threadsizes

12-24 Tap Drill Size

Unified coarse (UNC) · 24 threads per inch · ASME B1.1, class 2B

What size drill bit for a 12-24 tap?

#16number drill
0.1770inch
4.50mm
72%thread
No #16 drill? Use 4.5 mm (72%).

To scale section of the tapped hole

Section of a #12-24 tapped hole to scale: #16 (0.1770) drill, thread length 1.5 times the diameter. ⌀0.2160 P 0.0417 (24 TPI) DRILL #16 Thread 1.5 × D +5 P drill
Drawn from the #12-24 basic profile. Hatched area is the part; the thread runs 1.5 × D and the drill goes 5 pitches deeper for a plug tap.

Drill options % of thread

Drill options for #12-24
DrillInchmmThread2B
11/64" 0.1719 4.37 82% ✓
#17 0.1730 4.39 79% ✓
4.4 mm 0.1732 4.40 79% ✓
#16Use 0.1770 4.50 72% ✓
4.5 mm 0.1772 4.50 72% ✓
#15 0.1800 4.57 67% ✓
4.6 mm 0.1811 4.60 65% ✗
#14 0.1820 4.62 63% ✗
#13 0.1850 4.70 57% ✗

By material Aluminum, brass, mild steel: #16 (72%) · Stainless, tool steel: #15 (67%)

75% (orange tick) is the usual target. Hard or gummy metals like stainless are often tapped at 60 to 65% to spare the tap; aluminum, brass and plastic take 75% or a little more. The last column checks the hole against the 2B minor diameter limits (0.171 in to 0.181 in).

12-24 UNC ASME B1.1, class 2B

Major diameter0.2160 in / 5.49 mm
Threads per inch24 TPI (pitch 0.0417 in)
Pitch diameter (basic)0.1889 in
Minor diameter 2B0.1709–0.1807 in
Thread-forming tap drill#8 0.1990 in

Hole depth rule of thumb

  • 0.216 inthread length for full strength in steel (1 × D)
  • 0.324 inin cast iron or brass (1.5 × D), up to 0.432 in in aluminum
  • +0.208 inextra drill depth below the thread for a plug tap (5 pitches), +0.083 in with a bottoming tap

Shop notes

#12-24 sits in an awkward gap between #10 and 1/4 in, and its drills are rarely part of a basic set. The #16 (0.1770 in) is standard at 72%, and a 4.5 mm metric drill is a near twin at 71.7%.

Without either, the choice is easier than it looks. The 11/64 is the fractional drill most people have, and it is a usable one: about 82% thread and still inside the minor diameter limits. It costs tapping torque, so it belongs in aluminum and mild steel more than in stainless. On the large side, #15 (66.5%) is the last drill inside the limits. The 4.6 mm and #14 look like close neighbours but already put the hole above the 0.1807 in maximum minor diameter.

The usual mix-ups are easy to spot once you know them: a 10-24 screw in a 12-24 hole is visibly sloppy, and an M6 screw (0.236 in) is too big to enter a 12-24 hole (0.216 in) at all.

Questions

What size is a #16 drill bit?

A #16 drill is 0.1770 in (4.50 mm), between 11/64 (0.1719 in) and 3/16 (0.1875 in). It is the standard tap drill for #12-24.