HEIRING specifies the ring sizes by the inner circumference of the ring in millimetre (mm). For example size 48 corresponds to 48 mm.
You can easily determine your own ring size with a measuring tape or a string.
To measure simply wrap the string or measuring tape around the finger where you want to wear the ring, marking where the string first overlaps with a pen.
Then, line that up with a ruler and take down the length in millimetres
The chart below shows how the HEIRING sizes translate into other sizes: