Reference

Tap Drill & Thread Chart

Tap drill sizes for common inch and metric threads at about 75% thread engagement, plus how to size a clearance hole for the mating screw.

To cut an internal thread, you drill a hole slightly larger than the thread's minor diameter, then tap it. The standard target is about 75% thread engagement, which holds nearly all the strength of a full thread while cutting far less material and taking less tapping torque. The inch sizes below are the standard 75% tap drills; the metric coarse rule is simply tap drill = nominal diameter minus pitch. To pass a screw through a part instead of threading it, use a clearance hole sized larger than the screw.

Inch tap drills (UNC / UNF, ~75% thread)

These are the standard 75%-thread tap drills for common inch machine-screw and fractional threads. Coarse (UNC) and fine (UNF) share several sizes; both are listed where common.

Thread	Tap drill	Decimal (in)
#4-40 UNC	#43	0.0890
#6-32 UNC	#36	0.1065
#8-32 UNC	#29	0.1360
#10-24 UNC	#25	0.1495
#10-32 UNF	#21	0.1590
1/4-20 UNC	#7	0.2010
1/4-28 UNF	#3	0.2130
5/16-18 UNC	F	0.2570
3/8-16 UNC	5/16	0.3125
1/2-13 UNC	27/64	0.4219

Standard 75%-thread tap drills. Softer materials (aluminum, brass, plastics) tap well at 75%; in hard steels many shops drop to ~65% thread with a slightly larger drill to cut tapping torque and reduce broken taps.

Metric coarse tap drills (tap drill = nominal minus pitch)

For metric coarse threads the tap drill in millimeters equals the nominal diameter minus the pitch. For example, M6 x 1.0 uses a 6 - 1.0 = 5.0 mm drill.

Thread	Pitch (mm)	Tap drill (mm)
M3	0.5	2.5
M4	0.7	3.3
M5	0.8	4.2
M6	1.0	5.0
M8	1.25	6.8
M10	1.5	8.5

Coarse-pitch threads. Fine-pitch metric threads use a smaller pitch and therefore a larger tap drill (e.g. M8 x 1.0 uses a 7.0 mm drill).

Clearance holes (for passing a screw through)

When a screw passes through a part rather than threading into it, drill a clearance hole larger than the screw's major diameter. ISO 273 defines three fits for metric fasteners: close, normal (medium), and loose. Normal fit is the usual default.

Screw	Clearance drill (normal fit)
M3	3.4 mm
M4	4.5 mm
M5	5.5 mm
M6	6.6 mm
M8	9.0 mm
M10	11.0 mm

ISO 273 normal (medium) fit. Use close fit for precise location and loose fit for more play. For inch screws, a clearance hole is typically the screw major diameter plus about 0.010 to 0.030 in depending on fit.

Inch tap drills follow the standard 75%-thread chart; metric coarse follows tap drill = nominal minus pitch; clearance holes follow ISO 273. Confirm thread class, material, and fit for your part before cutting.

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Frequently Asked Questions

What size drill do I use for a 1/4-20 tap?

Use a #7 drill (0.201 in) for a standard 75%-thread 1/4-20 UNC tapped hole. In harder materials some shops step up slightly (toward ~65% thread) to reduce tapping torque and the risk of breaking the tap.

How do I calculate a metric tap drill size?

For metric coarse threads, tap drill diameter (mm) equals the nominal diameter minus the pitch. M6 x 1.0 uses 6 - 1.0 = 5.0 mm; M8 x 1.25 uses about 6.8 mm. Fine-pitch threads have a smaller pitch, so they need a larger tap drill.

What is 75% thread engagement?

It is the depth of thread the tap cuts relative to a theoretical full (100%) thread. A 75% thread holds nearly all the strength of a full thread while cutting much less material, so it is the standard target on most tap drill charts and lowers tapping torque and tap breakage.

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

A tap drill is sized just above the thread's minor diameter so the hole can be tapped with internal threads. A clearance hole is larger than the screw's major diameter so the screw passes through freely. Use the tap drill where threads are cut and the clearance drill where a screw only passes through.

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