We measure the stacking fault energy of a set of 20 at% Cr-austenitic stainless steels by means of transmission electron microscopy using the weak beam dark field imaging technique and the isolated dislocations method. The measurements are analyzed together with first principles calculations. The results show that experiment and theory agree very well for the investigated concentration range of Mn (0-8%) and Ni (11-30%). The calculations show that simultaneous relaxation of atomic and spin degrees of freedom is important in order to find the, global energy minimum for these materials. Our results clearly show the great potential of the weak beam dark field technique to obtain accurate measurements of the stacking fault energy of austenitic steels and that the reliable predictability of first principles calculations can be used to design new steels with optimized mechanical properties. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.