1/4-20 Tap Drill Size
Unified coarse (UNC) · 20 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 1/4-20 tap?
To scale section of the tapped hole
Drill options % of thread
| Drill | Inch | mm | Thread | 2B |
|---|---|---|---|---|
| #8 | 0.1990 | 5.06 | ✓ | |
| 5.1 mm | 0.2008 | 5.10 | ✓ | |
| #7Use | 0.2010 | 5.11 | ✓ | |
| 13/64" | 0.2031 | 5.16 | ✓ | |
| #6 | 0.2040 | 5.18 | ✓ | |
| 5.2 mm | 0.2047 | 5.20 | ✓ | |
| #5 | 0.2055 | 5.22 | ✓ | |
| 5.3 mm | 0.2087 | 5.30 | ✗ | |
| #4 | 0.2090 | 5.31 | ✗ |
By material Aluminum, brass, mild steel: #7 (75%) · Stainless, tool steel: #5 (69%)
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 (0.196 in to 0.207 in).
1/4-20 UNC ASME B1.1, class 2B
| Major diameter | 0.2500 in / 6.35 mm |
|---|---|
| Threads per inch | 20 TPI (pitch 0.0500 in) |
| Pitch diameter (basic) | 0.2175 in |
| Minor diameter 2B | 0.1959–0.2074 in |
| Thread-forming tap drill | #1 0.2280 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
- +0.250 inextra drill depth below the thread for a plug tap (5 pitches), +0.100 in with a bottoming tap
Shop notes
A #7 drill is the textbook answer and it earns it: 0.2010 in leaves about 75% of a full thread, which is where tap makers draw the line between holding power and tapping torque.
If the #7 is missing, a 13/64 is the drill to reach for. It is only 0.002 in bigger and still cuts about 72% thread, which no fastener you can buy for a 1/4-20 hole will notice. A 5.1 mm metric drill lands almost on top of the #7. Going smaller is where people get into trouble: a #9 (83%) works in aluminum but in steel it mostly makes the tap harder to turn and easier to snap.
In stainless or anything that work hardens, drop to a #5 or #6. You lose a few points of thread engagement and gain a tap that comes back out in one piece.
The M6 trap
An M6 x 1 bolt is 0.236 in across and a 1/4-20 is 0.250 in, close enough that they get mixed in the same bin. They are not interchangeable. An M6 will start in a 1/4-20 hole and then bind after a turn or two because the pitches differ (1.27 mm against 1 mm); forcing it strips the hole. If a 1/4-20 bolt will not start in a hole that looks right, check whether someone tapped it M6.
You will also find 1/4-20 under every camera and on most tripod heads: it is the standard camera mount thread, so the same drill and tap fix a stripped tripod plate.
Questions
Will a 13/64 drill bit work for a 1/4-20 tap?
Yes. A 13/64 drill is 0.2031 in and gives about 72% thread, against 75% for the standard #7. The difference is too small to matter for normal bolts, and it makes the tap a little easier to turn.
What size hole should I drill for a 1/4-20 bolt?
It depends on whether the hole is tapped or the bolt passes through. To tap it, drill #7 (0.201 in). For the bolt to pass through freely, drill 9/32 (0.281 in), or 17/64 (0.266 in) for a close fit that locates the part more accurately.
What size is a #7 drill bit?
A #7 drill is 0.2010 in (5.11 mm), between 3/16 (0.1875 in) and 13/64 (0.2031 in). It is the standard tap drill for 1/4-20.