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.
have a look at the ucsc table stsMap. For some markers it contains both the positions in cM and in pb. You could use those values to make the conversions.
#chromchromStartchromEndnamescoreidentNoctgAccotherAccgenethonChromgenethonPosmarshfieldChrommarshfieldPosgm99Gb4Chromgm99Gb4PosshgcTngChromshgcTngPosshgcG3ChromshgcG3PoswiYacChromwiYacPoswiRhChromwiRhPosfishChrombeginBandendBandlabdecodeChromdecodePos chr135848613585185AFM280WE510002AL136528.11-chr16.2chr14.2200000000000000chr14 chr144584014458794AFM344WE910003AL591916.8-chr111.1chr18.8500000000000000chr16.43 chr145916464591869AFM123XC310004Z98747.1-chr112.8chr110.78chr121.19chr12499chr112400000000chr16.43 chr148046234805011AFMA203YC110005AL023586.1-chr112.9chr110.7800000000000000chr17.09 chr155765055576824AFMB355ZH510007Z97988.1-chr114.2chr111.87chr123.8900chr11470000000000 chr160773796077732AFMA131YA510006AL035406.25-chr114.2chr111.8700000000000000chr110.11 chr162896036290000AFMA052WG1100010AL031847.17-chr116.4chr114.04chr123.8900chr120200000000chr111.37 chr163594346359798AFM254WB9100012AL031847.17-chr116.4chr114.59chr123.89000000chr123.6000000
there is no fixed ratio between a distance given in cM and a distance in bp: it varies according to chromosomes, organism, sex, etc...
If you need a number for a back-of-the-envelope estimation possibly you can use table 1 in this paper:
which gives you (sex-average on human genome) 1.30 cM/Mb (plus or minus 0.80 cM/Mb)
A fine scale analysis seems to be contained here :
(although I haven't read the paper myself)
If you need coordinates for the current human reference, you could get the HapMap data set from:
There's one file per chromosome there, listing the chromosomal bp positions for many cM coordinates. The map is dense - for example chr1 lists 256895 positions.
What exactly are you trying to achieve?
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).