Metric Tap Drill Size Chart: M2 to M24, and the Rule Behind It

Ask what drill to use for an M10 tap and you will hear three different answers in the same workshop. Only one of them — 8.5 mm — comes off the chart, and the chart comes from a rule so short it fits on a fingernail:

Tap drill = thread diameter − pitch

M10 has a 1.5 mm pitch, so 10 − 1.5 = 8.5 mm. M6 × 1.0 wants a 5.0 mm drill. M8 × 1.25 wants 6.75, which you round to the 6.8 mm bit that actually exists. The rule holds for every ISO metric thread, coarse or fine, and it is worth memorising because the day you need it, the chart is in the other toolbox.

Everything else on this page is the chart itself, plus the two or three decisions around it — clearance holes, blind holes, which tap style — that decide whether the thread goes in clean or the tap goes in the bin.

Tap drill chart — metric coarse

The clearance column is the hole for the part the bolt passes through — medium fit to ISO 273. Drill the tap size in the part that gets the thread, the clearance size in the part that does not. Mixing those two up is the oldest assembly mistake there is, and it still happens every day.

Fine pitch: the sizes that catch people out

Fine threads turn up on anything thin-walled, anything that vibrates, and most machine spindles. Same rule — diameter minus pitch — but the pitch is smaller, so the hole is bigger:

Note what happens if you grab the coarse chart by habit: an M12 × 1.5 hole drilled at 10.2 mm instead of 10.5 leaves the tap cutting far more metal than it was designed to. In mild steel it groans. In stainless it snaps.

Why the chart aims at 75%, not 100%

Diameter-minus-pitch gives roughly 75% thread engagement — the flanks are three-quarters formed, with a small flat left at the crest. That is not a compromise. It is the engineering optimum, and the numbers behind it are blunt:

• Going from 75% to 100% engagement roughly doubles the torque on the tap.
• The extra strength you buy with that torque is about 5% — a joint at 75% engagement already fails by bolt break or thread strip at practically the same load.
• A tap seized in a nearly-finished hole usually breaks off flush, and a snapped HSS tap cannot be drilled out with anything in the same toolbox.

So drilling a fraction over chart size is a legitimate move, not a bodge: in tough materials — stainless, spring steel — going to 65% engagement (about 0.1 mm over) cuts tapping torque by a third and costs nothing you can measure in the finished joint. In soft aluminium or thin sheet, stay on chart size; soft threads want all the flank they can get.

Match the tap to the hole, not the habit

Most people learn to tap with a straight-flute hand tap and never look at the label again. For production work the flute shape is the whole game, because it decides where the chips go:

• Spiral point ("gun") taps push the chip forward, out ahead of the tap. Fastest and strongest option — but only for through holes, where the chip has somewhere to go.
• Spiral flute taps pull the chip backward, up and out of the hole. This is the blind-hole tap: the chip never reaches the bottom, never packs, never seizes the tap.
• Straight flute taps just store the chip in the flutes. Fine for hand tapping, shallow holes and cast iron dust; wrong for deep blind holes in anything stringy.

From our range

Industrial HSS Spiral Point Machine Tap (Through Hole Dedicated) – OEM

Spiral point machine tap — the through-hole workhorse; chips exit forward.

From our range

Industrial M8x1.25 HSS Spiral Flute Machine Tap (Blind Hole) – OEM

Spiral flute machine tap — blind holes; chips lift back out of the hole.

Blind holes: drill deeper than you thread

A tap cannot cut thread to the bottom of a drilled hole — the chamfered lead of the tap needs somewhere to run out, and the chips need somewhere to sit. The working rule:

• Drill depth = thread depth you need + 4 × pitch as a minimum. For M8 × 1.25, add at least 5 mm of spare hole.
• Thread depth beyond 1.5 × diameter buys nothing — the first three threads carry most of the load, and past two diameters the extra engagement is decoration.
• In steel, a nut-height thread (0.8 × d) is already a full-strength joint. Deep threading is for soft materials, not strong ones.

And on the machine: tapping speed is a fraction of drilling speed. In mild steel an M6 machine tap wants somewhere around 300–400 RPM with cutting compound; in stainless, halve it and never pause mid-thread — see the same low-and-committed logic in the drill speed chart. Hand tapping, the old rhythm still stands: half a turn in, quarter back to break the chip.

For the counter and the container

If you stock taps for resale, the demand curve is short and steep: M6, M8 and M10 spiral point cover most of what a general workshop buys, with spiral flute M6–M10 behind them for blind work, and dies mostly bought singly for thread repair. HSS ground-thread taps hold size in a way carbon-steel cut-thread taps do not — the price gap is real, and so is the difference in warranty claims. A tap and die range specified that way sells on repeat, which is the only way tooling sells at all.

The other half of every threaded joint is the hole that does not get tapped — the clearance hole in the part the bolt passes through. Sizes for M1.6 to M24, plus counterbore dimensions for socket screws, are in the metric clearance hole chart.

To work backwards from a thread — and to set the engagement your material actually needs rather than the 75% this chart assumes — use the tap drill calculator.