How Tap Drill Charts Actually Work

Every tap drill chart on the internet gives you the same number for "M8" or "1/4-20" without saying why that number and not the one next to it. Here is where it comes from, and the two mistakes that break more taps than bad technique does.

A tap drill is a compromise, not a measurement

A tap cuts threads by displacing material outward from a pilot hole. Drill the hole exactly to the thread's minor diameter and there is no room for the tap to cut — it binds and snaps. Drill it to the major diameter and there is nothing left for the threads to bite into. The tap drill size sits between the two, sized so the resulting threads engage a chosen percentage of full depth.

Why 75%, and not 100%

Full (100%) thread engagement is almost never the target, for a reason that surprises people who assume more contact means more strength: past roughly 75% engagement, thread strength keeps climbing but the torque needed to cut it climbs faster, and tap breakage in a blind hole climbs faster still — a snapped tap turns a two-minute job into a drilled-and-plugged repair. At 75% engagement in most workpiece materials, the threaded joint is already stronger than the bolt that goes into it, so the extra 25% buys risk without buying usable strength. That is why 75% became the number stamped on shop-wall charts, not because it was rounded for convenience.

Why letters and numbers exist at all

Fractional drills step in 1/64 in (0.0156 in) increments. Below about a quarter inch, that gap is too coarse to land on the engagement percentage a tap actually needs — the nearest fraction is sometimes several points off 75%. Number drills (from #80, the smallest, up to #1) and letter drills (A up to Z) exist purely to fill those gaps with finer steps, in the exact range where small taps live. A "letter F drill" or a "#7 drill" isn't a different kind of drill bit — it's the same twist drill, just named from a size chart instead of a fraction, because the fraction that size would need doesn't exist.

The mistake that costs the most taps

Metric and inch threads don't share drill sizes, even when the threads look close. An M8×1.25 tap and a 5/16-18 tap cut threads of almost the same diameter, but their correct tap drills differ enough to matter — using one thread's chart entry for the other is the single most common way a close-enough-looking hole ends up under- or over-engaged. If a print calls out a thread, look up that exact thread, not the nearest one you recognize.

Blind holes need extra depth, not a bigger drill

A tap can't cut threads in the last bit of a blind hole's cone-shaped bottom, so drilling only to the length you need threaded leaves the last few threads unusable. The fix is depth, not diameter: drill deeper than the thread length by roughly the tap's own diameter, so the usable threaded length is what the print actually asked for.

Worked example

Take an M8×1.25 thread. Its 75%-engagement tap drill is 6.8 mm, and the clearance hole for the bolt itself — a different number, for a different job — is 9 mm for a normal fit. Both are on the tap drill chart, row by row for every common thread. If you already know the drill and want to work backwards — "what does a letter F drill actually tap?" — the drill size chart marks the tap each number, letter and fractional drill is for. And if you need the pitch and pitch diameter of an odd thread, the thread pitch calculator works it out.

A chart for the wall

The whole range — metric coarse and fine with their clearance holes, and UNC and UNF with the drill in inches and millimetres — is on one printable tap drill chart, sized for A4 and US Letter.