The circumference chart is a guideline only. This chart is used to help indicate the appropriate size. The most important indicator is the Mid Kneecap measurement. This measurement should be the basis for the size. To ensure a proper fit, circumference measurements should be taken directly on the skin with the leg straightened, using a soft tape measure. (If a soft tape measure is not available, a piece of string or yarn may be used to calibrate the leg's diameter, then measured against a hard tape measure). Locate the middle of the kneecap and measure around the leg. Measurements should be taken directly on the skin with a straight leg.