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Number and Letter Drill Sizes

Guide · Updated October 28, 2026

Fractional drills come in 1/64 in steps, which is too coarse for many tap drills and clearance holes. Number drills (#80, the smallest, to #1) and letter drills (A to Z, A the smallest) fill the gaps. Together with fractional drills they give a drill roughly every 0.002 to 0.005 in from about 0.0135 to 0.413 in. A jobber set of #1 to #60 and A to Z covers most shop tapping and clearance work.

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Step by step

Find the decimal you need: From a tap drill chart, clearance hole chart or drawing.
Pick the nearest drill: Look through number, letter, fractional and metric drills for the closest size.
Check the drill: Measure across the margins near the point with a micrometer.
Allow for oversize: Drills usually cut a little over size; pick slightly under if you will ream.
Mark the drill: Size marks on the shank wear off; keep drills in an indexed case.

Number drill sizes

#1 0.2280 in, #2 0.2210, #3 0.2130, #4 0.2090, #5 0.2055, #7 0.2010, #9 0.1960
#10 0.1935, #15 0.1800, #18 0.1695, #20 0.1610, #21 0.1590, #25 0.1495, #27 0.1440
#29 0.1360, #30 0.1285, #31 0.1200, #33 0.1130, #36 0.1065, #38 0.1015, #43 0.0890
#50 0.0700, #53 0.0595, #60 0.0400, #70 0.0280, #80 0.0135

Number sizes come from the old Stubs steel wire gauge, which is why the sizes get smaller as the number gets larger.

Letter drill sizes

A 0.234, B 0.238, C 0.242, D 0.246, E 0.250 (same as 1/4)
F 0.257, G 0.261, H 0.266, I 0.272, J 0.277, K 0.281
L 0.290, M 0.295, N 0.302, O 0.316, P 0.323, Q 0.332
R 0.339, S 0.348, T 0.358, U 0.368, V 0.377, W 0.386, X 0.397, Y 0.404, Z 0.413

Common uses

#43 (0.0890): tap drill for 4-40
#36 (0.1065): tap drill for 6-32
#29 (0.1360): tap drill for 8-32
#25 (0.1495): tap drill for 10-24; #21 (0.1590) for 10-32
#7 (0.2010): tap drill for 1/4-20
F (0.257): tap drill for 5/16-18
I (0.272): tap drill for 5/16-24
Q (0.332): tap drill for 3/8-24
U (0.368): tap drill for 7/16-14
#2 (0.221): normal clearance for #10 screws; #9 (0.196) for #8

Tap drills here are the common values for about 75 percent thread from standard tap drill charts.

Worked example: closest drill

You need a 0.201 in hole. The candidates are #7 at 0.2010, 13/64 at 0.2031 and 5.1 mm at 0.2008. #7 is exact, which is why it is the standard 1/4-20 tap drill. For a 0.260 in hole, the closest are G at 0.261 and 6.6 mm at 0.2598.

Worked example: drill before reaming

To ream 0.250 in, leave about 0.010 to 0.015 in of stock on the diameter. A letter C drill (0.242) leaves 0.008 in and D (0.246) leaves 0.004 in, which is too little; 15/64 (0.2344, nearly the same as A at 0.234) leaves about 0.016 in. Check the reamer guide for the stock your reamer maker recommends.

Common problems

Reading a worn shank mark wrong, confusing #1 (0.228) with 1/4, assuming a drill cuts exact size, and drilling a tap hole with a dull drill that cuts oversize. Measure drills across the margins, not across the flutes at the back.

FAQ

What size is a #7 drill?

0.2010 in, the standard tap drill for 1/4-20.

What size is an F drill?

0.257 in, used for 5/16-18 tapping.

Which number drill is biggest?

#1 at 0.2280 in. Letter drills continue from A at 0.234 in to Z at 0.413 in.

How do I check a drill size?

Measure across the margins with a micrometer checked on gage blocks; build stacks free at gageblockcalc.com.

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General shop information only — follow your drawing, tool maker's data and your quality system.