Number Drill Size Table

The series descends from wire gauge conventions of the nineteenth century, and the logic lives in the wire-drawing process: a rod gets pulled through progressively smaller dies, and the gauge number counted the draws - more passes, finer wire. Drill makers adopted the existing wire-size vocabulary wholesale, so the numbering arrived with its direction baked in: high count, small size. The same backwards habit explains the small end of the series reaching number 97 (about a third of a millimeter) in the full chart even though bits past 60 see little shop use. Is it intuitive? No. Is it changeable? Also no - every tap chart, every hardware drawer and every drawing note in American practice uses these numbers, so the practical response is one memorized sentence: number drills count down, so the smaller the number, the bigger the drill. The metric world sidestepped the whole mess by naming drills by their actual diameter in millimeters, which is why the millimeter column here is the most useful one: it is the only value that never needs a legend to decode.

A short list covers most American threading work. Number 7 (0.201 inch) is the 1/4-20 UNC tap drill - the single most-quoted number-drill fact in machining, drilled millions of times a day somewhere in the world. Number 21 (0.159 inch) pairs with 10-32 UNF, the classic electronics-enclosure thread. Number 25 (0.1495 inch) covers 10-24, the general-purpose small thread, while its close cousin number 21 (0.159 inch) handles the fine-thread 10-32. Number 43 (0.089 inch) and number 50 (0.070 inch) cover the small machine-screw threads (4-40 and 2-56 respectively) that circuit boards and instrument work live on. Number 29 (0.136 inch) is the 8-32 answer, and 8-32 is everywhere in computer hardware. Notice the pattern in why these are numbers rather than fractions: tap drills must land within a couple of thousandths of the thread minor diameter, and the number series provides steps of roughly 0.003 to 0.010 inch - the only American system fine enough to hit those targets without grinding a bit. When a tap breaks, the post-mortem almost always finds the hole drilled one step too small or drilled with a worn bit; the chart exists to keep you out of that club.

Let the target decimal decide, then use the system that holds it best. Fractional bits are the shop language - when a drawing says drill 3/16 inch, the 3/16 bit is the answer regardless of what a neighboring number size measures. Number sizes win below about 1/4 inch when fine granularity matters: tapping, pilot holes for threaded inserts, drift-free pin holes, anything where 0.005 inch changes the outcome. Letter sizes take over from number 1 (0.228 inch) up to Z (0.413 inch) - the mid-range where tapping larger threads and drilling clearance for small bolts happens. The workflow: write down the exact diameter the job needs (from a tap chart, a fit table, or a measurement), convert if needed with the millimeter column, then look for the closest size in each system and pick the tightest match - not the closest nickname. One shop-floor caution: number and letter bits are usually Jobber-length and sold in gauge sets whose markings wear off; measure a mystery bit with calipers and identify it from the decimal column rather than guessing from the shank color code, because a one-step error is invisible until the thread strips or the pin will not seat.