Introducing a decentralized air traffic management system could potentially increase the capacity of enroute airspace. Previous research in this field shows that a layered airspace might have benefits compared to an unstructured airspace, but the exact extent of the benefits are unknown. This research compares four layered airspace concepts, with a variation in heading range per layer, to assess the effect on capacity using fast-time simulations. First, a theoretical model is presented and validated to predict the number of conflicts in a three-dimensional airspace. When performing a model fit to the simulation results at low densities, it can be used to accurately predict the number of conflicts at high densities. Secondly, the influence on capacity of layered airspace concepts is determined based on safety, stability and efficiency. The results show that the safety is the most limiting performance metric, due to a steep increase in the number of conflicts and intrusions for an increase in traffic density. A priori separation of traffic using a layered airspace positively influences the performance compared to an unstructured airspace. Comparing the different layered airspace concepts, a clear improvement in capacity is observed for a decreasing heading range per layer.