Converting ng to pmol for DNA and RNA (With the Formula)
5 min read · Updated June 10, 2026
Protocols mix units constantly: your nanodrop reads ng/µL, but a ligation calls for picomoles, qPCR standards are quoted in copies, and a stock is labelled in nM. Converting between mass, moles and molarity for DNA and RNA is one short formula once you know the molar mass. Here's how it works.
The core conversion
Moles are just mass divided by molar mass. For nucleic acids it is convenient to work in nanograms and picomoles, which keeps the numbers human-sized:
pmol = ng × 1000 / molar mass (g/mol)
So if you know the molar mass of your molecule, the conversion in either direction is a single multiplication. To go from pmol back to ng, rearrange: ng = pmol × molar mass / 1000.
Estimating molar mass from length
When you only know the length, average per-unit masses are accurate enough for most lab work:
- Double-stranded DNA: ≈ 650 g/mol per base pair.
- Single-stranded DNA (oligos): ≈ 330 g/mol per nucleotide.
- Single-stranded RNA: ≈ 340 g/mol per nucleotide.
When to use the exact molar mass
Average masses assume a balanced base composition. For short oligos, GC-skewed sequences or anything where precision matters, calculate the molar mass from the actual base composition instead — the difference is largest for short molecules where every base counts. A calculator that accepts the sequence handles this automatically.
Molarity and copy number
Once you have moles you can get concentration and copies. Molarity follows from dividing moles by volume — conveniently, pmol per µL is the same number as nM. Copy number is moles multiplied by Avogadro's number (6.022 × 10²³), which is how you turn a mass of plasmid or amplicon into the copies needed for a qPCR standard curve.
- Work out the molar mass from length (or sequence).
- Convert your mass in ng to pmol with the formula above.
- Divide pmol by volume in µL to get the concentration in nM.
- Multiply moles by 6.022 × 10²³ for the number of molecules.
Frequently asked questions
- What is the molar mass of a base pair of dsDNA?
- About 650 g/mol on average. For a sequence with skewed base composition, calculate the exact mass from the bases instead of using the average.
- Is pmol/µL the same as nM?
- Yes. One picomole per microlitre equals one nanomolar, because pmol/µL = nmol/mL = nmol/L when scaled, which is the definition of nanomolar. It makes converting an amount into a concentration easy.