The tap drill for 1/4-28 UNF is #3 (0.2130 in, 5.41 mm), which gives about 80% of full thread depth. The exact 75% hole is 0.2152 in (5.47 mm). The table lists every number, letter, fractional and metric drill that gives between 55% and 90% thread, so you can work from the bits you own.
| Drill | Type | in | mm | Thread | Note |
|---|---|---|---|---|---|
| 5.3 mm | metric | 0.2087 | 5.30 | 89% | deep, hard to tap |
| #4 | number | 0.2090 | 5.31 | 88% | deep, hard to tap |
| 5.4 mm | metric | 0.2126 | 5.40 | 81% | deep, hard to tap |
| #3 | number | 0.2130 | 5.41 | 80% | chart drill |
| 5.5 mm | metric | 0.2165 | 5.50 | 72% | |
| 7/32" | fraction | 0.2188 | 5.56 | 67% | easier to tap |
| 5.6 mm | metric | 0.2205 | 5.60 | 64% | easier to tap |
| #2 | number | 0.2210 | 5.61 | 63% | easier to tap |
| 5.7 mm | metric | 0.2244 | 5.70 | 55% | easier to tap |
About the 1/4-28 thread
1/4-28 is the fine (UNF) thread at a quarter inch: the same 0.250 in diameter as the everyday 1/4-20, but 28 threads per inch instead of 20. It turns up where a fine adjustment, a thin wall or a short engagement matters: adjusting and set screws, fixtures and instruments, and automotive and small-engine parts. A finer pitch puts more threads into a thin plate or tube wall, and each turn moves the screw a shorter distance, which makes it easier to set precisely.
#3 (0.213 in) is the chart drill and gives a deep thread of about 80%. Many machinists drill 7/32 in (0.219 in) instead, which gives about two thirds of full depth and taps more easily; the loss of stripping strength from the shallower thread is small next to the gain from a longer engagement. In aluminium and brass, where the thread is the weak part, the #3 is the usual choice.
1/4-28 thread dimensions
| Dimension | Value | Also |
|---|---|---|
| Nominal diameter | 0.2500 in | 6.35 mm |
| Threads per inch | 28 | pitch 0.0357 in (0.907 mm) |
| Basic pitch diameter | 0.2268 in | 5.761 mm |
| Basic minor diameter (nut) | 0.2113 in | 5.368 mm |
| Tensile stress area | 0.0364 in² | 23.47 mm² |
| 1/4-20 UNC (same diameter): tap drill | #7 (0.2010 in) | 75% thread |
Clearance hole for 1/4
The tapped hole is in the part that holds the thread; the part being clamped gets a clearance hole the bolt passes through. For 1/4 a normal fit is 17/64" (0.2656 in). Never drill the clearance part with the tap drill: the screw will not pass.
| Fit | Drill | Diameter |
|---|---|---|
| Close fit | F | 0.2570 in |
| Normal fit | 17/64" | 0.2656 in |
| Loose fit | 9/32" | 0.2812 in |
How deep to drill a blind 1/4-28 hole
| Part material | Full thread wanted | Drill depth (full diameter) | Depth to drill tip (118°) |
|---|---|---|---|
| Steel or cast iron (1 × diameter) | 0.250 in | 0.500 in (about 1/2″) | 0.565 in (about 9/16″) |
| Aluminium or brass (2 × diameter) | 0.500 in | 0.750 in (about 3/4″) | 0.815 in (about 13/16″) |
Wrench size and torque for 1/4
- Hex cap screw: 7/16 in wrench
- Socket head cap screw: 3/16 in hex key
| Bolt grade | Dry | Lubricated |
|---|---|---|
| Grade 2 | 6 ft-lb | 5 ft-lb |
| Grade 5 | 10 ft-lb | 7 ft-lb |
| Grade 8 | 14 ft-lb | 10 ft-lb |
Threads easily mistaken for 1/4-28
| Thread | Diameter (in) | Diameter (mm) | Pitch |
|---|---|---|---|
| 1/4-28 UNF (this page) | 0.2500 | 6.35 | 28 TPI (0.91 mm) |
| 1/4-20 UNC | 0.2500 | 6.35 | 20 TPI (1.27 mm) |
| M6 x 1 | 0.2362 | 6.00 | 1 mm (25.4 TPI) |
Tips for tapping 1/4-28
- Back the tap off every half to full turn to break the chip, and use cutting fluid; fine-pitch taps clog quickly in gummy aluminium.
- Check the pitch before tapping a repair: a 1/4-20 bolt will not start in a 1/4-28 hole. A thread gauge or a nut of known pitch tells them apart in seconds.
- In aluminium, aim for at least 1/2 in of engagement (2 x diameter) so the screw, not the thread, is the weak link.
- Hex cap screws of this size take a 7/16 in wrench, the same as 1/4-20.
Frequently asked questions
What size drill bit for a 1/4-28 tap?
#3 (0.2130 in), which gives about 80% thread. The exact 75% size is 0.2152 in (5.47 mm).
How deep should a blind 1/4-28 hole be?
For a full thread of 1 × diameter in steel, drill about 0.500 in (about 1/2″) at full diameter with a plug tap, or 0.565 in (about 9/16″) to the drill tip; in aluminium, about 0.750 in (about 3/4″).
What is the 1/4-28 tap drill in mm?
5.5 mm is the nearest 0.1 mm drill to the 75% size and gives 72% thread.
Is 7/32 OK for a 1/4-28 tap?
Yes for most work. 7/32 in (0.2188 in) gives about 67% thread, easier to tap than the #3 chart drill (about 80%), with a shallower thread.
What is the difference between 1/4-28 and 1/4-20?
Both are 0.250 in in diameter; 1/4-28 UNF has 28 threads per inch and 1/4-20 UNC has 20. They do not interchange. The fine thread has a slightly larger stress area and suits thin material; the coarse thread is more forgiving in soft material and assembles faster.
Sources and method
Every dimension and percentage on this page is computed from the formulas listed, not copied from another chart. Values are basic (nominal); real parts are made to tolerance classes (2A/2B for inch, 6g/6H for metric).
- ASME B1.1 and FED-STD-H28/2B (Unified inch), ISO 68-1 and ISO 261 (metric): basic thread profile and stress area formulas.
- Machinery’s Handbook (Industrial Press): percentage-of-thread relation used by our tap drill size calculator.
- ASME B94.11M twist drill series (number, letter, fractional); 0.1 mm metric drill steps.
- ISO 273 metric clearance holes; inch close, normal and loose fits as published in the ASME B18.2.8 clearance classes.
- SAE J429 and ISO 898-1 proof strengths; T = K × D × F torque relation (see our bolt torque chart for the full method).
- Wrench and hex key sizes: ASME B18.2.1, ASME B18.3, ASME B18.6.3, ISO 4014/4017, DIN 933 and ISO 4762 values as widely published.
More thread and fastener tools
- All thread and fastener charts
- UNC and UNF thread chart
- Metric thread pitch chart
- Tap drill chart (inch and metric)
- Clearance hole chart
- Nail size chart
- Wood screw size chart
- Lag screw pilot hole chart
- Bolt torque chart
- Wrench size chart
- Drill bit size chart
- Thread engagement chart
- Nail gun nail sizes by gauge
- Screw and nail size calculator
- Tap drill size calculator
- Clearance hole size calculator
- Tapped hole depth calculator
Last reviewed: 24 October 2026. How we build and check our data: methodology and corrections. Spotted an error? Please let us know.