In bit patterned media (BPM), the medium is patterned into nanometer-sized magnetic islands where each island stores one bit. Although BPM samples of credible densities have been made, many problems remain, one of which is the synchronization of the write head switching position with respect to the targeted island. An accurate but efficient model has been developed to calculate the timing margin available for a given required write bit error rate (BER). The model predicts the write-error performance of BPM composed of populations of islands with distributions of magnetic, position, and geometric parameters, and can be used to calculate the write-window for a given BER. The effect of distributions of island position, geometric and magnetic properties has been investigated, and it has been shown that island position and magnetic properties have a much more significant effect upon BER than geometric (shape/size) variations. This model enables the relationship between servo requirements and raw BER to be established for disk drives using BPM.