Adapters range from A1 - A7, sizes are given in BSP (British Standard Pipe) - Example 1/8 BSP is not the diameter of the adapter.
1/8 BSP (British Standard Pipe) measures approx 9mm diameter.
1/4 BSP (British Standard Pipe) measures approx 12mm diameter
1/8 or 1/4 BSP MALE = External threads
1/8 or 1/4 BSP FEMALE = Internal threads