There is no fixed relationship between physcial and genetic distance, so as @vitis points out you need to use empirical estimates from your species/population. If you have this data from markers you can generate a MareyMap to interpolate the relationship between genetic and physical maps (see also this paper).
Linkage distances (centimorgans) have to be estimated from the populations your study (i. e. counting the recombinants), while physical distances can be identified by mapping your markers back to the genome, if you're lucky enough to have the reference and markers.
This number varies along each eukaryotic chromosome because the meiotic recombination rate can vary along each chromosome (see, for example Barnes at al., Genetics, 1995; for an early analysis comparing genetic map distances in cM to the C. elegans physical map in bp). So there is no simple "conversion" between the two. The previous posts describe the correct approach to calculate the Marey Map (which is what you want).