1/4-28 tap drill size
Drill a #3 hole (0.2130" / 5.41 mm) for a 1/4-28 UNF tap. That leaves about 80% thread, the standard for general-purpose tapping.
1/4-28 tap drill: #3 (0.2130 in / 5.41 mm), 80% thread1/4-28 thread specs
| Series | Unified fine (UNF) |
|---|---|
| Major diameter | 0.2500" (6.35 mm) |
| Pitch | 28 TPI (0.91 mm) |
| Basic minor diameter | 0.2113" (5.37 mm) |
| Common internal class | 2B |
Every drill that works for 1/4-28
Drills from all four sets that give 50–85% thread, largest first. Lower percentages tap easier; see how much thread you need.
| Drill | Thread | in | mm | Set | Good for |
|---|---|---|---|---|---|
| 5.7 mm | 55% | 0.2244 | 5.70 | Metric | deep holes only |
| #2 | 63% | 0.2210 | 5.61 | Number | hard or tough metals |
| 5.6 mm | 64% | 0.2205 | 5.60 | Metric | hard or tough metals |
| 7/32" | 67% | 0.2188 | 5.56 | Fractional | hard or tough metals |
| 5.5 mm | 72% | 0.2165 | 5.50 | Metric | general use |
| #3standard | 80% | 0.2130 | 5.41 | Number | general use |
| 5.4 mm | 81% | 0.2126 | 5.40 | Metric | more torque, more broken taps |
1/4-28 clearance hole
The hole a 1/4-28 screw passes through. Close fit locates parts; free fit is forgiving.
| Fit | Drill | in | mm |
|---|---|---|---|
| Close fit | F | 0.2570 | 6.53 |
| Free fit | H | 0.2660 | 6.76 |
1/4-28 counterbore and countersink
For a socket head cap screw to sit flush, and for a flat head socket screw.
| Socket head counterbore | Diameter | Depth |
|---|---|---|
| AMES | 7/16" 0.4375" | 0.2500" |
Flat head socket screw: countersink at 82° (inch screws, ASME B18.3) to 0.5310" (13.49 mm) across for a flush head. Metric flat heads use 90°, so don't mix countersinks.
1/4-28 holes in a 3D print
Printed holes usually come out undersize. Model the hole at the drill size plus what your printer loses, then tap or drill to size. Why printed holes shrink.
Print a test hole, measure it with calipers, enter the difference. 0 models the nominal size.
1/4-28 thread repair (Helicoil)
Stripped 1/4-28 thread? Drill out with the STI drill, tap with a 1/4-28 UNF STI tap, and fit a wire thread insert. The drill in your repair kit’s instructions wins over any chart.
| Chart | STI drill | in | mm |
|---|---|---|---|
| AMES | G | 0.2610 | 6.63 |
| MISUMI | 17/64" | 0.2656 | 6.75 |
Related sizes
Frequently asked questions
What size drill do I use for a 1/4-28 tap?
What metric drill can I use for a 1/4-28 tap?
What size clearance hole for a 1/4-28 screw?
What is the metric equivalent of 1/4-28?
Sources
- Engineers Edge: machinist screw thread tap drill chart (ANSI) checked 2026-10-19
- Penn MEDesign: tap drill chart checked 2026-10-19
- Fractory: imperial clearance hole sizes checked 2026-10-19
- AMES: counterbore hole dimensions, inch (Machinery's Handbook) checked 2026-10-19
- SC Fastening: flat head socket cap screw spec, ASME B18.3 (PDF) checked 2026-10-19
- Eugene Fastener: flat head socket screw countersink guide checked 2026-10-19
- AMES: Helicoil tap sizes (STI) checked 2026-10-19
- MISUMI: Helicoil tap and drill size chart checked 2026-10-19
Spotted a wrong number? Email contact@clove3d.com and it gets fixed.
% thread = (major − drill) ÷ (1.299 × pitch) × 100. The standard drill comes from published charts and is checked against this formula on every build.