DNA/RNA ng to pmol Calculator (With 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: pmol per µL is µM, and multiplying by 1000 gives 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 µM; multiply by 1000 to get nM.
- Multiply moles by 6.022 × 10²³ for the number of molecules.
Worked example: ng to pmol for a GC-rich oligo
The formulas above are exact once you have a molar mass — what moves the answer is whether that molar mass comes from a flat per-base average or from the real base composition. The gap is easiest to see with a short, deliberately skewed sequence.
Take a 20-nt oligo that's all G and C, no A or T at all: GCGCGCGCGCGCGCGCGCGC (10 G, 10 C), quantified at 100 ng.
- Flat average estimate: 20 nt × 330 g/mol ≈ 6,600 g/mol. pmol = 100 × 1000 / 6,600 ≈ 15.15 pmol.
- Exact mass from base composition, using standard per-base residue weights (A 313.21, T 304.2, C 289.18, G 329.21 g/mol, minus 61.96 for the free 5'-OH terminus): (10 × 289.18) + (10 × 329.21) − 61.96 = 6,121.94 g/mol.
- pmol using the exact mass: 100 × 1000 / 6,121.94 ≈ 16.33 pmol — about 8% higher than the flat-average estimate.
Worked example: dsDNA mass to qPCR copy number
Copy number follows the same chain as above — ng → pmol → moles → molecules — with one extra multiplication at the end. Say a 500 bp PCR amplicon is quantified at 50 ng total, and a qPCR standard curve needs to know how many copies that represents.
If that same 50 ng were eluted in 50 µL, the pmol figure below also gives you the concentration directly: 0.1538 pmol / 50 µL = 0.00308 µM = 3.08 nM. One mass measurement gets you copies, molarity, or both, once you commit to a molar mass and (if needed) a volume.
- Estimate molar mass from length: 500 bp × 650 g/mol per bp = 325,000 g/mol.
- Convert to pmol: 50 × 1000 / 325,000 ≈ 0.1538 pmol.
- Convert pmol to moles: 0.1538 pmol = 1.538 × 10⁻¹³ mol.
- Multiply by Avogadro's number: 1.538 × 10⁻¹³ mol × 6.022 × 10²³ /mol ≈ 9.26 × 10¹⁰ copies — about 93 billion molecules in that 50 ng.
Common mistakes when converting ng to pmol
- Using the ssDNA average (330 g/mol per nt) for a double-stranded fragment, or vice versa. The dsDNA average (650 g/mol per bp) already accounts for both strands, so mixing these up doesn't just nudge the answer — it roughly doubles or halves it.
- Plugging a concentration (ng/µL) straight into the ng → pmol formula as if it were a total mass. The arithmetic still runs, but the result is pmol per microlitre — a µM value — not the total picomoles in the tube. Multiply concentration by volume to get total ng first if a protocol or ligation calculator is asking for total pmol.
- Applying DNA's molar-mass constants (330 or 650) to RNA. RNA's extra 2'-hydroxyl pushes the average up to roughly 340 g/mol per nt for ssRNA, so using the DNA constants introduces a small but real, length-scaled error.
- Treating the flat per-base average as exact for short, composition-skewed oligos, gRNAs or probes, then being surprised a ligation or assembly comes in off-ratio — as the worked example above shows, that estimate can be off by several percent for exactly the short, GC- or AT-heavy sequences where hitting a precise molar ratio matters most.
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?
No. One picomole per microlitre equals one micromolar (µM), which is 1000 nM. Divide pmol by volume in µL to get µM, then multiply by 1000 for nM.
Do I need the exact sequence, or is the average per-base mass close enough?
For most routine work, the average per-base mass (650 g/mol per bp for dsDNA, 330 g/mol per nt for ssDNA, 340 g/mol per nt for ssRNA) is close enough — typically within a few percent for anything of reasonable length and roughly balanced composition. Use the exact molar mass calculated from the real base composition when precision matters: short oligos, strongly GC- or AT-skewed sequences, or steps like Golden Gate assembly and ligations where you're targeting a specific molar ratio.
How do I convert ng/µL directly to nM in one step?
nM = (ng/µL × 1,000,000) / molar mass (g/mol). This just chains the ng→pmol formula with the pmol/µL→nM step, so you don't need to compute a total mass and a total volume separately — plug in the concentration and the molar mass and you get molarity directly.
What happens if I use the ssDNA constant for a double-stranded fragment by mistake?
You'll be off by roughly 2x. The dsDNA average (650 g/mol per bp) already accounts for both strands, so using the ssDNA average (330 g/mol per nt) for the same fragment gives a molar mass about half the correct value — and since pmol is inversely proportional to molar mass, that understated mass makes your calculated pmol (and any copy number derived from it) come out roughly double what it should be.