The combined thermodynamic‐micromechanical model of Fischer [1] is applied to a low‐alloyed TRIP steel with a volume fraction of 16% of retained austenite. The model is implemented in a finite element code. The mesh consists of 9 by 9 by 9 cubical elements, each representing a single grain with a random crystallographic orientation. The retained austenite grains are randomly dispersed through the entire mesh. The extent of the martensitic transformation in all austenite grains is calculated. A large spread in transformation rate is observed. The most favourably oriented grains reach a full martensitic structure, while the martensite volume fraction of less favourably oriented grains is less than 50%. When the chemical driving force is more negative, the onset of the transformation is delayed and the increase of the martensite volume fraction is slower. The calculated results are compared with experimentally obtained values. Although in general a reasonable agreement is found, Fischer's approach leads to some discrepancies with experimental observations.