For a circular parachute, the diameter in feet should be approximately 2 * Squareroot (weight in pounds). For example a 9lb rocket (4kg) would need a 6 foot (72 inch, or 180cm) diameter parachute. A 4lb (1.8kg) rocket would need a 48inch (120cm) diameter parachute. Each brand of parachute has different performance, and the popular HPR parachutes will quote a range of weights (or masses) for which that parachute is suitable.