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. Spanish reference: perno 1/4-20. For tap drill size, thread types, and thread identifier workflows, pair this page with tools as you verify threads.
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. Cross-check loads with the bolt size chart and screw size conversion when you mix metric and inch bins.
Related tools
Related charts
1/4-20 vs 5/16-18 bolt size
Shank diameter and TPI change load capacity, wrench sizes, and required tap drill size before tapping. Compare tables on this page with the
Compare tables on this page with the 5/16-18 spec page.
Use the bolt size chart for full context.
Metric vs UNC thread size
Metric threads list diameter in millimeters and pitch in millimeters per thread; UNC lists nominal inch diameter and threads per inch. They are not interchangeable—taps, dies, and gauges differ.
Read metric vs unc thread context when series labels are unclear.
Use screw size conversion when you must match stocked parts.
How to choose the right bolt size
Choosing the correct bolt size depends on load, material, and thread type. Larger diameters increase strength, while thread pitch affects holding force.
Use the bolt size chart to compare shank and hex dimensions before you standardize hardware.
Use tap drill size guidance when you plan tapped holes.
Use the thread identifier when stamped sizes are missing.
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.