What size drill for an M12×1.25 tap?

10.8 mm (10.8 mm), which gives about 75% thread engagement.

What clearance hole for M12×1.25?

13 mm for a close fit — enough to pass the bolt without slop.

What spanner fits M12×1.25?

18 mm across the flats on a standard hex head.

M12×1.25 thread

M12×1.25 tap drill

10.8 mm

Clearance hole 13 mm · 18 mm hex

M12×1.25 is a fine metric thread with a 12 mm (0.472") major diameter and 1.25 mm pitch. Tap it with a 10.8 mm drill; drill 13 mm to pass a bolt through.

The numbers

Designation: M12×1.25
Major diameter: 12 mm / 0.472"
Minor diameter: 10.647 mm
Pitch: 1.25 mm pitch
Tap drill (75% thread): 10.8 mm — 10.8 mm
Clearance hole (close fit): 13 mm / 0.512"
Hex across flats: 18 mm

How to measure what you have

Measure across the thread crests with calipers. A used M12×1.25 reads a little under 12 mm — the crests are truncated by the standard, then worn and plated, so anything from about 11.6 mm up is consistent with this size.
Measure the pitch: put a rule against the threads and count the distance over ten crests, then divide by ten. This one should give 1.25 mm.
A thread pitch gauge settles it in seconds and costs very little. Guessing between a coarse and a fine thread of the same diameter is the usual way a thread gets stripped.

Tightening torque

Typical torque

78–106 N·m

57.5–78.2 lb·ft

Plain steel property class 8.8 or SAE grade 5, dry threads, tightened to about 65% of yield. Lubricated threads reach the same preload at 20-30% less torque. Where the manufacturer publishes a figure — spark plugs, wheel nuts, cylinder heads — use theirs.

Where this size turns up

Wheel studs on Nissan, Subaru, Mitsubishi and older Toyota
Modern 12 mm spark plugs
Fine-pitch suspension adjusters

Other threads at this diameter

Same diameter, different pitch. Forcing the wrong one strips the thread in a turn or two.
Thread	Pitch	Tap drill	Clearance
M12×1.75	1.75 mm	10.2 mm	13 mm
M12×1.5	1.5 mm	10.5 mm	13 mm
M12×1	1 mm	11 mm	13 mm

Common questions

What size drill for an M12×1.25 tap?: 10.8 mm (10.8 mm), which gives about 75% thread engagement.
What clearance hole for M12×1.25?: 13 mm for a close fit — enough to pass the bolt without slop.
What spanner fits M12×1.25?: 18 mm across the flats on a standard hex head.

M12×1.75 10.2 mm tap drill
M12×1.5 10.5 mm tap drill
M12×1 11 mm tap drill
M10×1.5 8.5 mm tap drill
M14×2 12 mm tap drill
M10×1.25 8.8 mm tap drill
M10×1 9 mm tap drill
M14×1.5 12.5 mm tap drill
M14×1.25 12.8 mm tap drill
M8×1.25 6.8 mm tap drill
M16×2 14 mm tap drill
M8×1 7 mm tap drill
M8×0.75 7.2 mm tap drill

Sources

Official vendor product documentation — At One Place
Typical range compiled from manufacturer specifications — At One Place

How these figures are compiled and checked