Tap drill size for 3/4-10
21/32"
0.6563 inches · 16.67 mm · 75% thread engagement
| Tap drill (75% thread) | 21/32" — 0.6563" / 16.67 mm |
| Major diameter | 0.75" / 19.05 mm |
| Drill for a 100% thread | 0.6201" / 15.75 mm |
| Threads per inch | 10 |
| Pitch | 0.1" / 2.54 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:
0.75 − 1.299 ÷ 10 = 0.6201 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:
0.75 − 0.974 ÷ 10 = 0.6526 inches
The nearest drill that actually exists is 21/32" at 0.6563 inches, which is 0.0037 inches over the theoretical figure. That is the number printed on every 3/4-10 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% | 0.6851" | 11/16" (0.6875") | 17.46 mm |
| 65% | 0.6656" | 43/64" (0.6719") | 17.07 mm |
| 75% — the chart value | 0.6526" | 21/32" (0.6563") | 16.67 mm |
| 85% | 0.6396" | 41/64" (0.6406") | 16.27 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.
3/4-10 in metric
3/4-10 has a major diameter of 19.05 mm and a pitch of 2.54 mm. The nearest metric tap is M20 at 20 mm, which is 0.95 mm larger — far enough apart that there is no confusing the two. An M20 bolt will not enter a 3/4-10 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: 21/32" is 16.67 mm, and a metric drill index will offer 16.7 mm as the nearest size.
What 3/4-10 is used for
Structural steelwork, foundation bolts and large threaded rod.
The same 0.75" screw is also made as 3/4-16, the fine version, which takes a 11/16" 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 3/4-10 tap?
A 21/32" drill, which is 0.6563 inches or 16.67 mm. That is the standard 75% thread engagement figure, and it is what every published tap chart gives for 3/4-10.
What size hole do I need for a 3/4-10 thread?
The same 21/32" hole — 0.6563 inches. The number comes from subtracting the thread height from the major diameter: 0.75 − 0.974 ÷ 10 = 0.6526 inches, and 21/32" is the nearest drill that exists.
What is the 3/4-10 tap drill size in mm?
16.67 mm. There is no metric drill of exactly that size, so a metric drill index gives you 16.7 mm as the nearest — close enough for 3/4-10 in most materials, though it changes the thread engagement slightly.
Can I use a 43/64" instead?
Yes, and in hard material you should. 43/64" 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
- 3/4-16 tap drill size — 11/16"
- 7/8-9 tap drill size — 49/64"
- 7/8-14 tap drill size — 13/16"
- 1-8 tap drill size — 7/8"
- 1-12 tap drill size — 59/64"
- 0-80 tap drill size — 3/64"
- 1-64 tap drill size — #53
- 1-72 tap drill size — #53
- 2-56 tap drill size — #50
- 2-64 tap drill size — #50