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Letter Size Drill Chart A to Z with Decimal Inch and mm Equivalents

Letter drills fill the gap between number drills and fractional drills. This chart gives each letter from A to Z as a decimal inch and in millimetres, so you can find the right drill for a reamer allowance, a clearance hole or a tapped hole.

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LetterDiameter (in)Diameter (mm)Note
A
0.234
5.94
Smallest letter drill
B
0.238
6.05
C
0.242
6.15
D
0.246
6.25
E
0.25
6.35
Same as 1/4 in
F
0.257
6.53
Tap drill for 5/16-18 UNC (about 75 percent thread)
G
0.261
6.63
H
0.266
6.76
I
0.272
6.91
Tap drill for 5/16-24 UNF
J
0.277
7.04
K
0.281
7.14
Same as 9/32 in
L
0.29
7.37
M
0.295
7.49
N
0.302
7.67
O
0.316
8.03
P
0.323
8.2
Q
0.332
8.43
Tap drill for 3/8-24 UNF
R
0.339
8.61
S
0.348
8.84
T
0.358
9.09
U
0.368
9.35
Tap drill for 7/16-14 UNC
V
0.377
9.58
W
0.386
9.8
X
0.397
10.08
Y
0.404
10.26
Z
0.413
10.49
Largest letter drill

Diameters are the standard nominal sizes. Letter drills continue the number drill series (#80 to #1), which end at 0.228 in for #1. Verify tap drill selections against your tap maker chart and the percent of thread required by your drawing.

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How to use the chart

Search for the letter, for example Q, or type a decimal such as 0.332 to find the matching drill. The mm column is for metric shops that stock imperial drills, or for CAM programmers who need to enter the exact diameter in a tool library. Export to Excel and drop it into the tool crib spreadsheet or the tool master in your ERP so that the description reads Q (0.332 in) rather than just Q.

  • Number drills run from #80 (smallest) to #1 (0.228 in).
  • Letter drills run from A (0.234 in) to Z (0.413 in).
  • Fractional drills continue after Z.

Where letter sizes show up

Letter drills are most common as tap drills and reamer starter drills. A 5/16-18 UNC tap drill is F, and a 3/8-24 UNF tap drill is Q. For reaming, leave 0.010 to 0.015 in for a typical hole, so a 0.250 in reamed hole starts with a 0.238 in drill, which is a B. Letter drills are also used when a drawing calls out a specific size such as .266 DIA or when a pin has a specific press fit.

Why the gaps matter

Drill sizes are spaced in steps of only a few thousandths of an inch, so choosing the wrong letter can change a thread from 75 to 60 percent engagement, or a press fit from tight to loose. For aluminium you often drill slightly oversize, while for stainless you may want a larger drill to reduce tap torque. Always check the drill runout in your holder, because a drill that wobbles by 0.003 in will cut a hole that large over its nominal diameter.

Common mistakes

Mixing up letter and number drills is the usual error. A number 7 drill is 0.201 in, and an E drill is 0.250 in, but they sit at the opposite ends of their series. Another mistake is entering a letter size in a CAM tool library as a fraction of an inch. Always store the decimal. In an ERP tooling master, keep the letter and the decimal in two separate fields so searches by either work.

Frequently Asked Questions

What size is a Q drill?

A Q drill is 0.332 inches, which is 8.43 mm. It is the standard tap drill for a 3/8-24 UNF thread. It sits between P (0.323 in) and R (0.339 in).

Which letter drill equals 1/4 inch?

E equals 0.250 inches, which is 1/4 inch. It is the only letter size that exactly matches a common fractional size, apart from K at 0.281 inches, which equals 9/32 inch.

What comes after Z in drill sizes?

After Z at 0.413 inches the series continues with fractional drills, starting at 27/64 inch (0.4219 in) and up. There are fractional sizes below A, but the number and letter series fill the gaps.

What is the largest number drill?

Number 1 is the largest number drill at 0.228 inches. Number 80 is the smallest at 0.0135 inches. Letter drills start just above #1 with A at 0.234 inches.

If your tool crib items are not searchable by diameter in your ERP, Netray can help clean up and standardise the tool master.

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