Tap drill size for 1/2-13
27/64"
0.4219 inches · 10.72 mm · 75% thread engagement
| Tap drill (75% thread) | 27/64" — 0.4219" / 10.72 mm |
| Major diameter | 0.5" / 12.7 mm |
| Drill for a 100% thread | 0.4001" / 10.16 mm |
| Threads per inch | 13 |
| Pitch | 0.0769" / 1.95 mm |
| Series | UNC (coarse) |
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 ÷ 13 = 0.4001 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 ÷ 13 = 0.4251 inches
The nearest drill that actually exists is 27/64" at 0.4219 inches, which is 0.0032 inches under the theoretical figure. That is the number printed on every 1/2-13 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% | 0.45" | 29/64" (0.4531") | 11.51 mm |
| 65% | 0.4351" | 7/16" (0.4375") | 11.11 mm |
| 75% — the chart value | 0.4251" | 27/64" (0.4219") | 10.72 mm |
| 85% | 0.4151" | Z (0.413") | 10.49 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.
1/2-13 in metric
1/2-13 has a major diameter of 12.7 mm and a pitch of 1.95 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 1/2-13 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: 27/64" is 10.72 mm, and a metric drill index will offer 10.7 mm as the nearest size.
What 1/2-13 is used for
Structural steel, anchor bolts, machine bases and threaded rod.
The same 0.5" screw is also made as 1/2-20, the fine version, which takes a 29/64" drill instead. Coarse threads tap faster, tolerate damage better and hold better in soft material.
Frequently asked questions
What size drill bit do I need for a 1/2-13 tap?
A 27/64" drill, which is 0.4219 inches or 10.72 mm. That is the standard 75% thread engagement figure, and it is what every published tap chart gives for 1/2-13.
What size hole do I need for a 1/2-13 thread?
The same 27/64" hole — 0.4219 inches. The number comes from subtracting the thread height from the major diameter: 0.5 − 0.974 ÷ 13 = 0.4251 inches, and 27/64" is the nearest drill that exists.
What is the 1/2-13 tap drill size in mm?
10.72 mm. There is no metric drill of exactly that size, so a metric drill index gives you 10.7 mm as the nearest — close enough for 1/2-13 in most materials, though it changes the thread engagement slightly.
Can I use a 7/16" instead?
Yes, and in hard material you should. 7/16" 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
- 1/2-20 tap drill size — 29/64"
- 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"