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Nucleic acid mass, moles and molarity converter

Mass, moles and molarity for a nucleic acid of a given length, both ways.

Nucleic acid

Result

How do you convert between mass, moles and molarity?

pmol = mass (µg) × 1,000,000 ÷ molecular weight
nM = concentration (ng/µL) × 1,000,000 ÷ molecular weight
molecular weight = length × weight per base (660 dsDNA, 330 ssDNA, 340 RNA)

Worked example. A 500 bp dsDNA has a molecular weight of 500 × 660 = 330,000 g/mol. On the Amount tab, 10 µg of it is 30.3 pmol, which is60.61 pmol of ends. On the Concentration tab, a solution of it at50 ng/µL is 151.5 nM. Typing into the right-hand box of either pair runs the conversion the other way.

Why does this matter for oligos, plasmids and RNA?

Reactions such as ligation, Gibson assembly, PCR and in vitro transcription are specified in moles or molarity, not mass, because they depend on the number of molecules or ends present, not on how much they weigh. A short oligo and a long plasmid at the same mass concentration can differ in molarity by orders of magnitude, which is why a protocol that mixes a primer and a template by mass alone can go badly wrong.

To go from mass or concentration straight to a copy number instead of a molar amount, for example for a qPCR standard, use the DNA copy number calculator. For a molarity calculation from a solute’s formula weight rather than a sequence length, see themolarity calculator.

Frequently asked questions

How do I convert micrograms to picomoles?

Divide the mass in micrograms by the molecular weight in g/mol, then multiply by a million: pmol = µg × 1,000,000 ÷ molecular weight. The molecular weight is the length in bases times the weight per base for the nucleic acid type you pick.

How do I convert ng/µL to nanomolar?

The same relationship applies to a concentration: nM = (ng/µL) × 1,000,000 ÷ molecular weight. This is how a NanoDrop or Qubit reading in ng/µL is turned into the molarity a downstream protocol, such as a ligation or an in vitro transcription, is often specified in. It is on the Concentration tab.

What is "pmol of ends" and why does it only apply to dsDNA?

Every linear double-stranded DNA molecule has exactly two ends, so pmol of ends is simply twice the pmol of molecule. It matters for reactions that act on DNA ends, such as ligation or end labelling, where the working unit is the end, not the whole molecule. A single-stranded molecule or an RNA is usually not described this way, so the calculator only reports it for dsDNA.

Why do dsDNA, ssDNA and RNA give different results for the same mass?

They differ in weight per base: about 660 g/mol per base pair for dsDNA, 330 g/mol per nucleotide for ssDNA and 340 g/mol per nucleotide for RNA. A shorter, lighter molecule has more moles in the same mass, so switching the type without changing the mass changes the pmol and nM results.

Which value do I type in, and which is calculated?

Either one. Type into one box of a pair and the other fills in, and the calculated value is repeated as the headline of the result. Enter a mass to get moles, or moles to get a mass; on the Concentration tab, enter a concentration to get molarity, or a molarity to get a concentration.