Letter Drill Size Table

Blame the manufacturing gap. Number drills run from 0.228 inch (number 1) downward into tiny wire-gauge territory, and fractional bits jump in coarse 1/64-inch steps - between number 1 at 0.228 and 1/4 inch at 0.250, the fractional system offers exactly nothing, and above 1/4 inch its steps are far coarser than precision work wants. The letter series fills that band with roughly 0.004-to-0.010-inch steps, which happens to be exactly where mid-size fastener work lives: tapping 1/4-20 threads wants a 0.201-inch bit (number 7 territory), but tapping larger coarse threads, drilling for 5/16-inch hardware, and opening out holes in 0.005-inch increments all land in letter land. The system is old - it descends from the same nineteenth-century gauge-numbering habits that produced wire gauges and number drills - and it survives because the intermediate steps are genuinely useful, not because anyone loves the naming. The practical takeaway: when a drawing or a tap chart calls out a letter size, this chart converts it to an exact decimal and millimeter value, which is the only identity that survives across brands and measurement systems.

The geometry is the same - a 6.5 mm letter-adjacent bit and an E bit (6.528 mm per this chart) are both twist drills - but the sizes are parallel systems, not translations. Letter sizes descend from US gauge conventions and land on decimal-inch dimensions that mostly do not match metric nominal sizes: G is 6.629 mm, close to a 6.6 mm bit you will never find stocked, while real metric steps come in 0.1 or 0.5 mm increments (6.0, 6.5, 7.0). Practically: use this chart to translate either direction. Holding a metric bit and need the letter nearest? Read the millimeter column and take the closest letter. Told to drill a 6.75 mm hole with only letter bits? H (6.756 mm) is your answer within a hundredth of a millimeter. What you should not do is treat close as equal in precision work - J at 7.036 is not a 7.0 mm bit when a reamed fit or a tap depends on the difference, and the whole point of the decimal column is that it tells you exactly how close close is.

By which system actually contains the size you need - and the tiebreaker is precision. Fractional sizes are the shop shorthand: when a drawing says drill 1/4 inch, use the 1/4 inch bit; letter E may be the same diameter, but the drawing language, the tool tolerance and the inspection expectation all assume the named size. Number and letter bits earn their spot when the size you need falls between fractional steps or when you need a fine adjustment: tapping is the classic case, because the correct tap drill is the thread minor diameter plus a percentage of the thread depth, and that number almost never lands on a fractional step. The workflow that works: compute or look up the exact decimal your job needs (tap charts give it directly), then find it in whichever series is nearest - if a fractional bit is within about 0.002 inch for a clearance hole, the fractional bit is fine; if the difference matters (tapped holes, press fits, reaming allowances), take the letter or number bit that lands closest. And whatever the lettering, verify by measurement once: gauge a new letter bit with calipers when the job is precise, because a mislabeled Chinese-market letter set is one of the more common sources of mysterious hole-size errors.

A handful of letter sizes earn their keep constantly. E (0.250 inch) is the identity size: exactly 1/4 inch, it drills clearance holes for 1/4-inch fasteners and opens out undersized pilot holes to a clean quarter. Q (0.332 inch) is the famous NPT answer: the nominal drill for 1/8-27 National Pipe Taper threads, one of the most-reached letter bits in plumbing-adjacent metalwork. F (0.257 inch) and I (0.272 inch) are common 10-32 and 12-24 clearance-adjacent sizes for electrical work. R (0.339 inch) lands near 8.5 mm, the standard pilot for M10 metric fasteners in softer material. N (0.302), O (0.316) and P (0.323) cluster around 5/16-inch work, where fractional steps are too coarse to thread-tap accurately. The general pattern: the letters you memorize are the ones matching the tap and clearance holes of the fasteners you use weekly - and for everything else, this chart plus calipers beats memory, because a one-letter error is a two-tenths-of-a-millimeter hole error, which is the difference between a bolt that threads and a part in the scrap bin.