Tap Drill Size Chart

M18 x 2.5 Tap Drill Size

Tap drill

15.5 mm 0.6102 in

Use a 15.5 mm drill (0.6102 in) for 77.0% thread.

Fine thread M18 x 2? Use 16 mm. (0.6299 in, 77.0% thread)

The drill size for a M18 x 2.5 tap is 15.5 mm (0.6102 in). If you are wondering what size drill bit a M18 x 2.5 tap needs, that is the one: it leaves 77.0% of thread, which is the figure standard tap and drill charts are built around. Below are the other holes you might want for this size - a larger drill for tough material, the form-tap hole, and the clearance holes for the bolt that goes in.

Every drill for M18 x 2.5

Tap drills and clearance drills for M18 x 2.5 (Metric coarse, 2.5 mm pitch). Percent of thread is the figure stored for each drill, which reproduces from the major diameter and pitch of M18 x 2.5.
Use Drill Decimal in mm % thread
Standard tap drill Cut tap, general use (D - P) 15.5 mm 0.6102 15.50 77.0%
Inch substitute Closest inch drill if you have no metric set 39/64″ 0.6094 15.48 77.7%
Hard material Stainless, tool steel, anything that fights the tap 16.1 mm 0.6339 16.10 58.5%
Form (roll) tap Theoretical 16.9 mm - starting point, check your tap maker's chart 16.9 mm 0.6654 16.90 —

Clearance holes for a M18 x 2.5 bolt

A clearance hole is not tapped. It is the hole in the part the bolt passes through, so it is always larger than the thread.

Clearance holes for M18 x 2.5 per ISO 273. Clearance depends on the bolt diameter alone, so every M18 thread shares these figures.
Fit Hole mm Hole in
Fine 19.00 0.748
Medium 20.00 0.7874
Coarse 21.00 0.8268

M18 x 2.5 dimensions

Nominal dimensions for M18 x 2.5. The basic internal minor diameter is the hole a 100% thread would start from, which is why no tap drill is ever that small.
Dimension Inch mm
Major diameter 0.7087 18.00
Basic internal minor diameter 0.6021 15.29
Pitch 0.0984 2.50
Thread designation M18 x 2.5

What the hole looks like

Cross-section of a M18 x 2.5 tapped hole, drawn to scale The drilled hole is 15.5 mm, 0.6102 inches or 15.50 millimetres across. The tap cuts outward from that wall to the major diameter of 0.7087 inches. A 100 percent thread would reach in to the basic minor diameter of 0.6021 inches, shown as a dashed line; the difference is why this drill gives 77.0% of thread. major 18 mm drill 15.5 mm · 15.5 mm dashed = 100% thread at 15.29 mm
M18 x 2.5 in section, to scale. The accent line is the thread the tap cuts from a 15.5 mm hole; the dashed line is where the crest would sit at 100% thread. The gap between them is the 23.0% of thread this drill leaves behind.

Work out a different hole for M18 x 2.5

Pre-filled with M18 x 2.5. Change the material, the tap type, or which drills you actually own, and it picks the closest real drill and tells you the percent of thread you end up with. The tables above are the published figures and do not depend on this.

The general-purpose target

Tap type
Drills you have

Best drill you have, cut tap, 70% target

15.7 mm0.6181 in70.8% thread

Theoretical hole 15.73 mm

One size down, deeper thread15.6 mm 0.6142 in · 73.9%

One size up, easier tapping15.8 mm 0.622 in · 67.7%

For comparison, the published chart drill for M18 x 2.5 is 15.5 mm at 77.0% thread.

Notes on M18 x 2.5

Coarse pitch
M18 x 2.5 is the coarse pitch at 18 mm. It is the usual choice for general work, and it tolerates a slightly off-size hole better than the finer pitches do. Finer options at the same diameter: M18 x 2, M18 x 1.5, M18 x 1.
What the standard drill gives you
15.5 mm leaves 77.0% of thread (D - P). That is in the normal 65-80% band most shop charts aim for.
Stainless, tool steel and other tough material
16.1 mm opens the hole to 58.5% engagement. Less material for the tap to cut means less torque and less heat, at the cost of some thread depth.
Using a form (roll) tap
A form tap displaces metal instead of cutting it, so it needs a larger hole: 16.9 mm, against a theoretical 16.9 mm. Treat that as a starting point and check your tap maker's chart.
Closest inch size
3/4-10 is the nearest inch thread by diameter (19.05 mm against 18 mm). It is a planning reference only. The two are not interchangeable: the pitches differ, and a bolt from one will not run into a hole tapped for the other.

Related sizes

Same diameter, different pitch

Next size up and down

Closest inch size

3/4-10 21/32″ · 0.6562 in · 16.67 mm · 72.2% 19.05 mm major against 18 mm

Not interchangeable. 3/4-10 is the nearest size by diameter, nothing more. The pitches differ, so a 3/4-10 fastener will not run into a M18 x 2.5 hole and forcing one will strip both.

Guides for tapping M18 x 2.5

Longer reads on the decisions behind the numbers above.

All guides →

M18 x 2.5 tap drill: common questions

What size drill bit do I need for a M18 x 2.5 tap?
A 15.5 mm drill, 0.6102 in. That leaves 77.0% of thread, which is the standard chart figure for M18 x 2.5.
What is the drill size for a M18 x 2.5 tap in inches?
The 15.5 mm tap drill measures 0.6102 in. If you only have inch drills, 39/64 in (0.6094 in) is the closest, giving 77.7% thread.
What drill should I use for M18 x 2.5 in stainless steel?
Use 16.1 mm instead of 15.5 mm. It leaves 58.5% of thread rather than 77.0%, so there is less material for the tap to cut and less chance of snapping it.
Is M18 x 2.5 the same as M18 x 2?
No. Both are 18 mm threads, but the pitch differs, so the drills differ: 15.5 mm for M18 x 2.5 and 16 mm for M18 x 2. The taps and the fasteners are not interchangeable.
What is the clearance hole for an M18 x 2.5 bolt?
For a medium fit, 20 mm per ISO 273. A clearance hole is not tapped; the bolt passes straight through it.
A 4 × 6 in card with the drills for this size.