The tap drill for a 1/4-20 UNC thread is a #7 drill (5.11 mm), which leaves about 75% of full thread with a cutting tap. The ideal hole for 75% thread is 0.2013 in, and #7 is the nearest standard inch drill.
| Percent of thread | Ideal hole | Nearest drill | Drill size | Thread achieved |
|---|---|---|---|---|
| 50% | 0.2175 in | 7/32" | 0.2188 in / 5.56 mm | 48% |
| 60% | 0.2110 in | #3 | 0.2130 in / 5.41 mm | 57% |
| 65% | 0.2078 in | #4 | 0.2090 in / 5.31 mm | 63% |
| 70% | 0.2045 in | #6 | 0.2040 in / 5.18 mm | 71% |
| 75% | 0.2013 in | #7 | 0.2010 in / 5.11 mm | 75% |
The drills either side of the target are 3 points of thread apart, so either is a reasonable choice. #7 (0.2010 in) gives 75% and a stronger but harder-to-tap thread; 13/64" (0.2031 in) gives 72% and less tapping torque. On 1/4-20 UNC the choice is mostly about tap life in hard materials.
1/4-20 is probably the most common inch bolt: camera tripods, fixtures, furniture, machine guards and general assembly all use it. Its basic pitch diameter is 0.2175 in and its tensile stress area is 0.0318 in². A bolt of this size passes through a 0.2810 in normal-fit clearance hole (ASME B18.2.8).
Example: 1/4-20 UNC at 75% thread
Result: #7 (0.2010 in), 75% thread
| Major (nominal) diameter | 0.2500 in / 6.35 mm |
|---|---|
| Pitch | 20 TPI (0.0500 in) |
| Basic pitch diameter (d − 0.6495·P) | 0.2175 in |
| Basic minor diameter, internal (d − 1.0825·P) | 0.1959 in |
| Tensile stress area | 0.0318 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 UNC sizes:
Use #7 (0.2010 in). It leaves 75% of full thread with a cutting tap; the ideal 75% hole is 0.2013 in. For an easier-to-tap 50% thread, 7/32" (0.2188 in) gives 48%.
#7 is 5.11 mm. The nearest metric drill to the 75% target is 5.1 mm (0.2008 in), which gives 76% thread.
A forming tap needs a bigger hole: 0.2279 in for 65% thread, so drill #1 (0.2280 in). Forming taps work in ductile metals such as aluminum and mild steel.
For 1/4-20 UNC, 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 1/4-28 UNF uses the same sizes.
0.0318 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. 1/4-20 UNC takes #7 (0.2010 in); 1/4-28 UNF takes #3 (0.2130 in). Check the pitch on the drawing before you drill.
COPLAIN AI PLATFORM
Put 1/4-20 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