Unified Thread Standard (UTS)

The Unified Thread Standard (UTS) is the inch-based thread system used to define and standardize screw and bolt threads for many fasteners in the United States and Canada. UTS specifies the fundamental thread form and naming convention so parts from different manufacturers will fit together consistently. Under UTS, threads use a 60° thread angle and are typically designated by nominal diameter and threads per inch (TPI), such as 1/4-20 (a 1/4 inch major diameter with 20 TPI).

UTS also includes different thread series that describe common pitch groupings for a given diameter, including UNC (Unified National Coarse), UNF (Unified National Fine), UNEF (Unified National Extra Fine), and UNS (Unified National Special). In addition, UTS defines fit/tolerance classes that control how tight or loose the assembly will be, commonly 1A/2A/3A for external threads (bolts) and 1B/2B/3B for internal threads (nuts/tapped holes). Together, the series and class help engineers and buyers select threads that balance strength, ease of assembly, vibration resistance, and manufacturability.