The most accurate way to find your ring size in the UK is to visit a local jeweller and have your finger professionally measured. Most jewellery stores offer this service quickly and free of charge. UK ring sizes are measured using letters, so once you know your size it can easily be used when ordering rings online.
Alternatively you can get a piece of string, wrap it around your finger and measure the circumference which will give you the inside diameter. I've added a handy table below with all the sizes and measurements for you to compare it to.
If none of these are an option you are more than welcome to purchase a ring sizer here. The instructions for how to use it will be on the listing.
