How do I work out the tap drill size for a metric thread?

For a metric thread the tap drill is the nominal diameter minus the pitch. An M8 x 1.25 thread takes a 6.8 mm drill. That rule of thumb targets roughly 75 percent thread engagement; for harder materials a slightly larger drill reduces the risk of tap breakage.

What tap drill do I use for a unified (inch) thread?

For unified threads the tap drill is the major diameter minus one divided by the thread per inch. A 1/4-20 thread takes 0.250 minus 0.050, which is 0.200 inch. The calculator returns the nearest common drill size and the exact decimal equivalent.

What is the pitch diameter of a thread?

The pitch diameter is the diameter where the thread flanks and the groove are equal in width. For a 60-degree thread with the ISO 68 basic profile it is the major diameter minus 0.649519 times the pitch. Pitch diameter, not major diameter, is what governs fit between a screw and a nut.

Should I use a different tap drill for stainless or titanium?

Yes. Thread engagement above roughly 65 percent raises tapping torque sharply, and work-hardening alloys such as 304 stainless and titanium are far less forgiving. Machining handbooks generally recommend a slightly larger tap drill plus a proper cutting fluid for those materials.

Thread & Tap Drill Size Chart

Basic thread dimensions and the tap drill to use before threading, for metric and unified series. The table covers common sizes; the calculator handles any diameter and pitch you type in.

Thread dimensions

Series

Nominal / major diameter

Millimetres.

Pitch

Millimetres per thread.

Thread engagement (%)

Calculated locally in your browser from the ISO 68 / ASME B1.1 basic profile. Class-of-fit limits are not included — consult the standard tables for those.

Result

Pitch diameter— Minor diameter (external)— Thread height H— Tap drill— Nearest common drill—

H = 0.866025 × P. Pitch dia = major − 0.649519 × P. Minor dia (external) = major − 1.226869 × P. Tap drill = major − (engagement ÷ 100) × 1.29904 × P.

Copy-ready text

Reference

Common metric coarse threads

Thread	Pitch (mm)	Pitch dia (mm)	Tap drill (mm)
M3 x 0.5	2.675	2.51	2.5 mm
M4 x 0.7	3.545	3.32	3.3 mm
M5 x 0.8	4.480	4.22	4.2 mm
M6 x 1.0	5.350	5.03	5.0 mm
M8 x 1.25	7.188	6.78	6.8 mm
M10 x 1.5	9.026	8.54	8.5 mm
M12 x 1.75	10.863	10.29	10.2 mm
M16 x 2.0	14.701	14.05	14.0 mm
M20 x 2.5	18.376	17.56	17.5 mm

Values computed from the ISO 68 basic profile at 75 percent thread engagement. For stainless, titanium and other work-hardening alloys, a drill one step larger is usually the safer choice.

Reference

Common unified inch threads (UNC / UNF)

Thread	Major (in)	Pitch dia (in)	Tap drill (in)
#6-32 UNC	0.1177	0.1076	#36
#8-32 UNC	0.1437	0.1336	#29
#10-24 UNC	0.1629	0.1494	#25
1/4-20 UNC	0.2175	0.2013	#7
5/16-18 UNC	0.2764	0.2584	F (0.257)
3/8-16 UNC	0.3344	0.3141	5/16 in
1/2-13 UNC	0.4500	0.4251	27/64 in
1/4-28 UNF	0.2268	0.2152	#3
3/8-24 UNF	0.3479	0.3344	Q (0.332)
1/2-20 UNF	0.4675	0.4513	29/64 in

Pitch diameter = major − 0.649519 ÷ TPI. Tap drill = major − 1 ÷ TPI, which corresponds to roughly 75 percent engagement. Class 2A/2B limits are defined in ASME B1.1 and are not reproduced here.

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Basis & references: ASME B1.1 — Unified Inch Screw Threads · ISO 68-1 — ISO general purpose screw threads, basic profile · ISO 965-1 — ISO general purpose metric screw threads, tolerances

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