What percent thread engagement means
When a tap cuts a hole, it never cuts a full, theoretical thread. Percent thread engagement (often shortened to "percent thread") measures how much of that full thread depth actually gets cut, based on how large the pilot hole is relative to the tap. A smaller pilot hole means more material for the tap to cut, so more thread depth and a higher percentage. A larger pilot hole leaves less material, so a shallower thread and a lower percentage.
More is not automatically better. Past roughly 60% thread engagement, additional holding strength from a tighter fit is marginal, while the tap has to remove more material to get there. That means higher cutting torque and a real risk of snapping the tap, especially by hand, in harder materials, or in a blind hole where chips cannot clear easily. 75% thread is the traditional target for general work because it holds essentially the same load as a full thread while staying well inside a safe tapping margin. Lower it (50 to 65%) in tough materials like stainless steel or titanium, or when hand tapping a deep blind hole; there is little reason to go higher than 75% for ordinary fasteners.
How to read this table
Find your thread size in the Thread column (for example 1/4-20 or 10-24). Every valid tap drill for that thread is listed underneath it, each with its own percent thread figure. Pick the drill whose percentage matches what the job calls for: a higher number for maximum strength in a soft material or a through-hole, a lower number when you need to protect the tap in a harder material or a blind hole. The Decimal (in) column is the drill's exact diameter in inches, useful for confirming a drill by micrometer or for cross-referencing against the drill bit size chart, which shares this same verified dataset.
Why one thread has several valid drills
Percent thread is a range, not a single correct number, because different jobs sit at different points on that range. A soft material, a through-hole, or a low-stress fastener can tolerate a smaller drill (a higher percentage) without extra tapping risk. A hard material, a deep blind hole, or a hand-tapping job usually calls for a larger drill (a lower percentage) to keep torque manageable. Rather than force one compromise drill on every job, this chart lists every drill Machinery's Handbook validates for each thread, so the choice stays with whoever is doing the work.
Machine screw sizes vs fractional sizes
This chart covers two families of Unified thread. Machine screw sizes (numbered 0 through 12, like 4-40 or 10-24) are the small fasteners common in electronics, small machinery, and general assembly work. Fractional sizes (named by diameter in inches, like 1/4-20 or 1/2-13, up through 1 inch) cover everything from small bolts up to structural fasteners. Both families follow the same tap-drill logic and the same percent-thread math; they are separated here into filter chips because most jobs only ever need one or the other. Neither family covers pipe threads: those follow a different, tapered standard, covered separately in the NPT thread chart.
The corrections, and why they matter
Five rows in this chart differ from what Machinery's Handbook, 31st edition actually prints, documented in full in the corrections section above. Two are drill-decimal typos: a printed value that does not match any real drill size or, in one case, back-converts to a non-standard metric drill. Three are percentage errors: a printed percent-thread figure that disagrees with the standard's own formula, while every other drill for that same thread matches the formula closely.
These are not guesses or independent recalculations layered on top of the source. Every value in this table, corrected or not, was checked against the Handbook's own published thread-engagement formula. A published reference is not automatically self-consistent just because it is published, and cross-checking a table against its own formula is exactly the kind of check a straight transcription never performs. That gap is why these five errors survived in print for decades of reprints: nobody re-ran the math. Getting a tap drill wrong is not academic, either. A hole drilled to the Handbook's printed 0.0801" for a 3-56 tap is drilled to a drill size that does not exist; the actual, correct drill for that hole is #46 at 0.0810", a full standard size larger. Get that wrong by hand or by an uncorrected reference and the tap either binds or breaks.