The extrapolation distance ϵ at the surface of a grey cylindrical control rod of given size is found in terms of the blackness β of the rod and in terms of the extrapolation distance ϵb at the surface of a black rod of the same size. The formula is where λe is the transport mean free path in the region external to the rod. Tables are provided for β and an empirical expression for ϵb/λe is given.