Tap Drill Size Calculator

A tapped hole is a compromise. Drill it small and the tap has to cut a deep thread, which takes torque, generates heat, and in a blind hole or a tough alloy is the usual way taps break. Drill it large and the thread is shallow and shears more easily. "Percent of thread" is the name for where between those you have landed.

The formula every chart uses subtracts a fixed amount per pitch from the major diameter: 0.01299 inches per percent per thread per inch, or the same constant times the pitch in metric. It comes from tap manufacturers and it has been in print for a century.

What it is measured against is the part nobody mentions. The constant works out to 1.299 times the pitch for a nominal 100%, which is 2 x cos(30) x 3/4. The actual full form of an internal thread under ASME B1.1 and ISO 68-1 is 5H/8 deep per side, or 1.08253 times the pitch on diameter. Those are different reference points, and they differ by exactly 1.2.

Multiply any charted percentage by 1.2 and you have the percentage of the real thread form. The trade's 75% is 90% of what the standard calls a full thread. Its 60% is 72%. Its nominal 100% is 120%, which is why that end of the scale is fictional - a hole that size would be smaller than the thread's own minor diameter and no tap could cut it.

This resolves a common confusion. People who look up the minor diameter of a 1/4-20 internal thread find 0.1959 basic, compare it to the 0.201 tap drill, and calculate 90% engagement - then wonder why the chart says 75%. Both numbers are right; they are answers to different questions, and almost nothing in print says so.

It also explains a practical experience. Tapping a "75%" hole in stainless or a deep blind hole in steel is markedly harder than a figure like 75% suggests, and people conclude their taps are poor or their technique is wrong. The hole is simply deeper than the label. Dropping to a charted 60% - which is a genuine 72% of form - is not the compromise it sounds like.

Work every one of the thirty nine common inch sizes back through the same formula that produced them and the results do not cluster on 75% at all. They spread from 65.0% for 9/16-18 and 5/8-18 up to 80.7% for 0-80, with 1/4-20 at 75.4% and 1/2-13 at 78.2%.

That is not sloppiness. The traditional chart was assembled around drills shops actually stock, and stock sizes are not evenly spaced - the number series is fine in the small sizes and the fractional series is coarse in the large ones, so the achievable percentage jumps around. Where several drills were close, the list generally favoured the larger one and easier tapping.

The practical point is that "use the chart" and "tap at 75%" are two different instructions, and following the first does not accomplish the second. If a drawing or a procedure specifies a percentage, it has to be calculated rather than looked up.

The last correction is one the tap makers publish themselves, in a column headed "probable hole size" that sits next to the nominal drill in their charts and is quietly ignored everywhere else.

A twist drill is a two-point tool on a long, torsionally soft shank, guided by nothing but its own point until it is a diameter deep. It wanders, and it cuts a hole larger than itself - about 0.0015 inch on a 3/64, 0.0038 on a quarter inch, 0.006 on a 15/16. Those are ordinary figures for a sharp drill in a decent machine, not a symptom of anything wrong.

The effect on engagement is direct and always in the same direction. That #7 for a 1/4-20 nominally gives 75.4%, and the hole it really cuts gives 69.6%. Every stage of this - the constant, the chart, the drill - moves the answer the same way, which is why a tapped hole in practice is a good deal easier than the number on the chart implies.

Where the percentage genuinely matters - a thin section, a soft material, a joint being qualified - the answer is not a different drill. It is to bore or ream the hole to size, which removes the variability entirely.

For nearly all work this whole argument is secondary, because thread strength is dominated by engagement length rather than engagement percentage.

A steel bolt in a steel hole one diameter deep fails by breaking the bolt, not by stripping the thread. That is the design intent of the standard nut height, and once the thread is stronger than the fastener, more thread does nothing whatsoever. Going from 60% to 75% adds shear area to a thread that was already not the weak point.

What does change is depth. In aluminium, a rule of about one and a half diameters restores the same margin; in magnesium or plastic, about two. Below that, no percentage of thread saves the joint - the threads strip because there are not enough of them. Above it, none is needed.

So the sensible order of decisions is: get the depth right first, then choose a percentage low enough that the tap has an easy time. Industrial practice for steel sits at 60% to 65% for exactly this reason, and the 75% that dominates hobby charts is a habit rather than a specification.