Received... Accepted...

Context. A recent new approach to apply a simple dynamical mass estimate of tidally limited star clusters is based on the identification of the tidal radius in a King profile with the dynamical Jacobi radius. The application to an unbiased open cluster catalogue yields significantly higher cluster masses compared to the classical methods. Aims. We quantify the bias in the mass determination as function of projection direction and cluster age by analysing a simulated star cluster. Methods. We use direct N-body simulations of a star cluster including stellar evolution in an analytic Milky Way potential and apply a best fit to the projected number density of cluster stars. Results. We obtain significantly overestimated star cluster masses which depend strongly on the viewing direction. The overestimation is typically in the range of 10-50 percent and reaches a factor of 3.5 for young clusters. Mass segregation reduces the derived limiting radii systematically. Key words. Galaxy: open clusters and associations: general – methods: N-body simulations – stellar dynamics 1