Tap Drill Size Calculator — metric and imperial

Tap drill diameter and clearance hole for metric and imperial threads, at any thread engagement.

Example: M3 × 0.5 needs a 2.5 mm tap drill and a 3.4 mm clearance hole.

Inputs

Thread

Thread engagement(%)

75% is standard. Lower means an easier tap and only slightly less strength.

Result

Published tap drill

2.5 mm

Computed at 75%: 2.594 mm
Computed in inches: 0.1021 in
Clearance hole: 3.4 mm
Major diameter: 3 mm
Pitch: 0.5 mm

For metric coarse threads the shortcut is simply major diameter minus pitch — 3 − 0.5 = 2.5 mm, which is where the published 2.5 mm comes from.
These figures are for tapping into metal. Plastics, especially soft ones, generally want a slightly larger hole because the material displaces rather than cutting cleanly.

Hole size against thread engagement

Engagement	Tap drill (mm)	In inches	Notes
50%	2.729	0.1075	weak thread
60%	2.675	0.1053	weak thread
65%	2.648	0.1043
70%	2.621	0.1032
75%	2.594	0.1021	standard
85%	2.54	0.1	hard to tap
100%	2.459	0.0968	hard to tap

Common threads

Thread	Major (mm)	Pitch (mm)	Tap drill	Clearance
M2 × 0.4	2	0.4	1.6	2.4
M2.5 × 0.45	2.5	0.45	2.05	2.9
M3 × 0.5	3	0.5	2.5	3.4
M4 × 0.7	4	0.7	3.3	4.5
M5 × 0.8	5	0.8	4.2	5.5
M6 × 1.0	6	1	5	6.6
M8 × 1.25	8	1.25	6.8	9
M10 × 1.5	10	1.5	8.5	11
M12 × 1.75	12	1.75	10.2	13.5
#4-40 UNC	2.845	0.635	#43 (2.26)	#32 (2.95)
#6-32 UNC	3.505	0.794	#36 (2.7)	#27 (3.66)
#8-32 UNC	4.166	0.794	#29 (3.45)	#18 (4.31)
#10-24 UNC	4.826	1.058	#25 (3.8)	#9 (4.98)
#10-32 UNF	4.826	0.794	#21 (4.04)	#9 (4.98)
1/4"-20 UNC	6.35	1.27	#7 (5.11)	17/64″ (6.75)

Formula

tap drill = major diameter − (thread % / 100) × 1.0825 × pitch

Worked example

M3 × 0.5 needs a 2.5 mm tap drill and a 3.4 mm clearance hole.

tap drill = major − (% / 100) × 1.0825 × pitch
3 − (75/100) × 1.0825 × 0.5
2.594 mm
1.0825 is the full thread height for a 60° ISO or Unified profile.

Frequently asked questions

What size drill do I need for an M3 tap?

2.5 mm. For metric coarse threads the rule is major diameter minus pitch, so M3 × 0.5 gives 3 − 0.5 = 2.5 mm. The same shortcut works for M4 (3.3 mm), M5 (4.2 mm) and M6 (5.0 mm).

What is thread percentage and does more mean stronger?

It is how deeply the thread is cut into the hole wall. Barely: going from 75% to 100% adds only about 5% holding strength while roughly doubling the tapping torque and greatly raising the risk of snapping the tap. 75% is standard for that reason.

Why do imperial taps use numbered drills?

Historical accident. Numbered and lettered drill sizes predate the standardisation of screw threads, and the closest available bit to each theoretical tap drill just happened to be a numbered one. There is no pattern — you look it up.

What is the difference between a tap drill and a clearance hole?

The tap drill is smaller than the screw and leaves material for the thread to be cut into. The clearance hole is larger than the screw so it passes straight through. A joint normally has a clearance hole in the top part and a tapped hole in the bottom.

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