What Size Drill Bit for a Tap?

You have the tap in hand and a part on the bench, and the whole job hinges on one number: the drill size that comes first. Drill the hole a hair too small and the tap fights you and snaps off flush with the surface. Drill it too big and the threads come out shallow and strip the first time they take load. Get it right and the tap cuts clean, the thread holds, and you never think about it again.

Here is how to find the correct drill for any tap: the quick formula, the reason every chart lands on the same numbers, and the common sizes worth memorising.

The quick answer: two formulas

You do not need a chart if you remember one rule per system. Both give you the tap drill diameter for the standard 75% thread engagement.

Both formulas are the same idea in different units: subtract the depth of one thread from the outside diameter, and what is left is the hole you drill. The tap then cuts the threads into the wall of that hole.

What the hole is actually doing

A tap does not remove all the material inside the thread. It cuts a V-shaped groove into the wall of a hole you have already drilled. The drilled hole sits between the crest and root of the finished thread. Drill closer to the root (a smaller hole) and the tap cuts a taller, fuller thread but has to shift more metal. Drill closer to the crest (a bigger hole) and the tap barely cuts anything, leaving a weak, shallow thread.

Why every chart lands on 75%

You will notice tap drill charts almost always aim for 75% thread engagement, not 100%. That is not caution for its own sake; it is where the math turns against you. Going from 75% to 100% engagement adds only a few percent of thread strength, but it roughly doubles the tapping torque. The reason a full thread gains so little is that by 75% the thread already carries about as much load as the bolt itself; the joint fails in the bolt, not the thread, so a deeper thread has nothing left to protect.

So the last 25% of engagement buys almost no strength while sharply raising the chance of a broken tap. Every practical chart, including our own, settles on 75% for exactly this reason. The tap drill size guide works through the full derivation if you want the numbers behind it.

Common tap drill sizes

These are the sizes that come up most often. Keep them close and you will cover the majority of everyday tapping.

Metric coarse

Inch (UNC)

For any size not listed here (fine pitches, larger diameters, or pipe taps), the drill and tap chart has the full set, and every individual thread page (for example M8 or 1/4-20 UNC) lists its own tap drill alongside the rest of the spec.

A few things that save taps

• Tapping into aluminium or brass? The chart size is fine, but consider drilling a hair larger for a slightly looser thread, since soft metals grab taps.
• No numbered or lettered bits? Round to the nearest fractional size, erring very slightly larger, not smaller. A hole a touch too big beats a snapped tap.
• Use cutting fluid on steel and aluminium. It cuts torque and gives a cleaner thread, which matters most on the small taps that break easily.
• Back off often on blind or deep holes to clear chips. Packed chips are a leading cause of broken taps, regardless of drill size.

The bottom line

The drill comes before the tap, and it decides everything that follows. Remember the two formulas (major minus pitch for metric, major minus 1/TPI for inch) and you can size any hole from memory. When you want the exact number without the arithmetic, the drill and tap chart and the individual thread pages have it ready.