1/2-28 Tap Drill Size
The tap drill for 1/2-28 UNEF is 15/32 in (0.4688 in, 11.91 mm). That is the standard chart size and leaves about 67% thread.
Drill a 15/32 in hole, then cut the thread with a 1/2-28 tap. The hole is 0.0313 in narrower than the screw, so the tap cuts 0.0156 in into each side.
A full-height thread would be 0.0232 in deep on each side. The 15/32 in drill leaves 67% of that.
Try another material or drill
1/2-28 tap drill size by material
| Material | Target thread | Drill | Size | Thread left |
|---|---|---|---|---|
| Aluminum, brass, plastics | 75% | 15/32 in | 0.4688 in (11.91 mm) | 67% |
| Mild steel | 60% | 15/32 in | 0.4688 in (11.91 mm) | 67% |
| Stainless, titanium | 50% | 15/32 in | 0.4688 in (11.91 mm) | 67% |
The 75% row is the published chart drill. The 60% and 50% rows pick the biggest standard drill that still leaves close to that much thread.
Drill sizes are coarse at this thread, so one drill serves every material. On 1/2-28 no standard drill lands close to 75%, 60% or 50%, so those rows show the closest one (15/32 in leaves 67%). How these are worked out.
Notes on 1/2-28
Printed charts give 15/32 in for 1/2-28, which leaves 67%, less than the usual 75%. No inch drill lands closer. One size smaller is 29/64 in, the drill for 1/2-20, and on this thread it would leave more than a full thread, so the tap would bind.
The exact 75% hole is 0.4652 in, says the YG-1 tap chart. Emuge recommends a 0.4646 in hole, which is an 11.8 mm drill and leaves about 76%. If you own metric drills, 11.8 mm comes closer to 75%. 15/32 in is the drill most inch charts print.
At 28 threads per inch, 1/2-28 is the extra-fine (UNEF) thread for a 1/2 in screw. A full thread is only 0.0232 in deep on each side, so one 1/64 in drill step moves it by about 33.7 points.
How much thread do you need?
The chart prints one drill. The right amount of thread depends on the material, and the guides give slightly different ranges.
| Material | Texas Metalworks | ToolGrit | OSG (tap maker) | Drill for 1/2-28 |
|---|---|---|---|---|
| Aluminum | 70-75% | 60-65% | 65-70% | 15/32 in (67%) |
| Brass | 70-75% | 70-75% | 65-70% | 15/32 in (67%) |
| Plastic | not covered | 70-75% | not covered | 15/32 in (67%) |
| Mild steel | 60-65% | 60-65% | 65-70% | 15/32 in (67%) |
| Stainless steel | 50% | 55-60% | 60-70% | 15/32 in (67%) |
| Titanium | 50% | 50-55% | not covered | 15/32 in (67%) |
The three rows in the first table follow Texas Metalworks. OSG makes taps, and asks for more thread in stainless than either guide, so its range sits closer to our steel column. ToolGrit also suggests 50 to 55% for blind holes in stainless, titanium and hardened steel, and for small taps (#6 and below).
Why not a full thread?
A 100% thread is not possible in practice. The tap and the thread both have flat crests and roots (ToolGrit).
More thread also means more work for the tap: higher cutting force, more heat, poorer chip clearance and a bigger risk of breaking it (ToolGrit).
And it adds little strength. ToolGrit cites studies showing about 90% of a thread's shear strength at 60% thread. Texas Metalworks says going from 75% to 60% costs only about 5% of pull-out strength.
On 1/2-28, the 60% drill is the standard drill, 15/32 in. The next size up, 31/64 in, leaves only 34%.
If you do not have the 15/32 in drill
Here are the drills on either side, and the thread each one leaves.
| Drill | Size | Thread left | What changes |
|---|---|---|---|
| 7/16 in | 0.4375 in (11.11 mm) | 100%+ | Too small, the tap will bind |
| 29/64 in | 0.4531 in (11.51 mm) | 100%+ | Too small, the tap will bind |
| 15/32 in | 0.4688 in (11.91 mm) | 67% | Standard size |
| 31/64 in | 0.4844 in (12.30 mm) | 34% | Less thread, easier to tap |
| 1/2 in | 0.5000 in (12.70 mm) | 0% | Less thread, easier to tap |
A bigger drill removes thread and makes the tap easier to turn. A smaller drill keeps more thread and puts more torque on the tap.
The check behind the 15/32 in number: (0.5000 - 0.4688) / (1.299 x 0.0357) = 67.4%.
What 1/2-28 means
- 1/2
- The major diameter, the widest part of the screw: 0.5000 in (12.70 mm).
- 28
- Threads per inch, so one thread every 0.0357 in (0.907 mm).
- UNEF
- Unified National Extra Fine. The fine thread of the same diameter is 1/2-20.
The rest of the size, worked out from the major diameter and pitch:
| Property | Value |
|---|---|
| Major diameter | 0.5000 in (12.70 mm) |
| Pitch | 28 TPI (0.907 mm) |
| Pitch diameter (basic) | 0.4768 in (12.11 mm) |
| Minor diameter, internal thread (basic) | 0.4613 in (11.72 mm) |
The pitch and minor diameters are for checking a thread with gauges. You can skip them if you only need the drill.
How long to make the 1/2-28 thread
Aim for a thread at least 1 to 1.5 times the diameter long. ToolGrit's strength figures assume that length.
| Thread length | In threads | Drill a blind hole at least |
|---|---|---|
| 1 x diameter: 0.500 in | 14 | 0.643 in |
| 1.5 x diameter: 0.750 in | 21 | 0.893 in |
The blind hole depth adds 4 threads (4 x 0.0357 in) so the tap has room to finish and the chips have somewhere to go.
How to tap a 1/2-28 hole
- Drill the hole with the 15/32 in drill (0.4688 in, 11.91 mm).
- For a blind hole, drill 0.643 in deep if you need a 0.500 in thread. That is 4 threads past the thread (ToolGrit).
- Put cutting fluid on the tap. Do not tap dry (ToolGrit, Wikipedia).
- Hold the tap in line with the hole.
- Turn the tap forward, then back it off now and then to break the chip (Wikipedia).
- Need threads right to the bottom of a blind hole? Finish with a bottoming tap (Wikipedia).
Taps usually break for one of these reasons (Wikipedia, ToolGrit):
- Chips clog the flutes.
- The tap is not in line with the hole.
- The tap is pushed too hard or too little.
- There is not enough cutting fluid.
- The drill was too small, so the tap works against too much thread.
Which tap to use for 1/2-28
The three common hand taps differ in how many threads at the tip are tapered (Wikipedia).
- Taper tap: 8 to 10 tapered threads, about 0.2857 in to 0.3571 in on 1/2-28. For hard materials and very small taps.
- Plug tap: 3 to 5 tapered threads, about 0.1071 in to 0.1786 in on 1/2-28. The most common tap, easy to start and keep straight.
- Bottoming tap: 1 to 1.5 tapered threads, about 0.0357 in to 0.0536 in on 1/2-28. Cuts threads to the bottom of a blind hole, usually after a plug tap.
Spiral-point taps are usually used in through holes so the chips can escape. Spiral-flute taps work well in blind holes (Wikipedia).
Forming (roll) taps only work in malleable metals such as mild steel and aluminum (Wikipedia). They use different drill sizes, which we do not cover yet.
1/2-28 vs 1/2-20
These two share the same diameter. They differ in pitch, so they take different drills.
| 1/2-28 | 1/2-20 | |
|---|---|---|
| Series | UNEF (extra fine) | UNF (fine) |
| Pitch | 28 TPI (0.907 mm) | 20 TPI (1.270 mm) |
| Standard drill | 15/32 in | 29/64 in |
| Thread depth, each side | 0.0232 in (0.59 mm) | 0.0325 in (0.82 mm) |
Thread depth is 0.6495 x pitch. Do not mix the two: a 1/2-28 screw will not fit a 1/2-20 hole.
FAQ
What is the tap drill size for 1/2-28?
The standard drill is 15/32 in (0.4688 in, 11.91 mm). It leaves about 67% thread, which is the standard chart choice.
Can I use 31/64 in instead of 15/32 in for 1/2-28?
31/64 in (0.4844 in, 12.30 mm) leaves about 34% thread instead of 67%. The tap cuts more easily and the thread is weaker. That is below the 60% Texas Metalworks suggests for mild steel.
Is an 11.9 mm drill the same as 15/32 in?
It is very close. 11.9 mm is 0.4685 in, against 0.4688 in, 11.91 mm for 15/32 in. It leaves about 68% thread.
What drill size do I use for 1/2-28 in aluminum?
Use 15/32 in (67%). Texas Metalworks says 70 to 75% for soft aluminum and ToolGrit says 60 to 65% for aluminum alloys. Drill sizes are coarse at this thread, so both point to 15/32 in. OSG, a tap maker, gives 65 to 70% for average work in aluminum, brass and mild steel.
What is the tap drill size for 1/2-28 in stainless steel?
15/32 in (0.4688 in, 11.91 mm) leaves about 67% thread, more than the 50% Texas Metalworks recommends for stainless and titanium, but the next drill up leaves too little. ToolGrit gives 55 to 60% for 304 and 316 stainless. OSG, a tap maker, asks for 60 to 70% in stainless for average work.
Does a bigger drill make the 1/2-28 thread much weaker?
Not by much at first. ToolGrit cites studies showing about 90% of a thread's shear strength at 60% thread, if the thread is at least 1 to 1.5 times the diameter long. Texas Metalworks says going from 75% to 60% costs only about 5% of pull-out strength. For 1/2-28, the 60% drill is 15/32 in, the same as the standard drill.
How long should a 1/2-28 thread be?
Aim for at least 1 to 1.5 times the diameter, the length ToolGrit's strength figures assume. For 1/2-28 that is 0.500 in to 0.750 in, or about 14 to 21 threads.
How deep do I drill a blind 1/2-28 hole?
At least 4 threads deeper than the thread you need, says ToolGrit. A 0.500 in thread needs a hole at least 0.643 in deep.
Which tap do I use for a blind 1/2-28 hole?
Start with a plug tap, then finish with a bottoming tap if the thread has to reach the bottom. Wikipedia says a bottoming tap has 1 to 1.5 tapered threads, about 0.0357 in to 0.0536 in on 1/2-28.
What is 1/2-28 in mm?
1/2-28 is 12.70 mm across, with a pitch of 0.907 mm.
Is 1/2-28 a coarse or fine thread?
1/2-28 is an extra-fine thread (UNEF). The fine thread of the same diameter is 1/2-20, which takes 29/64 in.
Related sizes
Chart drill checked against the Fabworks tap drill chart and the Microform Precision tap drill chart, since the American Fastener chart has no extra-fine rows. Tap maker figures from the YG-1 tap drill chart and the Emuge tap drill wall chart. Diameters from Wikipedia: drill bit sizes. Thread percentages, thread length and blind-hole depth from Texas Metalworks and ToolGrit. The percentage formula is the one printed in the OSG tap drill chart, Vol 3 (2024). Tap types and tapping tips from Wikipedia. Last reviewed October 12, 2026.