Hi there,

With average weight of base pair of 650 Da in DNA, you can calculate plasmid MW if you first estimate plasmid size on gel (provided the plasmid is linearized) using DNA ladder.

Hi,

I find this link very useful: www.genscript.com/conversion.html 

Dominique's suggestion, 1bp = 650 Dalton will work well - as long as your plasmid DNA is linear. If not, you have to use a closed circular DNA ladder. It is impossible to estimate the size of circular DNA molecules from a linear ladder.