Letter Drill Bit Sizes (A to Z)

Letter drill sizes run from A (0.234", the smallest) to Z (0.413", the largest), filling the gap between fractional 1/4" (0.25") and 1/2" (0.5") in finer steps than the fractional system provides there. The 26 letters give a standardized name to diameters that do not correspond to a clean fraction.

Letter sizes matter most for tap drills. Several common bolt and machine screw tap sizes call for a specific letter drill rather than a fraction, because the correct pilot hole diameter for cutting clean threads happens to fall between two fractional sizes.

Letter E (0.25") is identical to fractional 1/4". From there, letters climb in roughly 0.004" to 0.015" steps up to Z (0.413"), just under fractional 27/64" (0.4219"). No other letter size lands exactly on a fractional value, which is the entire reason the letter system exists: it names the diameters a fractional set skips over.

A tap drill has to leave enough material for the tap to cut clean threads, without removing so much that the threads come out shallow or weak. For many common bolt sizes, the correct tap drill diameter is a letter size rather than a fraction. See the tap and clearance drill chart for the full pairing between thread size and tap drill, or the main drill size chart for the fractional and number systems.