Tap Drill Chart

Weight Calculator Thread Size Chart Tap Drill Chart Limits & Fits Insert Installation Alloy Density All tools

Tap-drill calculator — Metric · UNC · BSP · NPT

Select a thread series and size for the recommended tap-drill (~75% engagement for metric/UNC). The table follows your selected series.

Thread series

Thread size

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tap drill (mm)

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tap drill (inch)

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pitch / TPI

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clearance Ø (mm)

Thread	Pitch / TPI	Tap drill (mm)	Tap drill (inch)	Clearance (close / normal)

Metric/UNC tap-drill ≈ major Ø − pitch (≈75% engagement); use a slightly larger drill in free-machining brass to reduce tap load. BSP G drills are for parallel threads (ISO 228-1). NPT holes should be drilled then reamed with a 1:16 taper reamer before tapping for full thread form.

References: ISO 261 / ISO 724 / ISO 273 (metric), ASME B1.1 & Machinery's Handbook tapping data (UNC), ISO 228-1 (G), ASME B1.20.1 (NPT).

The 75% rule

For metric threads the tap drill is simply nominal Ø − pitch (M6 − 1.0 = 5.0 mm), giving ≈75% thread engagement. Beyond 75%, strength gains are marginal while tapping torque climbs steeply — 60–75% is the engineering sweet spot. The inch equivalent: drill = major Ø − (1/TPI).

Tapping brass specifically

Free-machining brass (CW614N/C36000) taps beautifully at 70–75% engagement with short, broken chips. Lead-free grades like CW724R produce tougher chips — use the larger drill of the range, sharp spiral-point taps and lower speeds. For NPT, drill then ream 1:16 before tapping; for parallel G threads the drilled bore is cylindrical and sealing happens on the face, not the thread.

From chart to finished part

On our CNC and sliding-head lathes tapped holes are produced in the same setup as turning — rigid tapping to 6H, thread milling for large or interrupted threads. See CNC machining capabilities.

More engineering tools

Weight Calculator

Instant weight of round, hex, square or octagon bar in 20 brass, copper and aluminium allo…

Thread Size Chart

Major, pitch and minor diameters for ISO metric, UNC, BSP (G) and NPT threads — with tap drill sizes.

Limits & Fits

Type a size and a class or fit — H7, g6, H7/g6, H7/p6 — and get limit dimensions and deviations.

Insert Installation

Boss hole diameters, heat-set tip temperatures and seating-force guidance for brass threaded inserts.

Alloy Density

Nominal densities of 20 brass, copper and aluminium alloys — CW614N 8.50, CW617N 8.47, C36000 8.49.

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Frequently asked questions

What size drill do I use for an M6 tap?

5.0 mm — the metric rule of thumb is tap drill = nominal diameter − pitch (M6 − 1.0 = 5.0), which gives ≈75% thread engagement.

What are the common metric tap drill sizes?

M3 → 2.50 mm, M4 → 3.30 mm, M5 → 4.20 mm, M6 → 5.00 mm, M8 → 6.80 mm, M10 → 8.50 mm, M12 → 10.20 mm (≈75% engagement, coarse pitch).

Why 75% thread engagement and not 100%?

Above ~75%, joint strength gains are marginal but tapping torque and tap breakage risk rise sharply. 60–75% is the practical range; in free-machining brass 70–75% taps cleanly.

What drill for a G1/2 BSP thread?

19.0 mm for the parallel G1/2 (ISO 228-1). BSP parallel threads seal on a face, so the hole is a plain cylindrical bore before tapping.

How do I prepare a hole for an NPT thread?

Drill to the chart size, then ream with a 1:16 taper reamer before tapping — NPT is tapered, so a straight drilled hole alone gives incomplete thread form at depth.