The tap drill for a 1/4-28 UNF thread is a #3 drill (5.41 mm), which leaves about 80% of full thread with a cutting tap. The ideal hole for 75% thread is 0.2152 in, and #3 is the nearest standard inch drill.
| Percent of thread | Ideal hole | Nearest drill | Drill size | Thread achieved |
|---|---|---|---|---|
| 50% | 0.2268 in | #1 | 0.2280 in / 5.79 mm | 47% |
| 60% | 0.2222 in | #2 | 0.2210 in / 5.61 mm | 63% |
| 65% | 0.2198 in | 7/32" | 0.2188 in / 5.56 mm | 67% |
| 70% | 0.2175 in | 7/32" | 0.2188 in / 5.56 mm | 67% |
| 75% | 0.2152 in | #3 | 0.2130 in / 5.41 mm | 80% |
Drill sizes are coarse relative to this pitch: the two drills either side of the target are 12 points of thread apart. #3 (0.2130 in) gives 80% and a stronger but harder-to-tap thread; 7/32" (0.2188 in) gives 67% and less tapping torque. On 1/4-28 UNF the choice is mostly about tap life in hard materials.
1/4-28 is used on aircraft hardware, automotive parts, bicycle components and grease fittings. Its basic pitch diameter is 0.2268 in and its tensile stress area is 0.0364 in². A bolt of this size passes through a 0.2810 in normal-fit clearance hole (ASME B18.2.8).
Example: 1/4-28 UNF at 75% thread
Result: #3 (0.2130 in), 80% thread
| Major (nominal) diameter | 0.2500 in / 6.35 mm |
|---|---|
| Pitch | 28 TPI (0.0357 in) |
| Basic pitch diameter (d − 0.6495·P) | 0.2268 in |
| Basic minor diameter, internal (d − 1.0825·P) | 0.2113 in |
| Tensile stress area | 0.0364 in² |
| Fit | Hole diameter |
|---|---|
| Close | 0.2660 in / 6.76 mm |
| Normal | 0.2810 in / 7.14 mm |
| Loose | 0.2970 in / 7.54 mm |
All UNF sizes:
Use #3 (0.2130 in). It leaves 80% of full thread with a cutting tap; the ideal 75% hole is 0.2152 in. For an easier-to-tap 50% thread, #1 (0.2280 in) gives 47%.
#3 is 5.41 mm. The nearest metric drill to the 75% target is 5.5 mm (0.2165 in), which gives 72% thread.
A forming tap needs a bigger hole: 0.2342 in for 65% thread, so drill 15/64" (0.2344 in). Forming taps work in ductile metals such as aluminum and mild steel.
For 1/4-28 UNF, ASME B18.2.8 gives close fit 0.2660 in, normal fit 0.2810 in and loose fit 0.2970 in. The hole depends only on the diameter, so 1/4-20 UNC uses the same sizes.
0.0364 in², from As = 0.7854 × (D − 0.9743 ÷ n)². Multiply it by the bolt material's proof or tensile strength to get the load the thread can carry.
No. The drill depends on the pitch, not only the diameter. 1/4-28 UNF takes #3 (0.2130 in); 1/4-20 UNC takes #7 (0.2010 in). Check the pitch on the drawing before you drill.
COPLAIN AI PLATFORM
Put 1/4-28 tap drill, depth and torque values straight into a shop-floor job aid: Coplain turns your setup notes into clear, audit-ready work instructions.
Generate yours automatically with Coplain →Related Tools & Templates