Tap drill size for 1/2-20
29/64"
0.4531 inches · 11.51 mm · 75% thread engagement
| Tap drill (75% thread) | 29/64" — 0.4531" / 11.51 mm |
| Major diameter | 0.5" / 12.7 mm |
| Drill for a 100% thread | 0.4351" / 11.05 mm |
| Threads per inch | 20 |
| Pitch | 0.05" / 1.27 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:
0.5 − 1.299 ÷ 20 = 0.4351 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:
0.5 − 0.974 ÷ 20 = 0.4513 inches
The nearest drill that actually exists is 29/64" at 0.4531 inches, which is 0.0018 inches over the theoretical figure. That is the number printed on every 1/2-20 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% | 0.4675" | 15/32" (0.4688") | 11.91 mm |
| 65% | 0.4578" | 29/64" (0.4531") | 11.51 mm |
| 75% — the chart value | 0.4513" | 29/64" (0.4531") | 11.51 mm |
| 85% | 0.4448" | 7/16" (0.4375") | 11.11 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.
1/2-20 in metric
1/2-20 has a major diameter of 12.7 mm and a pitch of 1.27 mm. The nearest metric tap is M12 at 12 mm, which is 0.7 mm smaller — far enough apart that there is no confusing the two. An M12 bolt will drop straight through a 1/2-20 hole without catching, so if a fastener nearly fits, neither of these is the standard you are looking for.
For the drill itself the conversion is straightforward: 29/64" is 11.51 mm, and a metric drill index will offer 11.5 mm as the nearest size.
What 1/2-20 is used for
Automotive and hydraulic fittings.
The same 0.5" screw is also made as 1/2-13, the coarse version, which takes a 27/64" 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 1/2-20 tap?
A 29/64" drill, which is 0.4531 inches or 11.51 mm. That is the standard 75% thread engagement figure, and it is what every published tap chart gives for 1/2-20.
What size hole do I need for a 1/2-20 thread?
The same 29/64" hole — 0.4531 inches. The number comes from subtracting the thread height from the major diameter: 0.5 − 0.974 ÷ 20 = 0.4513 inches, and 29/64" is the nearest drill that exists.
What is the 1/2-20 tap drill size in mm?
11.51 mm. There is no metric drill of exactly that size, so a metric drill index gives you 11.5 mm as the nearest — close enough for 1/2-20 in most materials, though it changes the thread engagement slightly.
Can I use a 29/64" instead?
Yes, and in hard material you should. 29/64" 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
- 9/16-12 tap drill size — 31/64"
- 9/16-18 tap drill size — 33/64"
- 5/8-11 tap drill size — 17/32"
- 5/8-18 tap drill size — 37/64"
- 3/4-10 tap drill size — 21/32"
- 3/4-16 tap drill size — 11/16"
- 7/8-9 tap drill size — 49/64"
- 7/8-14 tap drill size — 13/16"
- 1-8 tap drill size — 7/8"
- 1-12 tap drill size — 59/64"