SUMMARYThis paper proposes a new measure of rotatability of a response surface design. It is based on a comparison between the design moments of a given design and the design moments of a rotatable design closest to the given design in the least squares sense. This measure is easier to calculate than Khurľs measure. An alternative easier non‐geometrical way of deriving Khuri's measure is also presented. Both measures are calculated to illustrate some designs considered by Khu