Find your correct ring size at home using a strip of paper and a ruler.
Cut a thin strip of paper or string and wrap it snugly around the base of the finger you want to size. Wrap below the knuckle but make sure it can still slide over the knuckle.
Use a pen to mark the exact point where the strip meets itself, forming a full circle around your finger.
Lay the strip flat and measure from the start to your mark in millimeters. This is your finger's circumference.
Compare your millimeter measurement to a ring size chart (for example, ~57.8mm is US size 8.5). Measure at the end of the day when fingers are warm, and check 2-3 times for accuracy.