Nucleic Acid Concentration Calculator: OD260, ng/µL, and pmol Conversion
A260-to-mass-concentration conversion uses empirical extinction coefficients for each nucleic acid type (1 cm path length, A260 = 1):
| Nucleic acid type | Concentration at A260 = 1 |
|---|---|
| Double-stranded DNA (dsDNA) | 50 µg/mL |
| Single-stranded DNA (ssDNA) | 33 µg/mL |
| RNA | 40 µg/mL |
| Oligonucleotide | ~33 µg/mL |
Concentration = A260 × coefficient × dilution factor.
Mass–molar interconversion uses average molecular weights: ~660 g/mol per base pair for dsDNA, ~330 g/mol per nucleotide for single-stranded nucleic acids. Therefore:
dsDNA: pmol = ng × 1000 / (660 × bp)
ssDNA: pmol = ng × 1000 / (330 × nt)
On purity ratios: A260/A280 indicates protein contamination; values near 1.8 for DNA and 2.0 for RNA are acceptable. A260/A230 reflects residual salts, phenol, or guanidinium; the ideal range is 2.0–2.2. A low ratio suggests contamination, but a normal ratio does not guarantee nucleic acid integrity — degraded RNA can still give a perfectly respectable 260/280. Check gel electrophoresis or RIN values.
For oligonucleotides, using these average molecular weights (660/330) introduces error; if the sequence is known, use the exact extinction coefficient provided by your synthesis vendor.
FAQ
Does an A260/A280 of 1.8 mean the DNA is high quality?
It only indicates low protein contamination. To assess nucleic acid integrity, run a gel or check fragment distribution with a Bioanalyzer or TapeStation.
Why do Qubit and NanoDrop give very different readings?
NanoDrop measures all 260 nm-absorbing species — free nucleotides and RNA are both counted. Qubit uses a sequence-specific fluorescent dye and is generally a more accurate measure of actual dsDNA content.
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