When buying rings online, determining the ring size can be tricky. Best is to measure at a local jewellery shop. Do mention to the jeweller whether you're measuring for a small or a wide band.
If you want to measure at home, you can measure the inner diameter of a ring that fits you well. Use a calliper to measure to one-tenth of a millimetre precise. Please note that every finger has its own size and the width of the ringband plays a role.