SUMMARYThe finite-element (FE) approximation of linear elliptic eigenvalue problems is considered. An analysis based on a number of known estimates leads to the simple formula M =r 0 d/(2 p) N relating the total number of degrees of freedom N , the maximum relative error level desired for the eigenvalues, and the number of 'reliable' modes M. (Here d is the spatial dimension and p is the polynomial degree of the FE space.) Moreover, a rough estimate for the numerical value of the constant r 0 for a given application is found. This result supports a well-known rule of thumb.