Since you are ordering online it is important to make sure you accurately measure your wrist to ensure a proper fit. Measure your wrist just above the wrist bone (the side towards your elbow).
Use a flat tape measure or strip of paper. Mark where the strip just overlaps, then lay strip flat and measure up to that mark. Add 1/4″ to 1/2″ depending on how snug you want bracelet to fit.
Don’t rely on a favorite bracelet to guess-timate your size. The total length is not always the inner diameter. Certain factors such as beads or other components can add bulk and in turn not give you the proper sizing. Using something flat will give you a more precise and therefore accurate measurement.