threadsizes

1/2-20 Tap Drill Size

Unified fine (UNF) · 20 threads per inch · ASME B1.1, class 2B

For a 1/2 tap: 1/2-13 coarse takes a 27/64" drill, 1/2-20 fine takes a 29/64" drill.

What size drill bit for a 1/2-20 tap?

29/64"fractional drill
0.4531inch
11.51mm
72%thread
No 29/64" drill? Use 11.5 mm (73%).

To scale section of the tapped hole

Section of a 1/2-20 tapped hole to scale: 29/64" (0.4531) drill, thread length 1.5 times the diameter. ⌀0.5000 P 0.0500 (20 TPI) DRILL 29/64" Thread 1.5 × D +5 P drill
Drawn from the 1/2-20 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 1/2-20
DrillInchmmThread2B
11.3 mm 0.4449 11.30 85% ✗
11.4 mm 0.4488 11.40 79% ✓
11.5 mm 0.4528 11.50 73% ✓
29/64"Use 0.4531 11.51 72% ✓
11.6 mm 0.4567 11.60 67% ✓
11.7 mm 0.4606 11.70 61% ✗

By material Aluminum, brass, mild steel: 29/64" (72%) · Stainless, tool steel: 11.6 mm (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 (0.446 in to 0.457 in).

1/2-20 UNF ASME B1.1, class 2B

Major diameter0.5000 in / 12.70 mm
Threads per inch20 TPI (pitch 0.0500 in)
Pitch diameter (basic)0.4675 in
Minor diameter 2B0.4459–0.4574 in
Thread-forming tap drill31/64" 0.4844 in

Clearance hole ASME B18.2.8

Close17/32"0.531 in
Normal9/16"0.562 in
Loose39/64"0.609 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

  • 0.500 inthread length for full strength in steel (1 × D)
  • 0.750 inin cast iron or brass (1.5 × D), up to 1.000 in in aluminum
  • +0.250 inextra drill depth below the thread for a plug tap (5 pitches), +0.100 in with a bottoming tap

Shop notes

The 29/64 drill and an 11.5 mm drill are practically the same hole here, both at about 72% thread. Use whichever is on the bench. If you only have metric drills in 0.1 mm steps, 11.4 mm (about 79%) and 11.6 mm (about 67%) are both inside the 2B minor diameter limits too, so you can pick the tight one for aluminum and the loose one for steel.

What makes 1/2-20 unusual is that the limit bites on the small side as well. An 11.3 mm drill gives about 85% thread and leaves the hole under the 0.4459 in minimum minor diameter. Most threads tolerate a slightly small drill; this one is drawn closely enough that one step down is already out.

The coarse 1/2-13 uses a 27/64 drill, and grabbing it by mistake is worse still: at 0.4219 in it is far below that minimum. The fine tap will either refuse to start or break trying.

1/2-20 is a common wheel stud thread on many American cars and light trucks, and it is the mounting thread on many 1/2 in drill chucks. Neither of those is a hole you should be cutting with a hand tap, but it explains why the bolts are everywhere.