What Does 1/4-20 Mean?

1/4-20 is an imperial thread designation. The first number (1/4) is the bolt diameter in inches. The second number (20) is the thread count — the number of threads per inch (TPI). So a 1/4-20 bolt has a 0.250-inch diameter with 20 threads per inch.

How to Read Imperial Thread Sizes

The format is always diameter-TPI. Common examples:

• #6-32 — #6 diameter (0.138"), 32 threads per inch
• #10-24 — #10 diameter (0.190"), 24 TPI
• 1/4-20 — 1/4" diameter, 20 TPI (coarse thread)
• 1/4-28 — 1/4" diameter, 28 TPI (fine thread)
• 3/8-16 — 3/8" diameter, 16 TPI
• 1/2-13 — 1/2" diameter, 13 TPI

Coarse vs. Fine Thread

For any given diameter, there is typically a coarse thread (UNC) and a fine thread (UNF) option. Coarse threads like 1/4-20 are the default for most applications. Fine threads (1/4-28) offer slightly higher strength and are used where vibration resistance matters.

Why It Matters When Buying Fasteners

A 1/4-20 bolt requires a 1/4-20 nut. If the threads do not match, the fasteners will not thread together. Always confirm both the diameter and the thread count before ordering. On Fastenio, every product listing shows the exact thread designation in the specs.