This paper reports on a three-dimensional simulation of a commercial, thermally actuated bubble jet printhead using an appropriate pressure boundary condition for the bubble nucleation and expansion. The ink jet system has been used as a realistic show case for the application of the volume of fluid (VOF) method that is implemented in the simulation package ACE+ of CFDRC [1]. Comparing the results obtained by computational fluid dynamics (CFD) simulations with stroboscopic and gravimetric measurements, excellent agreement with experimental findings has been obtained. The differences in ejected droplet volume and droplet speed between simulation and experiment have been lower than 5 %. The simulation tool applied has been used to derive quantitative predictions not only in simplified two-dimensional test geometries, but also for the real three-dimensional device. It proves to be a valuable tool for cost- and time-efficient optimisation of printhead performance.