1. A programmer for an implantable electrical stimulator, the programmer comprising: a user interface that displays a graphical representation of at least one electrical stimulation zone and receives user input graphically manipulating at least one of a size and a shape of the representation of the at least one electrical stimulation zone; and a processor configured to define a program to control delivery of electrical stimulation therapy by the implantable electrical stimulator based on the user input, wherein the processor is configured to: define an intensity of the at least one electrical stimulation zone; define each of a plurality of electrodes, on at least one lead, to deliver at least a portion of the electrical stimulation therapy; automatically determine an electrical stimulation contribution of each of the plurality of defined electrodes to the at least one stimulation zone; and automatically determine a stimulation amplitude of the portion of the electrical stimulation therapy delivered by each of the plurality of defined electrodes based on the electrical stimulation contribution of the electrode and the intensity of the at least one electrical stimulation zone, wherein the amplitude of at least one of the plurality of defined electrodes is equal to an amplitude corresponding to the intensity of the at least one electrical stimulation zone, and wherein a sum of the amplitudes of the plurality of defined electrodes is greater than the amplitude corresponding to the intensity of the at least one electrical stimulation zone.