3/4-10 Tap Drill Size
Unified coarse (UNC) · 10 threads per inch · ASME B1.1, class 2B
For a 3/4 tap: 3/4-10 coarse takes a 21/32" drill, 3/4-16 fine takes an 11/16" drill.
What size drill bit for a 3/4-10 tap?
To scale section of the tapped hole
Drill options % of thread
| Drill | Inch | mm | Thread | 2B |
|---|---|---|---|---|
| 41/64" | 0.6406 | 16.27 | ✗ | |
| 16.5 mm | 0.6496 | 16.50 | ✓ | |
| 21/32"Use | 0.6563 | 16.67 | ✓ | |
| 17 mm | 0.6693 | 17.00 | ✗ | |
| 43/64" | 0.6719 | 17.07 | ✗ |
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 (0.642 in to 0.663 in).
3/4-10 UNC ASME B1.1, class 2B
| Major diameter | 0.7500 in / 19.05 mm |
|---|---|
| Threads per inch | 10 TPI (pitch 0.1000 in) |
| Pitch diameter (basic) | 0.6851 in |
| Minor diameter 2B | 0.6418–0.6628 in |
| Thread-forming tap drill | 45/64" 0.7031 in |
Clearance hole ASME B18.2.8
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
- 0.750 inthread length for full strength in steel (1 × D)
- 1.125 inin cast iron or brass (1.5 × D), up to 1.500 in in aluminum
- +0.500 inextra drill depth below the thread for a plug tap (5 pitches), +0.200 in with a bottoming tap
Shop notes
A 21/32 drill (0.6562 in) is the usual chart pick for 3/4-10, at about 72% thread. Unusually, the metric drill lands closer to the classic 75% target: a 16.5 mm gives about 77% and still sits inside the 2B limits, so it is a fine choice in aluminum or mild steel.
Either side of those two, the options fall off fast. 41/64 is too tight at 84%, below the minimum minor diameter. 17 mm (62%) and 43/64 (60%) both leave the hole oversize. With only two good drills, check the size stamped on the shank before you start; at this diameter there is no neighbor to fall back on.
A 3/4 tap at 10 threads per inch is heavy to turn by hand in steel, and that is where the 21/32 earns its place over the 16.5 mm. The lighter thread is easier on the tap, and 72% is well within what the standard accepts.
Do not reach for an 11/16 because it sounds right. That is the drill for 3/4-16, and in a coarse 3/4-10 hole it is too big to appear in the table at all.