The 24 hour a day, seven day a week biometric tracker sold on your finger has one requirement that no wrist worn device does. It has to fit properly. Not close to properly. Exactly. The sensors that measure your heart rate, HRV, temperature, sleep stages, and 60+ other biometrics are calibrated to sit in a specific relationship to your skin, and a ring that is even half a size wrong loses the accuracy that makes the Oura worth wearing. Your fingers change size across the day, across seasons, across weight fluctuations, and across your dominant vs non dominant hand. This is why every Oura Ring order starts with the sizing kit and not the ring.
The sizing kit contains eight plastic rings across sizes 6 to 13 that you wear for 24 hours (Oura recommends this specifically) to identify the size that fits correctly across your daily temperature range, activity level, and finger swelling patterns. Sleep in it. Train in it. Cook in it. Sit at your desk in it. Notice how it feels at 6am, at 2pm after a workout, and at 10pm before bed. Notice which finger it fits best on. Notice whether the fit remains snug without being tight or loose without slipping. Once you have identified your correct size, you use the code included in the kit to order your Oura Ring 5 at that exact size.
Oura charges for the sizing kit to prevent the fraud, waste, and sizing chaos that free sizing kits create, and because getting the size right the first time saves both you and Oura the cost and hassle of returns and exchanges on the actual ring. It is a small investment to ensure the £300+ device you are about to buy fits perfectly and delivers the biometric accuracy it is engineered to provide.
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