The Oura Ring 5 Sizing Kit allows you and your environment to find the ideal fit before you buy. A properly fitting Oura Ring 5 ensures the best possible data accuracy, comfort and overall experience. Oura Ring 5 sizes differ slightly from standard ring sizes and other Oura Ring generations. It is important that you determine your own size and follow the sizing instructions. The Oura Ring 5 works on all fingers and can be worn on both hands. For optimal accuracy, we recommend wearing the ring on the index, middle or ring finger.