3/8-16 Tap Drill Size
Unified coarse (UNC) · 16 threads per inch · ASME B1.1, class 2B
For a 3/8 tap: 3/8-16 coarse takes a 5/16" drill, 3/8-24 fine takes a Q drill.
What size drill bit for a 3/8-16 tap?
To scale section of the tapped hole
Drill options % of thread
| Drill | Inch | mm | Thread | 2B |
|---|---|---|---|---|
| 7.8 mm | 0.3071 | 7.80 | ✗ | |
| 7.9 mm | 0.3110 | 7.90 | ✓ | |
| 5/16"Use | 0.3125 | 7.94 | ✓ | |
| 8 mm | 0.3150 | 8.00 | ✓ | |
| O | 0.3160 | 8.03 | ✓ | |
| 8.1 mm | 0.3189 | 8.10 | ✓ | |
| 8.2 mm | 0.3228 | 8.20 | ✗ | |
| P | 0.3230 | 8.20 | ✗ | |
| 8.3 mm | 0.3268 | 8.30 | ✗ |
By material Aluminum, brass, mild steel: 5/16" (77%) · Stainless, tool steel: O (73%)
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 (0.307 in to 0.321 in).
3/8-16 UNC ASME B1.1, class 2B
| Major diameter | 0.3750 in / 9.53 mm |
|---|---|
| Threads per inch | 16 TPI (pitch 0.0625 in) |
| Pitch diameter (basic) | 0.3344 in |
| Minor diameter 2B | 0.3073–0.3214 in |
| Thread-forming tap drill | S 0.3480 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
- 0.375 inthread length for full strength in steel (1 × D)
- 0.563 inin cast iron or brass (1.5 × D), up to 0.750 in in aluminum
- +0.313 inextra drill depth below the thread for a plug tap (5 pitches), +0.125 in with a bottoming tap
Shop notes
3/8-16 has the easiest tap drill in the inch chart to remember: a 5/16 drill, 0.3125 in, about 77% thread. Nothing to look up, and it is in every fractional set.
When the 5/16 is dull or missing, an 8 mm drill (74%) or a letter O (73%) is the sensible stand-in, and both sit well inside the 2B limits. An 8.1 mm still works at 69% and is a fair pick for stainless or tough alloy steel, where a lighter thread means less torque on the tap. Avoid 21/64. It is the next fractional size up, and at about 58% it falls outside the minor diameter limits.
The metric neighbor is M10 x 1.5, larger than it looks (0.394 in against 0.375 in) and with a pitch of 16.9 tpi, close enough to 16 to fool the eye. They are not the same hole: the 8.5 mm drill for M10 is too big to even appear in the 3/8-16 table. The fine version, 3/8-24, needs a Q drill instead.
3/8-16 is also the larger of the two camera mount threads in ISO 1222, found on bigger tripods and tripod heads, so a 5/16 drill and a 3/8-16 tap are what repair a stripped one.