Tap drill size for 1-8
7/8"
0.875 inches · 22.23 mm · 75% thread engagement
| Tap drill (75% thread) | 7/8" — 0.875" / 22.23 mm |
| Major diameter | 1" / 25.4 mm |
| Drill for a 100% thread | 0.8376" / 21.28 mm |
| Threads per inch | 8 |
| Pitch | 0.125" / 3.18 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:
1 − 1.299 ÷ 8 = 0.8376 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 ÷ 8 = 0.8782 inches
The nearest drill that actually exists is 7/8" at 0.875 inches, which is 0.0032 inches under the theoretical figure. That is the number printed on every 1-8 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% | 0.9188" | 59/64" (0.9219") | 23.42 mm |
| 65% | 0.8945" | 57/64" (0.8906") | 22.62 mm |
| 75% — the chart value | 0.8782" | 7/8" (0.875") | 22.23 mm |
| 85% | 0.862" | 55/64" (0.8594") | 21.83 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.
1-8 in metric
1-8 has a major diameter of 25.4 mm and a pitch of 3.18 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 1-8 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: 7/8" is 22.23 mm, and a metric drill index will offer 22.2 mm as the nearest size.
What 1-8 is used for
Foundation bolts, large machine anchorage and heavy threaded rod.
The same 1" screw is also made as 1-12, the fine version, which takes a 59/64" 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 1-8 tap?
A 7/8" drill, which is 0.875 inches or 22.23 mm. That is the standard 75% thread engagement figure, and it is what every published tap chart gives for 1-8.
What size hole do I need for a 1-8 thread?
The same 7/8" hole — 0.875 inches. The number comes from subtracting the thread height from the major diameter: 1 − 0.974 ÷ 8 = 0.8782 inches, and 7/8" is the nearest drill that exists.
What is the 1-8 tap drill size in mm?
22.23 mm. There is no metric drill of exactly that size, so a metric drill index gives you 22.2 mm as the nearest — close enough for 1-8 in most materials, though it changes the thread engagement slightly.
Can I use a 57/64" instead?
Yes, and in hard material you should. 57/64" 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
- 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
- 3-48 tap drill size — #47
- 3-56 tap drill size — #45
- 4-40 tap drill size — #43
- 4-48 tap drill size — #42