threadsizes

10-24 Tap Drill Size

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

What size drill bit for a 10-24 tap?

#25number drill
0.1495inch
3.80mm
75%thread
No #25 drill? Use 3.8 mm (75%).

To scale section of the tapped hole

Section of a #10-24 tapped hole to scale: #25 (0.1495) drill, thread length 1.5 times the diameter. ⌀0.1900 P 0.0417 (24 TPI) DRILL #25 Thread 1.5 × D +5 P drill
Drawn from the #10-24 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 #10-24
DrillInchmmThread2B
3.7 mm 0.1457 3.70 82% ✓
#26 0.1470 3.73 79% ✓
#25Use 0.1495 3.80 75% ✓
3.8 mm 0.1496 3.80 75% ✓
#24 0.1520 3.86 70% ✓
3.9 mm 0.1535 3.90 67% ✓
#23 0.1540 3.91 67% ✓
5/32" 0.1563 3.97 62% ✗
#22 0.1570 3.99 61% ✗

By material Aluminum, brass, mild steel: #25 (75%) · Stainless, tool steel: #23 (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 (0.145 in to 0.155 in).

10-24 UNC ASME B1.1, class 2B

Major diameter0.1900 in / 4.83 mm
Threads per inch24 TPI (pitch 0.0417 in)
Pitch diameter (basic)0.1629 in
Minor diameter 2B0.1449–0.1555 in
Thread-forming tap drill11/64" 0.1719 in

Clearance hole ASME B18.2.8

Close#50.206 in
Normal#20.221 in
LooseB0.238 in

For a screw that passes through the part. Normal fit is the default; close fit locates the part more accurately but leaves no room for hole position error.

Hole depth rule of thumb

  • 0.190 inthread length for full strength in steel (1 × D)
  • 0.285 inin cast iron or brass (1.5 × D), up to 0.380 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

The #25 drill (0.1495 in) gives #10-24 almost exactly 75% thread, and a 3.8 mm metric drill matches it to within a fraction of a point. Either one is the right answer. There is no reason to look further unless both are missing.

If they are, a #24 (70%) or a #26 (79%) both sit comfortably inside the minor diameter limits. Coarse pitch is forgiving that way. For stainless, the #23 (66.5%) is the largest drill still inside the limits and a reasonable way to take load off the tap.

The mistake worth warning about is drilling for 10-32 when the tap is 10-24. The usual 10-32 drills, #21 and 5/32, are too big here: they leave about 57% and 62% thread and put the hole above the 0.1555 in maximum minor diameter. The two screws are both #10 and look the same, and so do their rows in a drill chart at a glance. Read the thread count on the tap shank before the drill comes out.

An M5 screw is a different problem: at 0.197 in it is slightly bigger than a #10 (0.190 in) and will not go into a 10-24 hole at all.

Questions

What size is a #25 drill bit?

A #25 drill is 0.1495 in (3.80 mm), between 9/64 (0.1406 in) and 5/32 (0.1563 in). It is the standard tap drill for #10-24.