1/4-28 Tap Drill Size
Unified fine (UNF) · 28 threads per inch · ASME B1.1, class 2B
For a 1/4 tap: 1/4-20 coarse takes a #7 drill, 1/4-28 fine takes a #3 drill.
What size drill bit for a 1/4-28 tap?
To scale section of the tapped hole
Drill options % of thread
| Drill | Inch | mm | Thread | 2B |
|---|---|---|---|---|
| 5.4 mm | 0.2126 | 5.40 | ✓ | |
| #3Use | 0.2130 | 5.41 | ✓ | |
| 5.5 mm | 0.2165 | 5.50 | ✓ | |
| 7/32" | 0.2188 | 5.56 | ✓ | |
| 5.6 mm | 0.2205 | 5.60 | ✗ | |
| #2 | 0.2210 | 5.61 | ✗ | |
| 5.7 mm | 0.2244 | 5.70 | ✗ |
By material Aluminum, brass, mild steel: #3 (80%) · Stainless, tool steel: 7/32" (67%)
75% (orange tick) is the usual target. Hard or gummy metals like stainless are often tapped at 60 to 65% to spare the tap; aluminum, brass and plastic take 75% or a little more. The last column checks the hole against the 2B minor diameter limits (0.211 in to 0.220 in).
1/4-28 UNF ASME B1.1, class 2B
| Major diameter | 0.2500 in / 6.35 mm |
|---|---|
| Threads per inch | 28 TPI (pitch 0.0357 in) |
| Pitch diameter (basic) | 0.2268 in |
| Minor diameter 2B | 0.2113–0.2198 in |
| Thread-forming tap drill | 15/64" 0.2344 in |
Clearance hole ASME B18.2.8
For a screw that passes through the part. Normal fit is the default; close fit locates the part more accurately but leaves no room for hole position error.
Hole depth rule of thumb
- 0.250 inthread length for full strength in steel (1 × D)
- 0.375 inin cast iron or brass (1.5 × D), up to 0.500 in in aluminum
- +0.179 inextra drill depth below the thread for a plug tap (5 pitches), +0.071 in with a bottoming tap
Shop notes
The chart says #3, and for 1/4-28 that drill sits on the full side: 0.2130 in leaves about 80% thread. It is inside the 2B minor diameter limits, so nothing is wrong with it, but a 28-pitch thread does not need that much engagement and the tap has to work for every point of it.
The drill worth having for this thread is a 5.5 mm. It gives about 72%, and the easier cut is welcome in steel and more so in stainless. The 7/32 that is in every fractional set also passes at 67%, and it is the loosest drill that keeps the hole within limits. Do not go one more step: the #2 (0.2210 in) drops to about 62% and falls outside them. On a fine pitch, the gap between a tight hole and an out-of-spec one is a single drill size.
Not the 1/4-20 drill
Both threads are 0.250 in across, and the #7 for 1/4-20 is often the drill already in the chuck. It is too small here: at 0.2010 in it is under the 0.2113 in minimum minor diameter, so the 1/4-28 tap ends up cutting metal that should have been drilled away. Grease fittings commonly use a tapered 1/4-28 thread, which is a variant of this one and not the same straight thread.
Questions
What size is a #3 drill bit?
A #3 drill is 0.2130 in (5.41 mm), between 13/64 (0.2031 in) and 7/32 (0.2188 in). It is the standard tap drill for 1/4-28.