Quick answer
For a 1/2-13 UNC tap, use a 27/64 inch drill bit. The decimal equivalent is 0.4219 inches, or about 10.72 mm.
Thread
1/2-13 UNC
Tap drill
27/64 in drill bit
Decimal size
0.4219 in
Metric equivalent
10.72 mm
Where this size is used
1/2-13 threads are common on larger fixtures, heavy brackets, machine bases, and general shop hardware.
Before tapping
Large taps take more force. Clamp the work securely, keep the tap straight, use cutting fluid, and clear chips often.
Useful tools for this job
A 27/64 inch bit, 1/2-13 tap, and quality tap handle are worth buying as a matched setup for this job.
Frequently asked questions
Can I use 7/16 inch for a 1/2-13 tap?
7/16 inch is larger than 27/64 inch and will reduce thread engagement. Use 27/64 inch when possible.
Is 1/2-13 coarse thread?
Yes. 1/2-13 is the common UNC coarse-thread size.