1/4-20 Bolt Size and Thread Specifications
1/4-20 is one of the most common imperial (UNC) fastener sizes. A 1/4 inch nominal diameter with 20 threads per inch is used in automotive, machinery, and general hardware across North America.
Specifications
| Specification | Value |
|---|---|
| Nominal diameter | 0.25 in (6.35 mm) |
| Threads per inch (TPI) | 20 |
| Tap drill size | #7 (0.201 in) |
| Closest metric equivalent | M6 |
Hex head size
| Bolt size | Hex head size |
|---|---|
| 1/4-20 | 7/16 in |
Common uses
1/4-20 bolts are used in automotive assemblies, machinery, furniture, electronics, and general hardware. They are the standard choice for many North American OEM and aftermarket applications.
Related tools
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FAQ
What does 1/4-20 mean?
1/4 is the nominal diameter in inches (0.25 in). 20 is the number of threads per inch (TPI). It is a common UNC coarse thread.
What drill size for 1/4-20 tap?
Use a #7 drill (0.201 in) for a standard 1/4-20 tap. Use our Tap Drill Calculator for metric equivalents.
What metric bolt is closest to 1/4-20?
M6 is the closest metric equivalent to 1/4-20 in diameter and typical use. See M6 bolt size for full specs.