Tap Drill Size Chart: Metric (M2-M30) & UNC/UNF (#4-1") with the Formula Behind It

The tap drill is not the bolt’s size — it is smaller, and by a specific, computable amount. Get it wrong in either direction and you either strip the threads before the bolt ever sees full load, or snap the tap off inside the hole. Here is the exact formula, the full metric and inch charts it produces, and the two decisions — material hardness and hole depth — that the chart alone will not make for you.

Every number in the two tables below comes from one line of arithmetic, run in this article’s build script with numpy, not typed in by hand. If you already know your thread’s pitch or TPI — use the identify the thread pitch guide or our thread pitch chart if not — you can also just do the subtraction yourself.

The formula, and where it comes from

The rule that every machinist chart on the wall is built from is short enough to do in your head:

• Inch: tap drill = major diameter − (1 ÷ TPI)
• Metric: tap drill = major diameter − pitch

Both lines are the same formula — inch threads just specify pitch indirectly, as “threads per inch” instead of a millimetre distance. Wikipedia’s Tap and die article states this exact rule and notes plainly that it “ultimately results in an approximate 75% thread” — not exactly 75%, because the formula is a shop shortcut for a more involved thread-height calculation, but close enough that nobody bothers with the longer version by hand.

Why 75% and not 100%? Because 100% thread — drilling exactly to minor diameter — gives you almost no extra holding strength over 75%, but multiplies the torque needed to cut the thread and the odds of snapping the tap off in the hole. 75% has been the standard compromise for so long that it is simply what “the tap drill size” means unless a print says otherwise.

When to drop to 50–60%: hard materials

The 75% figure assumes a workpiece the tap can cut through without a fight — aluminum, brass, mild steel, plastic. Steel, stainless and cast iron cut harder, and a tap that is already working at 75% engagement in a soft metal can bind and snap in a hard one. Little Machine Shop’s own published tap chart heads its two drill columns exactly this way: “75% Thread for Aluminum, Brass, & Plastics” next to “50% Thread for Steel, Stainless, & Iron.” Dropping to 50–60% engagement means a slightly larger drill, a shallower thread, and a tap that survives the cut — a trade worth making any time the hole is blind, the material is hard, or you are tapping by hand without a drill press to keep the tap square.

Metric tap drill chart (M2–M30)

Coarse pitch first — this is what a bare metric size (“M8”, no second number) means unless a print specifies fine:

Fine pitch, where it is commonly used (M8 and up):

Fine threads use a shallower, more closely spaced thread and therefore a slightly larger tap drill than the coarse version of the same size. See the bolt size chart for the major diameters these pitches attach to.

UNC / UNF tap drill chart (#4–1″)

UNC (coarse, the inch default):

UNF (fine):

One quirk worth knowing: 8-32 (UNC) and 8-36 (UNF) both call for a #29 drill. Same drill, different tap, different depth of thread — the drill size alone never tells you which thread you are cutting.

Cutting taps vs. forming (roll) taps: the drill gets bigger

Everything above assumes a standard cutting tap, which removes metal with flutes the same way a drill bit does, and produces chips you have to clear. A forming tap — also called a roll tap, cold-form tap, or fluteless tap — cuts nothing at all. It has no flutes and no cutting edges; instead, lobes spaced around the tap forge the thread by displacing metal sideways into shape as the tap advances, the way a rolling die forms threads on a bolt blank.

Because a forming tap has to push material into the thread crest rather than cut it away, it needs a larger pre-drilled hole than a cutting tap of the same nominal size and %thread — there has to be somewhere for the displaced material to go. Tap manufacturers publish separate drill charts for form taps for exactly this reason; do not use a cutting-tap drill size from the tables above and expect a form tap to thread it cleanly. In exchange, form taps leave the surrounding grain structure compressed instead of cut, which is why they commonly outlast cutting taps several times over and rarely produce loose chips inside a blind hole — a real advantage in aluminum castings and thin sheet.

Blind-hole depth: drill deeper than the thread you need

A tap cannot cut full-depth thread all the way to its own tip. The first several threads on every tap are ground on a taper — the “chamfer” — so the tap can start square and work its way to full depth gradually instead of trying to cut a complete thread in one bite. A standard plug tap chamfers its first 3–5 threads; a bottoming tap, ground with almost no taper at all, chamfers only 1–1.5 threads so it can finish threads right to the bottom of a blind hole.

That chamfer has to go somewhere, and in a blind hole it has to go past the last full thread you need, into extra drilled depth the tap cannot use. Drill only exactly as deep as the thread length you want and you will get several partially-formed, useless threads at the bottom instead. The practical fix: drill the hole deeper than the thread engagement you need by at least the chamfer length of whichever tap finishes the hole, then finish with a bottoming tap if you need thread genuinely to the bottom. Starting with a plug tap and switching to a bottoming tap to finish — the sequence machinists actually use on a hard blind hole — needs less extra depth than trying to finish with a plug tap alone.

Try it yourself

No shop required for the first two.

1. Do the subtraction on a bolt you already have. Read its size and pitch (a thread pitch gauge or the identify the thread pitch guide gets you there in seconds), subtract the pitch, and check your answer against the tables above.
2. Sanity-check a drill you own. Measure it with calipers, and see which row of the chart it lands closest to — that tells you which tap it is sized to start, without reading the stamp on the shank.
3. Test-drill scrap before you commit. Drill and tap a test hole in the same material as the real job before you cut the good part — the five minutes it costs is always cheaper than a broken tap stuck in a finished piece.

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Frequently asked questions

Keep going

Bottom line: the tap drill is major diameter minus pitch, for a standard 75% thread in ordinary material — drop to 50–60% (a slightly bigger drill) in hard material or a blind hole, use the manufacturer’s own chart for a forming tap, and drill past your needed thread depth by the finishing tap’s chamfer length any time the hole does not go all the way through.

Sources

• Wikipedia: Tap and die — the tap drill formula (both inch and metric forms), the approximate-75%-thread note, plug/bottoming tap chamfer lengths, and the forming (roll) tap description.
• Little Machine Shop: Tap & Clearance Drill Sizes — the published metric and inch (UNC/UNF) 75%/50% drill charts used to verify every computed row, and the 75%-soft-material vs. 50%-hard-material framing.
• Newman Tools: Tap Drill Sizes — a second independently published metric and inch chart, and its explicit statement of the major-diameter-minus-pitch rule.
• Wikipedia: Drill bit sizes — the standard number, letter and fractional twist-drill decimal equivalents used for the nearest-metric-drill conversion column.