Tap drill size for 1-12
59/64"
0.9219 inches · 23.42 mm · 75% thread engagement
| Tap drill (75% thread) | 59/64" — 0.9219" / 23.42 mm |
| Major diameter | 1" / 25.4 mm |
| Drill for a 100% thread | 0.8918" / 22.65 mm |
| Threads per inch | 12 |
| Pitch | 0.0833" / 2.12 mm |
| Series | UNF (fine) |
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:
1 − 1.299 ÷ 12 = 0.8918 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:
1 − 0.974 ÷ 12 = 0.9188 inches
The nearest drill that actually exists is 59/64" at 0.9219 inches, which is 0.0031 inches over the theoretical figure. That is the number printed on every 1-12 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% | 0.9459" | 61/64" (0.9531") | 24.21 mm |
| 65% | 0.9296" | 59/64" (0.9219") | 23.42 mm |
| 75% — the chart value | 0.9188" | 59/64" (0.9219") | 23.42 mm |
| 85% | 0.908" | 29/32" (0.9063") | 23.02 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.
1-12 in metric
1-12 has a major diameter of 25.4 mm and a pitch of 2.12 mm. The nearest metric tap is M24 at 24 mm, which is 1.4 mm smaller — far enough apart that there is no confusing the two. An M24 bolt will drop straight through a 1-12 hole without catching, so if a fastener nearly fits, neither of these is the standard you are looking for.
For the drill itself the conversion is straightforward: 59/64" is 23.42 mm, and a metric drill index will offer 23.4 mm as the nearest size.
What 1-12 is used for
Fine-pitch one-inch fittings.
The same 1" screw is also made as 1-8, the coarse version, which takes a 7/8" drill instead. Fine threads hold better in thin material, resist vibration loosening and give finer adjustment.
Frequently asked questions
What size drill bit do I need for a 1-12 tap?
A 59/64" drill, which is 0.9219 inches or 23.42 mm. That is the standard 75% thread engagement figure, and it is what every published tap chart gives for 1-12.
What size hole do I need for a 1-12 thread?
The same 59/64" hole — 0.9219 inches. The number comes from subtracting the thread height from the major diameter: 1 − 0.974 ÷ 12 = 0.9188 inches, and 59/64" is the nearest drill that exists.
What is the 1-12 tap drill size in mm?
23.42 mm. There is no metric drill of exactly that size, so a metric drill index gives you 23.4 mm as the nearest — close enough for 1-12 in most materials, though it changes the thread engagement slightly.
Can I use a 59/64" instead?
Yes, and in hard material you should. 59/64" 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
- 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
- 3-48 tap drill size — #47
- 3-56 tap drill size — #45
- 4-40 tap drill size — #43
- 4-48 tap drill size — #42
- 5-40 tap drill size — #38