Up to about 10% recombination, 1% recombination = 1 centimorgan (cM). Centimorgans are calculated from observed % recombination, and attempt to take into account double crossovers. The cM is thus a statistical unit of measure without a physical basis (i.e. DNA bases). Examples are Kosambi and Haldane cM.

Observed % recombination is converted to Kosambi cM as

cM = (0.25)LN ((1+ 2(observed recombination)/1-2 (observed recombination)).
note: LN = natural log

Observed % recombination is converted to Haldane cM as

cM =-0.5*LN(1-2*r)
note: LN = natural log

Graphic representation of the relationship between cM and % recombination