The relationship between maximum continuous torque and size for permanent magnet machines has been studied for a variety of configurations using rare-earth magnets. The published and theoretical results show a remarkably consistent value of about 2.2-2.3 for the ratio T/V or the experimental range considered (nearly two orders of magnitude). The value 2.2 to 2.3 applies to the working torque of a cylindrical machine with rare-earth magnets working at high efficiency with economic use of magnet material and realistic values of temperature rise. For less conventional machines using a wide range of dimensions, materials and construction, the normalised parameter T/(BH) averaging just above unity (with observed range from about 1/3 to 3) for three orders ofmagnitude in machine size, and for machines ranging from wind generators to brushless DC servo motors. This general scaling rule should be of particular value to potential users and to those considering rating standards