Dna Molecular Weight
Calculator

For double-stranded DNA, molecular weight is commonly approximated as the number of base pairs times about 650 daltons per base pair (average mass of a nucleotide pair including the phosphate backbone); for single-stranded DNA, roughly 330 daltons per base is used instead.

The 650 Da/bp figure is an average across the four bases weighted by their typical occurrence, since A-T and G-C pairs have slightly different exact masses; for most cloning and molar conversion purposes this average is precise enough, though exact-sequence calculations exist for higher precision work.

Divide the mass of DNA you have (in nanograms or micrograms) by its molecular weight to get moles, then apply the appropriate unit conversion; this is the standard step for calculating how many picomoles of an insert or vector you have from a nanogram quantity on a gel or nanodrop reading.

The per-base-pair mass constant is the same, so molecular weight is still base pairs x 650 Da regardless of topology; what differs for plasmids is that you must use the full plasmid length (backbone plus insert) rather than just the insert length.

Gel-based size estimates from migration distance are approximate and can be skewed by DNA topology (supercoiled plasmids migrate faster than their true size suggests) or by the reference ladder used, whereas the calculator gives the theoretical mass from the known sequence length.