Analogously to the traditional preset count rate meter algorithm, the error estimate of the modified preset count ratemeter algorithm has been defined as the range around measured mean count rate containing 2/3 of the measurement results. The coefficients determining this range have been calculated for a set of values of the modification parameter (m) and preset count (NT). The calculations cover the ranges of these parameters which are of practical interest (m 0 25 1 5 NT 12 150). By applying the piecewise-linear interpolation scheme it is possible to calculate measurement error for any pair m NT within the ranges of these parameters covered by the present calculations.