What Is American Thread & US Thread Sizes | Differences Between British and American Threads


American thread is an important fastener thread standard, in this article, we’ll explore the US thread sizes and differences between British and American threads and also show you the chart of American thread dimensions.

What Is American Thread?

American thread refers to screw thread standards that are used in the United States, but there are various thread standards employed in the US. Sometimes, the American thread is used interchangeably with the Imperial thread. One of the most common American thread standards is the Unified Thread Standard (UTS), which includes two main specifications, that are Unified National Thread (UNC) and Unified Fine Thread (UNF). UNC threads have a coarser thread pitch, which means the spacing between threads is greater, and UNF comes with a finer thread pitch.

Related Read: Difference Between UNF and UNC Thread

US Thread Sizes Explained

The US thread sizes or dimensions are typically expressed using the inch measurement system, thread diameters and pitch are delineated using a combination of fractional inch measurements along with the number of threads per inch. For example, a 1/4-20 thread has a diameter of 1/4 inch (0.25 inches) with 20 threads per inch.

Differences Between British and American Threads

1. Types: Thread fasteners in the Unified Thread Standard use UNC/UNF threads, while in the British Standard, BSW (British Standard Whitworth), BSF (British Standard Fine), and BA (British Association) are used.
2. Thread designation: British threads are designated by the thread diameter followed by the number of threads per inch (e.g. 1/2 BSW). American threads use fractional inches and the number of threads per inch (e.g. 1/4-20).
3. Thread Angle: British Standard Whitworth (BSW) uses a 55-degree thread angle, while the most common American standard (UN/UNF) uses 60 degrees.
4. Profile: BSW has a steeper thread profile than American. British threads are not as sharp at the root.
5. Direction: BSW threads are designated Right-Hand (RH) or Left-Hand (LH). American threads increase in diameter when turned clockwise.
6. Tolerance: British standard tolerances are looser than comparable American standards like UNF. BSW threads may not be as precisely interchangeable.
7. Use: BSW was commonly used in the UK and British Empire. UN/UNF became dominant in the US and is now widely adopted internationally in many industries.

American (US) Thread Size Chart

These sizes are inch-based thread dimensions used in the United States. Currently, the American National Series has been replaced by the Unified Thread series.