Tap Drill Size Chart

M20 x 2.5 Tap Drill Size

Tap drill

17.5 mm 0.689 in

Use a 17.5 mm drill (0.689 in) for 77.0% thread.

Fine thread M20 x 2? Use 18 mm. (0.7087 in, 77.0% thread)

The drill size for a M20 x 2.5 tap is 17.5 mm (0.689 in). If you are wondering what size drill bit a M20 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 M20 x 2.5

Tap drills and clearance drills for M20 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 M20 x 2.5.
Use Drill Decimal in mm % thread
Standard tap drill Cut tap, general use (D - P) 17.5 mm 0.689 17.50 77.0%
Inch substitute Closest inch drill if you have no metric set 11/16″ 0.6875 17.46 78.2%
Hard material Stainless, tool steel, anything that fights the tap 18.1 mm 0.7126 18.10 58.5%
Form (roll) tap Theoretical 18.9 mm - starting point, check your tap maker's chart 18.9 mm 0.7441 18.90 —

Clearance holes for a M20 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 M20 x 2.5 per ISO 273. Clearance depends on the bolt diameter alone, so every M20 thread shares these figures.
Fit Hole mm Hole in
Fine 21.00 0.8268
Medium 22.00 0.8661
Coarse 24.00 0.9449

M20 x 2.5 dimensions

Nominal dimensions for M20 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 0.7874 20.00
Basic internal minor diameter 0.6809 17.29
Pitch 0.0984 2.50
Thread designation M20 x 2.5

What the hole looks like

Cross-section of a M20 x 2.5 tapped hole, drawn to scale The drilled hole is 17.5 mm, 0.689 inches or 17.50 millimetres across. The tap cuts outward from that wall to the major diameter of 0.7874 inches. A 100 percent thread would reach in to the basic minor diameter of 0.6809 inches, shown as a dashed line; the difference is why this drill gives 77.0% of thread. major 20 mm drill 17.5 mm · 17.5 mm dashed = 100% thread at 17.29 mm
M20 x 2.5 in section, to scale. The accent line is the thread the tap cuts from a 17.5 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 M20 x 2.5

Pre-filled with M20 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

17.7 mm0.6969 in70.8% thread

Theoretical hole 17.73 mm

One size down, deeper thread17.6 mm 0.6929 in · 73.9%

One size up, easier tapping17.8 mm 0.7008 in · 67.7%

For comparison, the published chart drill for M20 x 2.5 is 17.5 mm at 77.0% thread.

Notes on M20 x 2.5

Coarse pitch
M20 x 2.5 is the coarse pitch at 20 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: M20 x 2, M20 x 1.5, M20 x 1.
What the standard drill gives you
17.5 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
18.1 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: 18.9 mm, against a theoretical 18.9 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 20 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 20 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 M20 x 2.5 hole and forcing one will strip both.

Guides for tapping M20 x 2.5

Longer reads on the decisions behind the numbers above.

All guides →

M20 x 2.5 tap drill: common questions

What size drill bit do I need for a M20 x 2.5 tap?
A 17.5 mm drill, 0.689 in. That leaves 77.0% of thread, which is the standard chart figure for M20 x 2.5.
What is the drill size for a M20 x 2.5 tap in inches?
The 17.5 mm tap drill measures 0.689 in. If you only have inch drills, 11/16 in (0.6875 in) is the closest, giving 78.2% thread.
What drill should I use for M20 x 2.5 in stainless steel?
Use 18.1 mm instead of 17.5 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 M20 x 2.5 the same as M20 x 2?
No. Both are 20 mm threads, but the pitch differs, so the drills differ: 17.5 mm for M20 x 2.5 and 18 mm for M20 x 2. The taps and the fasteners are not interchangeable.
What is the clearance hole for an M20 x 2.5 bolt?
For a medium fit, 22 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.