Tap drill size for 12-28
#14
0.182 inches · 4.62 mm · 75% thread engagement
| Tap drill (75% thread) | #14 — 0.182" / 4.62 mm |
| Major diameter | 0.216" / 5.49 mm |
| Drill for a 100% thread | 0.1696" / 4.31 mm |
| Threads per inch | 28 |
| Pitch | 0.0357" / 0.91 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:
0.216 − 1.299 ÷ 28 = 0.1696 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:
0.216 − 0.974 ÷ 28 = 0.1812 inches
The nearest drill that actually exists is #14 at 0.182 inches, which is 0.0008 inches over the theoretical figure. That is the number printed on every 12-28 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% | 0.1928" | #10 (0.1935") | 4.91 mm |
| 65% | 0.1858" | #13 (0.185") | 4.7 mm |
| 75% — the chart value | 0.1812" | #14 (0.182") | 4.62 mm |
| 85% | 0.1766" | #16 (0.177") | 4.5 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.
12-28 in metric
12-28 has a major diameter of 5.49 mm and a pitch of 0.91 mm. The nearest metric tap is M5 at 5 mm, which is 0.49 mm smaller — far enough apart that there is no confusing the two. An M5 bolt will drop straight through a 12-28 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: #14 is 4.62 mm, and a metric drill index will offer 4.6 mm as the nearest size.
What 12-28 is used for
Fine-pitch #12; uncommon outside specific equipment.
The same 0.216" screw is also made as 12-24, the coarse version, which takes a #16 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 12-28 tap?
A #14 drill, which is 0.182 inches or 4.62 mm. That is the standard 75% thread engagement figure, and it is what every published tap chart gives for 12-28.
What size hole do I need for a 12-28 thread?
The same #14 hole — 0.182 inches. The number comes from subtracting the thread height from the major diameter: 0.216 − 0.974 ÷ 28 = 0.1812 inches, and #14 is the nearest drill that exists.
What is the 12-28 tap drill size in mm?
4.62 mm. There is no metric drill of exactly that size, so a metric drill index gives you 4.6 mm as the nearest — close enough for 12-28 in most materials, though it changes the thread engagement slightly.
Can I use a #13 instead?
Yes, and in hard material you should. #13 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
- 1/4-20 tap drill size — #7
- 1/4-28 tap drill size — #3
- 5/16-18 tap drill size — F
- 5/16-24 tap drill size — I
- 3/8-16 tap drill size — 5/16"
- 3/8-24 tap drill size — Q
- 7/16-14 tap drill size — U
- 7/16-20 tap drill size — 25/64"
- 1/2-13 tap drill size — 27/64"
- 1/2-20 tap drill size — 29/64"