An alternate procedure, the run time integral method, is suggested for precise determinations of molar mass distributions of polymers. The method is based on the fact that modern analytical ultracentrifuges directly measure the variable angular velocity ω as a function of time. This allows an exact calculation of the run time integral ∫ ω 2 dt and replaces the physically uncorrect zero time correction t 0 by ∫ ω 2 dt. The procedure is demonstrated by narrow and broad distributed polymers.