M18 Tap Drill Size
The tap drill for M18 x 2.5 is 15.5 mm (0.6102 in). That is the standard major-minus-pitch size and leaves about 77% thread.
Drill a 15.5 mm hole, then cut the thread with an M18 tap. The hole is 2.50 mm narrower than the screw, so the tap cuts 1.25 mm into each side.
A full-height thread would be 1.62 mm deep on each side. The 15.5 mm drill leaves 77% of that.
Try another material or drill
M18 tap drill size by material
| Material | Target thread | Drill | Size | Thread left |
|---|---|---|---|---|
| Aluminum, brass, plastics | 75% | 15.5 mm | 0.6102 in | 77% |
| Mild steel | 60% | 16.1 mm | 0.6339 in | 59% |
| Stainless, titanium | 50% | 16.4 mm | 0.6457 in | 49% |
The 75% row is the major-minus-pitch drill. The 60% and 50% rows pick the biggest standard drill that still leaves close to that much thread.
Notes on M18
M18 x 2.5 has the chart drill 15.5 mm (77%). Steel takes 16.1 mm (59%) and stainless takes 16.4 mm (49%).
M18 also comes in fine pitches, which take much larger drills: see the fine-pitch table on the metric chart. Check the pitch on your tap before you drill.
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 M18 |
|---|---|---|---|---|
| Aluminum | 70-75% | 60-65% | 65-70% | 15.5 mm (77%) to 16.1 mm (59%) |
| Brass | 70-75% | 70-75% | 65-70% | 15.5 mm (77%) |
| Plastic | not covered | 70-75% | not covered | 15.5 mm (77%) |
| Mild steel | 60-65% | 60-65% | 65-70% | 16.1 mm (59%) |
| Stainless steel | 50% | 55-60% | 60-70% | 16.1 mm (59%) to 16.4 mm (49%) |
| Titanium | 50% | 50-55% | not covered | 16.4 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 M18, that step is from 15.5 mm to 16.1 mm. The two drills differ by 0.60 mm.
If you do not have the 15.5 mm drill
Here are the drills on either side, and the thread each one leaves.
| Drill | Size | Thread left | What changes |
|---|---|---|---|
| 15.3 mm | 0.6024 in | 83% | More thread, harder to tap |
| 15.4 mm | 0.6063 in | 80% | More thread, harder to tap |
| 15.5 mm | 0.6102 in | 77% | Standard size |
| 15.6 mm | 0.6142 in | 74% | Less thread, easier to tap |
| 15.7 mm | 0.6181 in | 71% | 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 15.5 mm number: (18.00 - 15.50) / (1.299 x 2.50) = 77.0%.
What M18 means
- M18
- Metric thread. The 18 is the major diameter in mm (0.7087 in).
- x 2.5
- The pitch: 2.5 mm from one thread to the next. This is the coarse pitch for M18, so it is often left off.
The rest of the size, worked out from the major diameter and pitch:
| Property | Value |
|---|---|
| Major diameter | 18.000 mm (0.7087 in) |
| Pitch | 2.50 mm |
| Pitch diameter (basic) | 16.376 mm (0.6447 in) |
| Minor diameter, internal thread (basic) | 15.294 mm (0.6021 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 M18 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: 18.0 mm | 7.2 | 28.0 mm |
| 1.5 x diameter: 27.0 mm | 10.8 | 37.0 mm |
The blind hole depth adds 4 threads (4 x 2.50 mm) so the tap has room to finish and the chips have somewhere to go.
How to tap an M18 hole
- Drill the hole with the 15.5 mm drill (0.6102 in).
- For a blind hole, drill 28.0 mm deep if you need a 18.0 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 M18
The three common hand taps differ in how many threads at the tip are tapered (Wikipedia).
- Taper tap: 8 to 10 tapered threads, about 20.00 mm to 25.00 mm on M18. For hard materials and very small taps.
- Plug tap: 3 to 5 tapered threads, about 7.50 mm to 12.50 mm on M18. The most common tap, easy to start and keep straight.
- Bottoming tap: 1 to 1.5 tapered threads, about 2.50 mm to 3.75 mm on M18. 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.
FAQ
What is the tap drill size for M18?
The standard drill is 15.5 mm (0.6102 in). It leaves about 77% thread, which is the standard chart choice.
Can I use 15.6 mm instead of 15.5 mm for M18?
15.6 mm (0.6142 in) leaves about 74% thread instead of 77%. 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 39/64 in drill the same as 15.5 mm?
It is very close. 39/64 in is 0.6094 in, 15.48 mm, against 0.6102 in for 15.5 mm. It leaves about 78% thread.
What drill size do I use for M18 in aluminum?
Use 15.5 mm (77%) or 16.1 mm (59%), depending on the guide. Texas Metalworks says 70 to 75% for soft aluminum, which is 15.5 mm. ToolGrit says 60 to 65% for aluminum alloys, which is 16.1 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 M18 in stainless steel?
16.4 mm (0.6457 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 16.1 mm and 16.4 mm. The standard drill, 15.5 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, 16.1 mm.
Does a bigger drill make the M18 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 M18, the 60% drill is 16.1 mm.
How long should an M18 thread be?
Aim for at least 1 to 1.5 times the diameter, the length ToolGrit's strength figures assume. For M18 that is 18.0 mm to 27.0 mm, or about 7.2 to 10.8 threads.
How deep do I drill a blind M18 hole?
At least 4 threads deeper than the thread you need, says ToolGrit. A 18.0 mm thread needs a hole at least 28.0 mm deep.
Which tap do I use for a blind M18 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 2.50 mm to 3.75 mm on M18.
What is M18 in inches?
M18 is 0.7087 in across, with a pitch of 2.5 mm.
Is M18 a coarse or fine thread?
M18 x 2.5 is the coarse pitch for M18.
Related sizes
The charts we read stop at M20, so the M18 drill uses the major-minus-pitch rule (Wikipedia: ISO metric screw thread for the pitch). 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.