M8 Tap Drill Size
The tap drill for M8 x 1.25 is 6.8 mm (0.2677 in). That is the standard chart size and leaves about 74% thread.
Drill a 6.8 mm hole, then cut the thread with an M8 tap. The hole is 1.20 mm narrower than the screw, so the tap cuts 0.60 mm into each side.
A full-height thread would be 0.81 mm deep on each side. The 6.8 mm drill leaves 74% of that.
Try another material or drill
M8 tap drill size by material
| Material | Target thread | Drill | Size | Thread left |
|---|---|---|---|---|
| Aluminum, brass, plastics | 75% | 6.8 mm | 0.2677 in | 74% |
| Mild steel | 60% | 7.0 mm | 0.2756 in | 62% |
| Stainless, titanium | 50% | 7.2 mm | 0.2835 in | 49% |
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.
Notes on M8
M8 (8.00 mm) and 5/16 in (7.94 mm) are only 0.06 mm apart. Only the pitch separates them: 1.25 mm for M8, 1.411 mm for 5/16-18. So do the drills: 6.8 mm for M8, letter F for 5/16-18.
Steel takes 7.0 mm (62%) and stainless 7.2 mm (49%) on M8.
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 M8 |
|---|---|---|---|---|
| Aluminum | 70-75% | 60-65% | 65-70% | 6.8 mm (74%) to 7.0 mm (62%) |
| Brass | 70-75% | 70-75% | 65-70% | 6.8 mm (74%) |
| Plastic | not covered | 70-75% | not covered | 6.8 mm (74%) |
| Mild steel | 60-65% | 60-65% | 65-70% | 7.0 mm (62%) |
| Stainless steel | 50% | 55-60% | 60-70% | 7.0 mm (62%) to 7.2 mm (49%) |
| Titanium | 50% | 50-55% | not covered | 7.2 mm (49%) |
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 M8, that step is from 6.8 mm to 7.0 mm. The two drills differ by 0.20 mm.
If you do not have the 6.8 mm drill
Here are the drills on either side, and the thread each one leaves.
| Drill | Size | Thread left | What changes |
|---|---|---|---|
| 6.6 mm | 0.2598 in | 86% | More thread, harder to tap |
| 6.7 mm | 0.2638 in | 80% | More thread, harder to tap |
| 6.8 mm | 0.2677 in | 74% | Standard size |
| 6.9 mm | 0.2717 in | 68% | Less thread, easier to tap |
| 7.0 mm | 0.2756 in | 62% | 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 6.8 mm number: (8.00 - 6.80) / (1.299 x 1.25) = 73.9%.
What M8 means
- M8
- Metric thread. The 8 is the major diameter in mm (0.3150 in).
- x 1.25
- The pitch: 1.25 mm from one thread to the next. This is the coarse pitch for M8, so it is often left off.
The rest of the size, worked out from the major diameter and pitch:
| Property | Value |
|---|---|
| Major diameter | 8.000 mm (0.3150 in) |
| Pitch | 1.25 mm |
| Pitch diameter (basic) | 7.188 mm (0.2830 in) |
| Minor diameter, internal thread (basic) | 6.647 mm (0.2617 in) |
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 M8 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: 8.0 mm | 6.4 | 13.0 mm |
| 1.5 x diameter: 12.0 mm | 9.6 | 17.0 mm |
The blind hole depth adds 4 threads (4 x 1.25 mm) so the tap has room to finish and the chips have somewhere to go.
How to tap an M8 hole
- Drill the hole with the 6.8 mm drill (0.2677 in).
- For a blind hole, drill 13.0 mm deep if you need a 8.0 mm 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 M8
The three common hand taps differ in how many threads at the tip are tapered (Wikipedia).
- Taper tap: 8 to 10 tapered threads, about 10.00 mm to 12.50 mm on M8. For hard materials and very small taps.
- Plug tap: 3 to 5 tapered threads, about 3.75 mm to 6.25 mm on M8. The most common tap, easy to start and keep straight.
- Bottoming tap: 1 to 1.5 tapered threads, about 1.25 mm to 1.88 mm on M8. 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.
Closest inch size
5/16-18 is close to M8, but it is not the same thread.
| Major diameter | Pitch | Standard drill | |
|---|---|---|---|
| M8 | 8.00 mm | 1.250 mm | 6.8 mm |
| 5/16-18 | 7.94 mm | 1.411 mm | letter F |
The diameters differ by 0.06 mm and the pitches are not the same. Do not swap them.
FAQ
What is the tap drill size for M8?
The standard drill is 6.8 mm (0.2677 in). It leaves about 74% thread, which is the standard chart choice.
Can I use 6.9 mm instead of 6.8 mm for M8?
6.9 mm (0.2717 in) leaves about 68% thread instead of 74%. 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 letter H drill the same as 6.8 mm?
It is very close. letter H is 0.2660 in, 6.76 mm, against 0.2677 in for 6.8 mm. It leaves about 77% thread.
What drill size do I use for M8 in aluminum?
Use 6.8 mm (74%) or 7.0 mm (62%), depending on the guide. Texas Metalworks says 70 to 75% for soft aluminum, which is 6.8 mm. ToolGrit says 60 to 65% for aluminum alloys, which is 7.0 mm. 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 M8 in stainless steel?
7.2 mm (0.2835 in) leaves about 49% 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 7.0 mm and 7.2 mm. The standard drill, 6.8 mm, 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, 7.0 mm.
Does a bigger drill make the M8 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 M8, the 60% drill is 7.0 mm.
How long should an M8 thread be?
Aim for at least 1 to 1.5 times the diameter, the length ToolGrit's strength figures assume. For M8 that is 8.0 mm to 12.0 mm, or about 6.4 to 9.6 threads.
How deep do I drill a blind M8 hole?
At least 4 threads deeper than the thread you need, says ToolGrit. A 8.0 mm thread needs a hole at least 13.0 mm deep.
Which tap do I use for a blind M8 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 1.25 mm to 1.88 mm on M8.
What is M8 in inches?
M8 is 0.3150 in across, with a pitch of 1.25 mm.
Is M8 a coarse or fine thread?
M8 x 1.25 is the coarse pitch for M8.
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.