inch

    20 for 1/4-20, 28 for 1/4-28, and so on.

  3. Step 3: Choose the engagement

    75% unless you have a specific reason. Softer materials sometimes take less.

  4. Step 4: Round up, not down

    Pick the nearest available drill above the figure. A slightly small hole breaks taps.

Example usageWorked figures06

Example usage

A common coarse thread
0.25 in at 20 TPI and 75% engagement gives 0.2013 in — 5.11 mm, which is why a #7 drill is the standard choice.
The fine version
0.25 in at 28 TPI gives 0.2152 in. Finer threads have shallower forms, so the hole is larger.
Reduced engagement in tough stock
The same 1/4-20 at 60% engagement gives 0.2110 in — noticeably easier tapping for very little loss of strength.
Frequently asked questionsCommon questions07

Frequently asked questions

What drill size for a 1/4-20 tap?

About 0.2013 in at the standard 75% engagement, which is why a #7 drill at 0.201 in is the conventional answer.

What thread engagement should I use?

Seventy-five per cent is the usual target. Higher adds very little strength while raising tapping torque steeply, and it is the commonest reason taps break.

How is tap drill size calculated?

Major diameter minus twice the full thread depth times the engagement fraction, where full depth per side is 0.64952 divided by threads per inch.

Should I round up or down?

Up, always. A slightly larger hole loses a little engagement; a slightly smaller one sharply increases the chance of snapping the tap in the work.

Does this work for metric threads?

The same geometry applies — convert pitch in millimetres to threads per inch by dividing 25.4 by the pitch, then enter the major diameter in inches.

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