Drill Size Chart

Four series of twist drill are in general use and they were never meant to interlock. The fractional sizes step by 1/64 in, so they are coarse — fifteen and a half thousandths apart. The number series is a wire gauge inherited from drawn wire, running backwards from #1 at 0.2280 in to #80 at 0.0135 in in irregular steps that tighten wherever tap drills cluster. The letter series exists only to fill the gap that leaves between the largest number drill and the fractional sizes above 0.4130 in, and it runs forwards, A to Z. The metric series is a stocking series in 0.05, 0.1 and 0.25 mm bands. Merge them and sort by diameter and 353 rows come out, 29 of them inside the fifty thousandths between 0.200 in and 0.250 in — which is why a bench chart that keeps the systems in four separate columns answers the wrong question.

The tap columns are an argument rather than a list. Percentage of full thread is defined against 1.299 pitches of diametral engagement, so the hole is the major diameter minus 1.299 × pitch × percentage/100, and that relation is what makes every published tap chart reproducible instead of memorized. It also exposes how loosely those charts hold their own headline: at the 75% setting the Unified rows here land anywhere from 65.0% to 80.8%, because the drill has to be a size that exists. On the metric side the shop rule “drill equals diameter minus pitch” comes out at 77.0% at every single pitch, since the relation is linear in pitch and one whole pitch is always the same fraction of 1.299 — which is why a metric shop does not own a chart. If you are working the other way, from a bolt in your hand to a designation, that is the thread pitch calculator; and the hole tolerance you can hold once you have picked the drill belongs to the tolerance calculator.

Three things move a tap drill and most charts print none of them. A form tap displaces metal rather than cutting it and needs a substantially larger hole from the tap maker’s own chart, not this one. A deep or blind hole wants a lower percentage, because chip packing rather than thread strength is what snaps taps. And a real twist drill in a hand-fed machine cuts two to five thousandths over its nominal size, so the finished thread comes out shallower than the column says — an effect worth more than the difference between two adjacent drills in the 0.2 in band. The speed and feed that produce a hole close to nominal are on the feed and speed calculator, and it is also why a hole that has to fit something is drilled undersize and then reamed or bored: a twist drill leaves a roughing feature, at the coarse end of the surface finish chart. Nothing on this page is a stamped design: it is a reference table and the arithmetic behind it, and what the tapped joint has to carry is still yours and your engineer’s to decide.