Why are there different sizes?

The UK and US knitting needle sizing systems evolved separately over time, leading to some differences in how sizes are designated. 

UK Knitting Needle Sizes:

The UK knitting needle sizing system traditionally used a numbering system based on the Standard Wire Gauge (SWG). This system was originally developed for industrial wire and sheet metal, but it was adapted for knitting needles. The higher the number, the smaller the needle size. This system is commonly referred to as the "old UK sizes."and was somewhat arbitrary and lacked a consistent correlation to actual needle diameters.

US Knitting Needle Sizes:

The US knitting needle sizing system is based on a different standard called the American Wire Gauge (AWG), which is used for electrical wiring. Different to the UK system, the higher the number, the larger the needle size. The US system provides a more consistent correlation between the gauge number and the actual needle diameter.

This system was formalized later than the UK system, and it was standardized by various knitting needle manufacturers in the United States. The US sizes are more consistent and easier to understand, especially for beginners.

Modernisation and Standardisation:

As knitting became more widespread globally and with the rise of international knitting patterns, there was a need for standard needle sizes. In more recent years, efforts have been made to harmonize knitting needle sizes across different countries and regions.

The Metric system (millimeters) has emerged as a universal standard for knitting needle sizing due to its simplicity and consistency. Many modern knitting patterns now include needle size recommendations in both Metric and US/UK sizes to accommodate knitters from different backgrounds.

While the traditional UK and US sizing systems are still used, especially by older patterns and in certain regions, the Metric system has become increasingly prevalent in modern knitting literature and needle manufacturing.