Tap drill size for 3/8-16
5/16"
0.3125 inches · 7.94 mm · 75% thread engagement
| Tap drill (75% thread) | 5/16" — 0.3125" / 7.94 mm |
| Major diameter | 0.375" / 9.52 mm |
| Drill for a 100% thread | 0.2938" / 7.46 mm |
| Threads per inch | 16 |
| Pitch | 0.0625" / 1.59 mm |
| Series | UNC (coarse) |
Where the number comes from
A tap cuts the thread; the drill leaves the hole it cuts into. Cutting the full thread form would mean drilling the major diameter less the whole height of the thread:
0.375 − 1.299 ÷ 16 = 0.2938 inches
That is the 100% thread, which nobody drills — the tap has to remove every scrap of metal and usually breaks trying. Charts give 75%, so three quarters of the thread height is cut away and the rest is left as clearance:
0.375 − 0.974 ÷ 16 = 0.3141 inches
The nearest drill that actually exists is 5/16" at 0.3125 inches, which is 0.0016 inches under the theoretical figure. That is the number printed on every 3/8-16 tap chart, and this is where it comes from.
The same hole at other engagements
Thread engagement is a choice, not a constant. Less engagement means an easier cut and a longer tap life for very little loss of strength, which is why a machinist working stainless will not drill the chart size.
| Engagement | Theoretical | Drill | Millimetres |
|---|---|---|---|
| 50% | 0.3344" | Q (0.332") | 8.43 mm |
| 65% | 0.3222" | P (0.323") | 8.2 mm |
| 75% — the chart value | 0.3141" | 5/16" (0.3125") | 7.94 mm |
| 85% | 0.306" | N (0.302") | 7.67 mm |
A 75% thread is close to the practical ceiling: going from 75% to 100% adds only a few per cent of strength and roughly doubles the torque needed to turn the tap. Going the other way, a 65% thread still holds about 95% of the load a 75% thread does, which is why 65% is the usual choice in anything harder than mild steel.
3/8-16 in metric
3/8-16 has a major diameter of 9.52 mm and a pitch of 1.59 mm. The nearest metric tap is M10 at 10 mm, which is 0.48 mm larger — far enough apart that there is no confusing the two. An M10 bolt will not enter a 3/8-16 hole at all, so if a fastener nearly fits, neither of these is the standard you are looking for.
For the drill itself the conversion is straightforward: 5/16" is 7.94 mm, and a metric drill index will offer 7.9 mm as the nearest size.
What 3/8-16 is used for
Machine mounts, hoisting eyes, heavy brackets and threaded rod.
The same 0.375" screw is also made as 3/8-24, the fine version, which takes a Q drill instead. Coarse threads tap faster, tolerate damage better and hold better in soft material.
Frequently asked questions
What size drill bit do I need for a 3/8-16 tap?
A 5/16" drill, which is 0.3125 inches or 7.94 mm. That is the standard 75% thread engagement figure, and it is what every published tap chart gives for 3/8-16.
What size hole do I need for a 3/8-16 thread?
The same 5/16" hole — 0.3125 inches. The number comes from subtracting the thread height from the major diameter: 0.375 − 0.974 ÷ 16 = 0.3141 inches, and 5/16" is the nearest drill that exists.
What is the 3/8-16 tap drill size in mm?
7.94 mm. There is no metric drill of exactly that size, so a metric drill index gives you 7.9 mm as the nearest — close enough for 3/8-16 in most materials, though it changes the thread engagement slightly.
Can I use a P instead?
Yes, and in hard material you should. P gives about 65% thread engagement instead of 75%. That costs very little strength — a 65% thread holds roughly 95% of what a 75% thread does — and it makes the tap markedly easier to turn, which is the trade every machinist makes in stainless or tough alloy.
Other tap sizes
- 3/8-24 tap drill size — Q
- 7/16-14 tap drill size — U
- 7/16-20 tap drill size — 25/64"
- 1/2-13 tap drill size — 27/64"
- 1/2-20 tap drill size — 29/64"
- 9/16-12 tap drill size — 31/64"
- 9/16-18 tap drill size — 33/64"
- 5/8-11 tap drill size — 17/32"
- 5/8-18 tap drill size — 37/64"
- 3/4-10 tap drill size — 21/32"