The centrality determination and the estimated fluctuations of number of participant nucleons N_{part} in Au–Au collisions at 1.23 AGeV beam kinetic energy suffers from severe model dependencies. Comparing the Glauber Monte Carlo (MC) and UrQMD transport models, it is shown that N_{part} is a strongly model dependant quantity. In addition, for any given centrality class, Glauber MC and UrQMD predicts drastically different N_{part} distributions. The impact parameter b and the number of charged particles N_{ch} on the other hand are much more correlated and give an almost model independent centrality estimator. It is suggested that the total baryon number balance, from integrated rapidity distributions, can be used instead of N_{part} in experiments. Preliminary HADES data show significant differences to both, UrQMD simulations and STAR data in this respect.