A model is presented to calculate the step coverage of blanket tungsten low pressure chemical vapor deposition (W‐LPCVD) from tungsten hexafluoride . The model can calculate tungsten growth in trenches and circular contact holes, in the case of the reduction by , , or both. The step coverage model predictions have been verified experimentally by scanning electron microscopy (SEM). We found that the predictions of the step coverage model for the reduction of are very accurate, if the partial pressures of the reactants at the inlet of the trench or contact hole are known. To get these reactant inlet partial pressures, we used a reactor model which calculates the surface partial pressures of all the reactants. These calculated surface partial pressures are used as input for our step coverage model. In this study we showed that thermodiffusion plays a very important role in the actual surface partial pressure. In the case where was present in the gas mixture trends are predicted very well but the absolute values predicted by the step coverage model are too high. The partial pressure of , which is a by‐product of the reduction reaction, may be very high inside trenches or contact holes, especially just before closing of the trench or contact hole. We found no influence of the calculated partial pressure on the step coverage. Differences between step coverage in trenches and contact holes, as predicted by the step coverage model, were found to agree with the experiments. It is shown that the combination of the step coverage and reactor model is very useful in the optimization towards high step coverage, high throughput, and low flow. We found a perfect step coverage (no void formation) in a 2 μm wide and 10 μm deep trench using an average flow of only 35 sccm, at a growth rate of 150 nm/min. In general, it is shown that the reduction of by offers no advantages over the reduction by as far as step coverage is concerned.