Tap Drill Size Chart

M8 x 1.25 Tap Drill Size

Tap drill

6.8 mm 0.2677 in

Use a 6.8 mm drill (0.2677 in) for 73.9% thread.

Fine thread M8 x 1? Use 7 mm. (0.2756 in, 77.0% thread)

The drill size for a M8 x 1.25 tap is 6.8 mm (0.2677 in). If you are wondering what size drill bit a M8 x 1.25 tap needs, that is the one: it leaves 73.9% 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 M8 x 1.25

Tap drills and clearance drills for M8 x 1.25 (Metric coarse, 1.25 mm pitch). Percent of thread is the figure stored for each drill, which reproduces from the major diameter and pitch of M8 x 1.25.
Use Drill Decimal in mm % thread
Standard tap drill Cut tap, general use (D - P) 6.8 mm 0.2677 6.80 73.9%
Inch substitute Closest inch drill if you have no metric set 17/64″ 0.2656 6.75 77.2%
Hard material Stainless, tool steel, anything that fights the tap 7 mm 0.2756 7.00 61.6%
Form (roll) tap Theoretical 7.45 mm - starting point, check your tap maker's chart 7.4 mm 0.2913 7.40 —

Clearance holes for a M8 x 1.25 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 M8 x 1.25 per ISO 273. Clearance depends on the bolt diameter alone, so every M8 thread shares these figures.
Fit Hole mm Hole in
Fine 8.40 0.3307
Medium 9.00 0.3543
Coarse 10.00 0.3937

M8 x 1.25 dimensions

Nominal dimensions for M8 x 1.25. 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 0.315 8.00
Basic internal minor diameter 0.2617 6.65
Pitch 0.0492 1.25
Thread designation M8 x 1.25

What the hole looks like

Cross-section of a M8 x 1.25 tapped hole, drawn to scale The drilled hole is 6.8 mm, 0.2677 inches or 6.80 millimetres across. The tap cuts outward from that wall to the major diameter of 0.315 inches. A 100 percent thread would reach in to the basic minor diameter of 0.2617 inches, shown as a dashed line; the difference is why this drill gives 73.9% of thread. major 8 mm drill 6.8 mm · 6.8 mm dashed = 100% thread at 6.65 mm
M8 x 1.25 in section, to scale. The accent line is the thread the tap cuts from a 6.8 mm hole; the dashed line is where the crest would sit at 100% thread. The gap between them is the 26.1% of thread this drill leaves behind.

Work out a different hole for M8 x 1.25

Pre-filled with M8 x 1.25. 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

6.9 mm0.2717 in67.7% thread

Theoretical hole 6.86 mm

One size down, deeper thread6.8 mm 0.2677 in · 73.9%

One size up, easier tappingI 0.272 in · 67.2%

For comparison, the published chart drill for M8 x 1.25 is 6.8 mm at 73.9% thread.

Notes on M8 x 1.25

Coarse pitch
M8 x 1.25 is the coarse pitch at 8 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: M8 x 1, M8 x 0.75.
What the standard drill gives you
6.8 mm leaves 73.9% of thread (D - P). That is in the normal 65-80% band most shop charts aim for.
Stainless, tool steel and other tough material
7 mm opens the hole to 61.6% 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: 7.4 mm, against a theoretical 7.45 mm. Treat that as a starting point and check your tap maker's chart.
Closest inch size
5/16-18 is the nearest inch thread by diameter (7.94 mm against 8 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

5/16-18 F · 0.257 in · 6.53 mm · 76.9% 7.94 mm major against 8 mm

Not interchangeable. 5/16-18 is the nearest size by diameter, nothing more. The pitches differ, so a 5/16-18 fastener will not run into a M8 x 1.25 hole and forcing one will strip both.

Guides for tapping M8 x 1.25

Longer reads on the decisions behind the numbers above.

All guides →

M8 x 1.25 tap drill: common questions

What size drill bit do I need for a M8 x 1.25 tap?
A 6.8 mm drill, 0.2677 in. That leaves 73.9% of thread, which is the standard chart figure for M8 x 1.25.
What is the drill size for a M8 x 1.25 tap in inches?
The 6.8 mm tap drill measures 0.2677 in. If you only have inch drills, 17/64 in (0.2656 in) is the closest, giving 77.2% thread.
What drill should I use for M8 x 1.25 in stainless steel?
Use 7 mm instead of 6.8 mm. It leaves 61.6% of thread rather than 73.9%, so there is less material for the tap to cut and less chance of snapping it.
Is M8 x 1.25 the same as M8 x 1?
No. Both are 8 mm threads, but the pitch differs, so the drills differ: 6.8 mm for M8 x 1.25 and 7 mm for M8 x 1. The taps and the fasteners are not interchangeable.
What is the clearance hole for an M8 x 1.25 bolt?
For a medium fit, 9 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.