To measure your finger for using a ring size chart, wrap a tape measure snugly around your finger and not where it overlaps to form a circle. Compare the measurement with a ring size chart to determine your ring size.
Alternative measurement tools include pieces of string or paper. Make sure whatever tool you use doesn't stretch. Wrap either around your finger, and mark the point of overlap, then measure it. If the base of your finger is smaller than your knuckle, measure both and use a size in between both measurements. That way, the ring fits over your knuckle but isn't loose.
Your fingers should be warm or at a normal body temperature when you measure them, because thermal expansion causes them to expand with heat and shrink with cold. Measure the finger where you plan to wear the ring, because different fingers could have different sizes. Fingers are at their biggest towards the end of the day.
A ring with a wide band style is typically a bit smaller, so you may want to buy it a size larger than usual. Rings are sized according to the circumference of the ring. Size charts vary depending on the country, with countries using either a numerical or alphabetical system.