Tap drill size for 10-32

Q: What size drill bit do I need for a 10-32 tap?

A #21 drill, which is 0.159 inches or 4.04 mm. That is the standard 75% thread engagement figure, and it is what every published tap chart gives for 10-32.

Q: What is the 10-32 tap drill size in mm?

4.04 mm. There is no metric drill of exactly that size, so a metric drill index gives you 4 mm as the nearest — close enough for 10-32 in most materials, though it changes the thread engagement slightly.

Q: Can I use a #19 instead?

Yes, and in hard material you should. #19 gives about 65% thread engagement instead of 75%. That costs very little strength — a 65% thread holds roughly 95% of what a 75% thread does — and it makes the tap markedly easier to turn, which is the trade every machinist makes in stainless or tough alloy.

#21

0.159 inches · 4.04 mm · 75% thread engagement

Tap drill (75% thread)	#21 — 0.159" / 4.04 mm
Major diameter	0.19" / 4.83 mm
Drill for a 100% thread	0.1494" / 3.79 mm
Threads per inch	32
Pitch	0.0313" / 0.79 mm
Series	UNF (fine)

Where the number comes from

A tap cuts the thread; the drill leaves the hole it cuts into. Cutting the full thread form would mean drilling the major diameter less the whole height of the thread:

0.19 − 1.299 ÷ 32 = 0.1494 inches

That is the 100% thread, which nobody drills — the tap has to remove every scrap of metal and usually breaks trying. Charts give 75%, so three quarters of the thread height is cut away and the rest is left as clearance:

0.19 − 0.974 ÷ 32 = 0.1596 inches

The nearest drill that actually exists is #21 at 0.159 inches, which is 0.0006 inches under the theoretical figure. That is the number printed on every 10-32 tap chart, and this is where it comes from.

The same hole at other engagements

Thread engagement is a choice, not a constant. Less engagement means an easier cut and a longer tap life for very little loss of strength, which is why a machinist working stainless will not drill the chart size.

Engagement	Theoretical	Drill	Millimetres
50%	0.1697"	#18 (0.1695")	4.31 mm
65%	0.1636"	#19 (0.166")	4.22 mm
75% — the chart value	0.1596"	#21 (0.159")	4.04 mm
85%	0.1555"	5/32" (0.1563")	3.97 mm

A 75% thread is close to the practical ceiling: going from 75% to 100% adds only a few per cent of strength and roughly doubles the torque needed to turn the tap. Going the other way, a 65% thread still holds about 95% of the load a 75% thread does, which is why 65% is the usual choice in anything harder than mild steel.

10-32 in metric

10-32 has a major diameter of 4.83 mm and a pitch of 0.79 mm. The closest metric tap is M5, and it is close: 5 mm across against 4.83, 0.8 mm of pitch against 0.79. Close is the problem. An M5 bolt will start in a 10-32 hole and then jam, because it is 0.17 mm fatter than the hole was cut for. The pitches are near enough identical — 0.8 mm against 0.79 — which is precisely why it goes far enough in to feel right. That is the failure people mistake for a damaged thread when it is simply the wrong standard.

For the drill itself the conversion is straightforward: #21 is 4.04 mm, and a metric drill index will offer 4 mm as the nearest size.

What 10-32 is used for

Rack-mount equipment, aerospace fittings and anywhere a #10 has to bite into thin material. More common than 10-24 in some trades.

The same 0.19" screw is also made as 10-24, the coarse version, which takes a #25 drill instead. Fine threads hold better in thin material, resist vibration loosening and give finer adjustment.

Frequently asked questions

What size drill bit do I need for a 10-32 tap?

A #21 drill, which is 0.159 inches or 4.04 mm. That is the standard 75% thread engagement figure, and it is what every published tap chart gives for 10-32.

What size hole do I need for a 10-32 thread?

The same #21 hole — 0.159 inches. The number comes from subtracting the thread height from the major diameter: 0.19 − 0.974 ÷ 32 = 0.1596 inches, and #21 is the nearest drill that exists.

What is the 10-32 tap drill size in mm?

4.04 mm. There is no metric drill of exactly that size, so a metric drill index gives you 4 mm as the nearest — close enough for 10-32 in most materials, though it changes the thread engagement slightly.

Can I use a #19 instead?

Yes, and in hard material you should. #19 gives about 65% thread engagement instead of 75%. That costs very little strength — a 65% thread holds roughly 95% of what a 75% thread does — and it makes the tap markedly easier to turn, which is the trade every machinist makes in stainless or tough alloy.

Other tap sizes

Every unified tap drill size · Metric thread sizes