The present invention discloses a solution for allocating hall calls in an elevator system, which comprises call input devices for registering hall calls at the floors, a group control system responsive to said hall calls, and a number of elevators controlled by elevator-specific elevator controllers on the basis of commands issued by the group control system. A number of route alternatives are generated on the basis of the calls active, and allocation calculation is decentralized by calculating elevator-specific cost terms associated with the route alternatives in the elevator controllers. The cost terms are returned to the group control system, which allocates the hall calls to the elevators according to the route alternative giving the lowest allocation cost.