DNA/RNA Molecular Weight Converter

1How to use

1. Choose double-stranded DNA, single-stranded DNA or RNA. The unit of length follows: base pairs for the first, nucleotides for the other two.
2. Enter the length.
3. The average weight per unit is filled in for you. Change it if the protocol you are following uses a different figure — 660 g/mol per bp is as common as 650.
4. Optionally enter an amount in pmol to get its mass in µg and its number of molecules.

💡 Exponents can be entered with e — for example, 1.5×10⁻⁵ is entered as 1.5e-5.

2Formula & notes

A molecular weight for a nucleic acid is usually estimated rather than computed: take the length and multiply it by the average weight of one base pair or one nucleotide. That is what happens here, and the whole of the uncertainty in the answer is in the word average.

Practical notes

• ⚠️ 650 and 660 g/mol per base pair are both in common use for double-stranded DNA, and they give answers 1.5 % apart. The field is an input rather than a constant for exactly that reason. Whichever your source uses is the one to put in.
• The per-unit weights here are the same ones the ng ↔ pmol converter uses, read from the same place in the code. Two tools on one site giving different answers for the same molecule is a failure nothing would report, so the figures are shared rather than copied.
• For a short oligo the estimate is the weakest. A 20-mer of all A weighs meaningfully more than a 20-mer of all C, and averaging over four bases stops being a good approximation once there are only twenty of them. Suppliers give the exact weight on the specification sheet.
• Nothing here knows about the 5′ end. A phosphate adds about 80 g/mol, which matters for a 20-mer and disappears for a plasmid.
• kDa is just the molecular weight divided by a thousand, and it is how constructs get quoted — a 1 kb fragment is about 650 kDa. It is offered because suppliers use it, not because it is a different calculation.

3FAQs