7/8-9 Tap Drill Size
The tap drill for 7/8-9 UNC is 49/64 in (0.7656 in, 19.45 mm). That is the standard chart size and leaves about 76% thread.
Drill a 49/64 in hole, then cut the thread with a 7/8-9 tap. The hole is 0.1094 in narrower than the screw, so the tap cuts 0.0547 in into each side.
A full-height thread would be 0.0722 in deep on each side. The 49/64 in drill leaves 76% of that.
Try another material or drill
7/8-9 tap drill size by material
| Material | Target thread | Drill | Size | Thread left |
|---|---|---|---|---|
| Aluminum, brass, plastics | 75% | 49/64 in | 0.7656 in (19.45 mm) | 76% |
| Mild steel | 60% | 25/32 in | 0.7813 in (19.84 mm) | 65% |
| Stainless, titanium | 50% | 51/64 in | 0.7969 in (20.24 mm) | 54% |
The 75% row is the published chart drill. The 60% and 50% rows pick the biggest standard drill that still leaves close to that much thread.
How much thread do you need?
The chart prints one drill. The right amount of thread depends on the material, and the guides give slightly different ranges.
| Material | Texas Metalworks | ToolGrit | OSG (tap maker) | Drill for 7/8-9 |
|---|---|---|---|---|
| Aluminum | 70-75% | 60-65% | 65-70% | 49/64 in (76%) to 25/32 in (65%) |
| Brass | 70-75% | 70-75% | 65-70% | 49/64 in (76%) |
| Plastic | not covered | 70-75% | not covered | 49/64 in (76%) |
| Mild steel | 60-65% | 60-65% | 65-70% | 25/32 in (65%) |
| Stainless steel | 50% | 55-60% | 60-70% | 25/32 in (65%) to 51/64 in (54%) |
| Titanium | 50% | 50-55% | not covered | 51/64 in (54%) |
The three rows in the first table follow Texas Metalworks. OSG makes taps, and asks for more thread in stainless than either guide, so its range sits closer to our steel column. ToolGrit also suggests 50 to 55% for blind holes in stainless, titanium and hardened steel, and for small taps (#6 and below).
Why not a full thread?
A 100% thread is not possible in practice. The tap and the thread both have flat crests and roots (ToolGrit).
More thread also means more work for the tap: higher cutting force, more heat, poorer chip clearance and a bigger risk of breaking it (ToolGrit).
And it adds little strength. ToolGrit cites studies showing about 90% of a thread's shear strength at 60% thread. Texas Metalworks says going from 75% to 60% costs only about 5% of pull-out strength.
On 7/8-9, that step is from 49/64 in to 25/32 in. The two drills differ by 0.0156 in.
If you do not have the 49/64 in drill
Here are the drills on either side, and the thread each one leaves.
| Drill | Size | Thread left | What changes |
|---|---|---|---|
| 47/64 in | 0.7344 in (18.65 mm) | 97% | More thread, harder to tap |
| 3/4 in | 0.7500 in (19.05 mm) | 87% | More thread, harder to tap |
| 49/64 in | 0.7656 in (19.45 mm) | 76% | Standard size |
| 25/32 in | 0.7813 in (19.84 mm) | 65% | Less thread, easier to tap |
| 51/64 in | 0.7969 in (20.24 mm) | 54% | Less thread, easier to tap |
A bigger drill removes thread and makes the tap easier to turn. A smaller drill keeps more thread and puts more torque on the tap.
The check behind the 49/64 in number: (0.8750 - 0.7656) / (1.299 x 0.1111) = 75.8%.
What 7/8-9 means
- 7/8
- The major diameter, the widest part of the screw: 0.8750 in (22.22 mm).
- 9
- Threads per inch, so one thread every 0.1111 in (2.822 mm).
- UNC
- Unified National Coarse. Its fine twin is 7/8-14.
The rest of the size, worked out from the major diameter and pitch:
| Property | Value |
|---|---|
| Major diameter | 0.8750 in (22.22 mm) |
| Pitch | 9 TPI (2.822 mm) |
| Pitch diameter (basic) | 0.8028 in (20.39 mm) |
| Minor diameter, internal thread (basic) | 0.7547 in (19.17 mm) |
The pitch and minor diameters are for checking a thread with gauges. You can skip them if you only need the drill.
How long to make the 7/8-9 thread
Aim for a thread at least 1 to 1.5 times the diameter long. ToolGrit's strength figures assume that length.
| Thread length | In threads | Drill a blind hole at least |
|---|---|---|
| 1 x diameter: 0.875 in | 7.9 | 1.319 in |
| 1.5 x diameter: 1.313 in | 11.8 | 1.757 in |
The blind hole depth adds 4 threads (4 x 0.1111 in) so the tap has room to finish and the chips have somewhere to go.
How to tap a 7/8-9 hole
- Drill the hole with the 49/64 in drill (0.7656 in, 19.45 mm).
- For a blind hole, drill 1.319 in deep if you need a 0.875 in thread. That is 4 threads past the thread (ToolGrit).
- Put cutting fluid on the tap. Do not tap dry (ToolGrit, Wikipedia).
- Hold the tap in line with the hole.
- Turn the tap forward, then back it off now and then to break the chip (Wikipedia).
- Need threads right to the bottom of a blind hole? Finish with a bottoming tap (Wikipedia).
Taps usually break for one of these reasons (Wikipedia, ToolGrit):
- Chips clog the flutes.
- The tap is not in line with the hole.
- The tap is pushed too hard or too little.
- There is not enough cutting fluid.
- The drill was too small, so the tap works against too much thread.
Which tap to use for 7/8-9
The three common hand taps differ in how many threads at the tip are tapered (Wikipedia).
- Taper tap: 8 to 10 tapered threads, about 0.8889 in to 1.1111 in on 7/8-9. For hard materials and very small taps.
- Plug tap: 3 to 5 tapered threads, about 0.3333 in to 0.5556 in on 7/8-9. The most common tap, easy to start and keep straight.
- Bottoming tap: 1 to 1.5 tapered threads, about 0.1111 in to 0.1667 in on 7/8-9. Cuts threads to the bottom of a blind hole, usually after a plug tap.
Spiral-point taps are usually used in through holes so the chips can escape. Spiral-flute taps work well in blind holes (Wikipedia).
Forming (roll) taps only work in malleable metals such as mild steel and aluminum (Wikipedia). They use different drill sizes, which we do not cover yet.
7/8-9 vs 7/8-14
These two share the same diameter. They differ in pitch, so they take different drills.
| 7/8-9 | 7/8-14 | |
|---|---|---|
| Series | UNC (coarse) | UNF (fine) |
| Pitch | 9 TPI (2.822 mm) | 14 TPI (1.814 mm) |
| Standard drill | 49/64 in | 13/16 in |
| Thread depth, each side | 0.0722 in (1.83 mm) | 0.0464 in (1.18 mm) |
Thread depth is 0.6495 x pitch. Do not mix the two: a 7/8-9 screw will not fit a 7/8-14 hole.
Closest metric size
M22 is close to 7/8-9, but it is not the same thread.
| Major diameter | Pitch | Standard drill | |
|---|---|---|---|
| 7/8-9 | 22.22 mm | 2.822 mm | 49/64 in |
| M22 | 22.00 mm | 2.500 mm | 19.5 mm |
The diameters differ by 0.22 mm and the pitches are not the same. Do not swap them.
FAQ
What is the tap drill size for 7/8-9?
The standard drill is 49/64 in (0.7656 in, 19.45 mm). It leaves about 76% thread, which is the standard chart choice.
Can I use 25/32 in instead of 49/64 in for 7/8-9?
25/32 in (0.7813 in, 19.84 mm) leaves about 65% thread instead of 76%. The tap cuts more easily and the thread is weaker. That is still at or above the 60% Texas Metalworks suggests for mild steel.
Is a 19.4 mm drill the same as 49/64 in?
It is very close. 19.4 mm is 0.7638 in, against 0.7656 in, 19.45 mm for 49/64 in. It leaves about 77% thread.
What drill size do I use for 7/8-9 in aluminum?
Use 49/64 in (76%) or 25/32 in (65%), depending on the guide. Texas Metalworks says 70 to 75% for soft aluminum, which is 49/64 in. ToolGrit says 60 to 65% for aluminum alloys, which is 25/32 in. Either is a normal choice. OSG, a tap maker, gives 65 to 70% for average work in aluminum, brass and mild steel.
What is the tap drill size for 7/8-9 in stainless steel?
51/64 in (0.7969 in, 20.24 mm) leaves about 54% thread, close to the 50% Texas Metalworks recommends for stainless and titanium. ToolGrit gives 55 to 60% for 304 and 316 stainless, which is between 25/32 in and 51/64 in. The standard drill, 49/64 in, puts more torque on the tap. OSG, a tap maker, asks for 60 to 70% in stainless for average work, which is closer to the steel drill, 25/32 in.
Does a bigger drill make the 7/8-9 thread much weaker?
Not by much at first. ToolGrit cites studies showing about 90% of a thread's shear strength at 60% thread, if the thread is at least 1 to 1.5 times the diameter long. Texas Metalworks says going from 75% to 60% costs only about 5% of pull-out strength. For 7/8-9, the 60% drill is 25/32 in.
How long should a 7/8-9 thread be?
Aim for at least 1 to 1.5 times the diameter, the length ToolGrit's strength figures assume. For 7/8-9 that is 0.875 in to 1.313 in, or about 7.9 to 11.8 threads.
How deep do I drill a blind 7/8-9 hole?
At least 4 threads deeper than the thread you need, says ToolGrit. A 0.875 in thread needs a hole at least 1.319 in deep.
Which tap do I use for a blind 7/8-9 hole?
Start with a plug tap, then finish with a bottoming tap if the thread has to reach the bottom. Wikipedia says a bottoming tap has 1 to 1.5 tapered threads, about 0.1111 in to 0.1667 in on 7/8-9.
What is 7/8-9 in mm?
7/8-9 is 22.22 mm across, with a pitch of 2.822 mm.
Is 7/8-9 a coarse or fine thread?
7/8-9 is a coarse thread (UNC). The fine thread of the same diameter is 7/8-14, which takes 13/16 in.
Related sizes
Chart drill checked against the American Fastener tap and drill chart and the Fabworks tap drill chart. Diameters from Wikipedia: drill bit sizes. Thread percentages, thread length and blind-hole depth from Texas Metalworks and ToolGrit. The percentage formula is the one printed in the OSG tap drill chart, Vol 3 (2024). Tap types and tapping tips from Wikipedia. Last reviewed October 7, 2026.