What this calculator does that the chart cannot
The tap drill chart lists every drill Machinery's Handbook validates as a standard tap drill, 193 rows across 54 threads. This calculator computes percent thread for ANY drill you enter, standard or not: a worn drill that measures slightly undersize, a metric drill used in an inch thread, a fraction that isn't one of the table's listed sizes. If you already have a drill in your hand and want to know what it actually does in a given thread, this answers that directly instead of asking you to find the closest table entry.
The formula, in plain terms
Percent thread engagement equals the major diameter of the thread, minus your drill's diameter, divided by 1.299 times the thread's pitch, all times 100. Major diameter is just the nominal thread size (0.25" for a 1/4-20, for example). Pitch is 1 divided by threads per inch (0.05" for a 20 TPI thread). The 1.299 constant comes from the geometry of a standard 60 degree thread form; it is not something to intuit, just a fixed constant in the standard formula. Every part of that equation is shown with your actual numbers plugged in above, not hidden behind a single output number.
What actually drives percent thread
Two things, and only two: how much smaller your drill is than the thread's major diameter, and how coarse or fine the thread's pitch is. A smaller drill relative to the major diameter cuts deeper thread and a higher percentage. A finer pitch (more threads per inch, smaller pitch value) makes the same drill-to-major-diameter gap represent a larger share of the available thread depth, which also raises the percentage. Neither the drill's designation system (number, letter, fraction, or metric) nor the material being tapped changes the math; they only change which physical drill you reach for.
Reading the result: the practical bands
Under roughly 60% thread engagement is on the weak side: acceptable for low-load applications, but not where a joint needs to carry real force. Roughly 65 to 80% is the typical, solid range most general fastening work targets, balancing full practical holding strength against reasonable tapping effort. Above about 85%, the tap is cutting close to a full thread form, which takes disproportionately more torque for very little extra strength and meaningfully raises the odds of breaking the tap, especially by hand or in a blind hole. These are practical guidelines, not hard cutoffs; the exact right number depends on the material, the load, and whether the hole is blind or through.
Higher percent thread is not proportionally stronger
It's a common assumption that more thread engagement always means a stronger joint, and it's only true up to a point. Once a tapped hole is engaged enough that the bolt itself would fail (stretch or shear) before the threads strip, which for most standard fastener combinations happens somewhere around 60 to 65% thread engagement, adding more thread depth stops adding meaningful strength. What it keeps adding is tapping torque and tap-breakage risk. Treat this calculator's output as a tool for finding the drill that hits a sensible target, not as an argument for always drilling the smallest hole a tap will physically fit through.