DNA/RNA Molecular Weight

Double-stranded DNA is measured in base pairs, where each unit already accounts for both complementary bases plus their phosphate backbones, averaging about 660 Da. Single-stranded DNA is measured per individual nucleotide, so its average weight per unit is roughly half that of a base pair, about 330 Da, since only one strand's mass is being counted.

RNA nucleotides carry an extra hydroxyl group on the ribose sugar's 2' carbon that DNA's deoxyribose lacks, adding roughly 16 Da of mass per residue. That is why the standard average, about 340 Da for ssRNA versus 330 Da for ssDNA, is consistently higher even though both are single-stranded nucleic acids.

It is a close estimate, not an exact value, because it uses the average weight per nucleotide across all four bases rather than each base's individual weight. A sequence with an unusual base composition, heavily skewed toward guanine and cytosine or toward adenine and thymine, will have a true molecular weight that differs slightly from this average-based estimate.

It converts a spectrophotometer's UV absorbance reading at 260 nm into a nucleic acid concentration. This calculator reports a standard estimate based on nucleic acid type and length rather than a sequence-specific coefficient, which real quantification workflows calculate from the exact base composition for a more precise concentration.