ISO metric · fine

M12 × 1.25 tap drill size

For general work, OSG suggests 65–70% of full thread. Harder materials and deep holes want less; the calculator sets it from your material.

Cut tap

10.9 mm

0.4291" · 68% thread

Form tap

11.4 mm

0.4488" · 71% thread

Open M12 × 1.25 in the calculator

Pick your material for its % of thread, tapping RPM and the rigid-tap code.

Thread dimensions

Major Ø: 12.000 mm (0.4724")
Pitch: 1.25 mm
Pitch Ø (basic): 11.188 mm (0.4405")
Minor Ø (internal, basic): 10.647 mm (0.4192")
Thread depth (internal): 0.677 mm (0.0266")

Basic sizes of the 60° form. Tolerance classes (6H) set the limits around them.

Cut tap drills

For a cutting (fluted or spiral) tap. Every standard drill from 50% to 85% thread; suggested rows are in the 65–70% band.

Cut tap drills for M12 × 1.25
Drill	Ø in	Ø mm	Thread
10.7 mm	0.4213	10.70	80%
27/64	0.4219	10.72	79%
10.8 mm	0.4252	10.80	74%
10.9 mmsuggested	0.4291	10.90	68%
11 mm	0.4331	11.00	62%
11.1 mm	0.4370	11.10	55%
7/16	0.4375	11.11	55%

Form tap drills

A form (roll) tap displaces metal instead of cutting it, so it needs a bigger hole. Every standard drill from 50% to 85% thread; suggested rows are in the 65–70% band.

Form tap drills for M12 × 1.25
Drill	Ø in	Ø mm	Thread
11.3 mm	0.4449	11.30	82%
11.4 mm	0.4488	11.40	71%
11.5 mm	0.4528	11.50	59%
29/64	0.4531	11.51	58%

3-wire measurement

Best wire size: 0.722 mm (0.0284")
Over wires, at basic pitch Ø: 12.271 mm (0.4831")

For the external thread (the screw). The best wire touches the flanks at the pitch diameter.

Sources

ISO 261 / ISO 262 — ISO general purpose metric screw threads: general plan and selected sizes
OSG — Tap drill size and pitch diameter limit chart (formula, suggested % of full thread)
Cutting Tool Engineering — "Threads will roll": form tap hole size = D − 0.0068 × % / TPI
ASME B94.11M — Twist drills (fractional, number, letter and metric sizes)
NIST (NBS) Letter Circular 23 and Pratt & Whitney — three-wire thread measurement, best wire size