How to use
- Choose double-stranded DNA, single-stranded DNA or RNA. The unit of length follows: base pairs for the first, nucleotides for the other two.
- Enter the length.
- 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.
- 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.
Formula & 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.
Three lines, applied in order.
- Molecular weight
- MW (g/mol) = length × average weight per unit
- Mass of one picomole
- m (µg) = MW × 10⁻¹² mol × 10⁶ = MW × 10⁻⁶
- Molecules in one picomole
- N = 6.022 × 10¹¹
The averages this tool starts from
| Species | Counted in | g/mol per unit |
|---|---|---|
| Double-stranded DNA | base pairs (bp) | 650 |
| Single-stranded DNA | nucleotides (nt) | 330 |
| RNA | nucleotides (nt) | 340 |
The single-stranded figure is close to half the double-stranded one, and that is not a coincidence — a base pair is two nucleotides. The small discrepancy is the averaging, not an error.
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.
FAQs
Both are in use and neither is wrong; 650 is the rounder figure and 660 is closer to an equal-composition average including counterions. They differ by 1.5 %, which is under the error of most concentration measurements and above the error of a good one. Use whatever your protocol used, and write down which.
Because they calculated it from the actual sequence rather than from an average. For a 20-mer that difference can be several percent; for a 5 kb fragment it vanishes. Use the supplier figure when you have it — this tool is for when you only have a length.
About 0.65 µg, on the 650 g/mol per bp average. 1000 bp × 650 is 650 000 g/mol, and one picomole of anything weighs its molecular weight in picograms times 10⁶ — so 0.65 µg.
It is the same number as g/mol divided by a thousand, so it is meaningful in the sense that any unit is. It is used because suppliers and size markers use it. For nucleic acids the length in bp is the more informative figure, which is why it is the input here rather than the output.
Because both are averages over base composition and they were rounded independently, by different people, decades apart. 2 × 330 is 660, which is exactly the other double-stranded figure in circulation. The 10 g/mol gap between the two conventions is the whole of the 650-versus-660 argument.
No. The per-unit averages are for the residues, so a 5′ phosphate — about 80 g/mol — is not counted. That is a quarter of a percent on a 20-mer and nothing at all on a plasmid, but it is worth knowing when you are reconciling a mass spectrum.
References
Where the defaults 650, 330 and 340 g/mol come from. Length × unit molecular weight is a definition, and the value can be changed in the field.
- New England Biolabs. Nucleic Acid Data (usage guidelines). Read 2026-08-21.https://www.neb.com/en/tools-and-resources/usage-guidelines/nucleic-acid-datadsDNA, 650 g/mol per bp — "MW = number of base pairs × 650 daltons". The same page's "1 pmol of 1000 bp DNA = 0.66 µg" is the 660 this tool mentions as also in use
- Promega Corporation. Nucleotides and Nucleic Acids (Technical Reference, Part #GE634). 2010.https://www.promega.com/-/media/files/resources/technical-references/nucleotides-and-nucleic-acids.pdfssDNA 330 and RNA 340 g/mol per nt — its "Standards" entry. The same entry puts dsDNA at 660, which is why this tool says both 650 and 660 are in use
Often used together
Open →
ng ↔ pmol Converter
Converts between ng, pmol and copy number from nucleic acid length.
Open →
DNA/RNA Concentration Calculator (A260)
Calculates nucleic acid concentration and purity ratios from A260.
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Ligation Calculator
Works out what goes in a ligation reaction from the molar ratio you want.