Health Fitness Tracker Smart Ring Bluetooth Connection IPX8 Waterproof Rechargeable Smart Ring for Heart Rate Sleep Monitor Type 21 Item Type: Health Tracker Smart Ring Material: Aluminum alloy, ceramic, electronic components Battery Type: Lithium battery (shipped with built in battery) Battery Quantity: 18mAh Duration: 3 to 5 days Charging Time: 45 minutes Protection Grade: IPX8 Applicable size: Ring Number:No. 17, Finger Circumference:56mm/2.2in, International/US:7, Inside Diameter:17mm/0.67in Ring Number:No. 18, Finger Circumference:60mm/2.4in, International/US:8, Inside Diameter:18mm/0.71in Ring Number:No. 19, Finger Circumference:64mm/2.5in, International/US:9, Inside Diameter:19mm/0.75in Ring Number:No. 20, Finger Circumference:69mm/2.7in, International/US:10, Inside Diameter:20mm/0.79in Ring Number:No. 21, Finger Circumference:73mm/2.9in, International/US:11, Inside Diameter:21mm/0.83in Ring Number:No. 22, Finger Circumference:75mm/3.0in, International/US:12, Inside Diameter:22mm/0.87in