Tap Drill Size Calculator

Where this formula comes from

For a standard 60° thread form, the theoretical full thread height works out to about 1.299 times the pitch. Thread engagement percentage describes how much of that full triangle actually gets cut into the hole wall, so the pilot hole diameter is the major diameter minus that fraction of the full thread height: drill = major diameter − (1.299 × pitch × engagement% / 100), where pitch is 1÷TPI for imperial threads. Plug in a 1/4-20 thread at 75% engagement and this gives 0.2013 in — the long-standard #7 drill (0.201 in) used for that exact thread.

Why 75% instead of 100%

A 100% thread engagement is stronger on paper but dramatically harder to tap, especially by hand, since it removes less material and leaves the tap fighting more resistance the whole way down — it also increases the odds of breaking a tap in a blind hole. 75% engagement gives roughly 90% of the full thread's strength in most materials while tapping noticeably easier, which is why it's the default most machinists reach for unless a print specifically calls out a different percentage.

Rounding to a real drill

This calculator gives you the exact theoretical diameter; the actual drill you chuck up needs to be a real, physical drill size. Round up to the nearest standard fractional, number, or metric drill using the Drill Bit Size Chart — rounding down instead would push engagement above your target and make the hole harder to tap than intended.

How to Tap a Hole by Hand, Step by Step

Tapping is the last operation in the hole, and it's the one most likely to go wrong if you rush it — a broken tap stuck in a near-finished part is one of the most frustrating scrap events in a shop. The process below covers hand tapping on a drill press or bench vise; the same sequence (minus the wrench) applies to tapping on a mill with a tapping head or floating holder.

What you need

Item	Notes
Correct tap drill	Use the calculator above, then round up to a real drill size from the Drill Bit Size Chart.
Tap (taper, plug, or bottoming)	Match to hole type — see Step 2 below.
Tap wrench or T-handle	Sized to the tap's square shank. A T-handle gives better feel for small taps; a two-handled wrench gives more control on larger ones.
Cutting fluid	Tapping fluid or cutting oil — see Step 5 for matching it to your material.
Countersink or deburring tool	To break the sharp edge at the hole mouth before starting the tap.

Step 1: Drill and deburr the pilot hole

Drill the pilot hole to the diameter this calculator gives you, rounded to the nearest real drill size. Then knock the sharp edge off the hole mouth with a countersink or deburring tool — a few seconds of work here keeps the tap from catching on a burr and starting crooked, and it gives the first thread a clean lead-in instead of a ragged one. For a blind hole, drill it about two to three thread pitches deeper than the thread length you actually need, since the tap's tapered end and any accumulated chips both eat into usable depth at the bottom.

Step 2: Pick the right tap for the hole

Hand tap sets usually come in three styles, distinguished by how many of the tap's leading threads are tapered to ease it into the cut. Picking the wrong one for the job is a common reason a hole "won't tap easy" even with a correctly sized pilot hole.

Type	Taper	Best for
Taper tap	~8-10 threads	Starting a hole, especially by hand, and through-holes where full thread depth isn't needed right at the surface.
Plug tap	~3-5 threads	General-purpose — the most common single tap to reach for, works in most through-holes and shallow blind holes.
Bottoming tap	~1-1.5 threads	Finishing threads as close as possible to the bottom of a blind hole, after starting with a taper or plug tap first.

A common sequence for a blind hole that needs full-depth threads: start and cut most of the hole with a taper tap, then finish the last few threads at the bottom with a bottoming tap. Trying to start a hole cold with a bottoming tap is asking for a broken tap — it has almost nothing to ease it into alignment.

Step 3: Start the tap square to the hole

Set the tap in the hole by hand and turn it a few turns under light pressure before the wrench ever touches it — you're feeling for even resistance, which means the tap is going in straight. If one side feels tighter than the other, back it out and try again rather than forcing it; a tap that starts crooked will cut a crooked, oversized, or off-center thread, and correcting it partway through usually breaks it. On a drill press, you can use the quill (with the motor off, spindle free to turn by hand) as a square-alignment guide: bring the tap down close to the hole, engage it by hand, and let the quill hold it true for the first several turns.

Step 4: Cut with a turn-and-back-off rhythm

Once the tap has started true, turn it forward about half to one full turn, then back it off a quarter turn. That reversal breaks the chip instead of letting it curl continuously ahead of the tap, which is what actually causes most tap breakage — not the cutting force itself, but a long chip jamming in the flutes and locking the tap solid. Repeat forward-and-back all the way to depth, and never force a tap that suddenly goes hard; back it out, clear it, and inspect the hole rather than pushing through resistance.

Step 5: Match cutting fluid to the material

Cutting fluid reduces the friction and heat that cause galling and tap breakage, and different materials call for different approaches. Steel and stainless benefit from a dedicated tapping fluid or cutting oil, applied generously and reapplied as you work deeper. Aluminum taps well with light cutting oil or WD-40 in a pinch — avoid tapping it bone dry, since aluminum loves to gall onto tool steel. Cast iron and brass are usually tapped dry or with only a trace of oil, since their chips are short and brittle rather than stringy, and excess fluid mostly just makes a mess without helping the cut.

Step 6: Back the tap out and clean up

Reverse the tap all the way out rather than trying to run it through and pull it out the far side (unless it's a through-hole where that's the point). On the way out, the same turn-and-back-off idea in reverse helps clear trapped chips. Once it's out, blow or brush chips out of the hole — especially a blind hole, where chips packed at the bottom can jam a bolt short of full seat even though the threads themselves are cut correctly.

Common mistakes that break taps

Almost every broken tap traces back to one of a handful of causes: starting crooked and forcing it straight mid-cut, skipping the back-off rhythm and packing the flutes with a long chip, using a bottoming tap to start a hole cold, tapping a hole that's undersized because the pilot drill was wrong or worn oversize-compensating, or running out of fluid partway through a deep hole in steel or stainless. Small taps (below about #10-32 or M5) are the most fragile and the least forgiving of any of these — go slower and check alignment more often on small threads than you would on a 3/8" or M10 tap.