Nucleic Acid Quantification Calculator
Convert A260/A280/A230 readings into concentration, yield, purity ratios and molar concentration.
Concentration (ng/µL) = A260 × factor × dilution ÷ path length. Factors: dsDNA 50, ssDNA 33, RNA 40, oligo 33.
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What it does
Absorbance at 260 nm is the fastest way to estimate how much nucleic acid a sample contains, because the bases absorb strongly in the UV. Converting that reading into ng/µL needs the right extinction factor for the molecule, a correction for any dilution and for the light path, and a quick look at the 260/280 and 260/230 ratios to confirm the sample is clean enough for downstream work.
When to use it
Use it after a plasmid miniprep or a PCR cleanup to check recovery, before setting up a ligation or a sequencing reaction to normalise template amount, and when converting an OD-based measurement into pmol for primer or oligo stocks. The purity ratios also help decide whether a sample needs an extra wash or an ethanol precipitation before it is used in a sensitive enzymatic step.
FAQ
- Why does the conversion factor differ for DNA, RNA and oligos?
- The factor is the concentration that corresponds to an A260 of 1.0, and it depends on the average extinction per base: 50 ng/µL for double-stranded DNA, 33 ng/µL for single-stranded DNA and oligonucleotides, and 40 ng/µL for RNA. RNA absorbs more per unit mass because of its uracil content, and a duplex absorbs less per mass than a single strand because base stacking is reduced in the paired state.
- What A260/A280 ratio should a clean sample have?
- Purified DNA typically falls between 1.7 and 1.9, and purified RNA between 1.9 and 2.1. A lower ratio usually means protein, phenol or ethanol carryover; a markedly higher DNA ratio can indicate RNA contamination. The ratio is only a quick check, so confirm anything suspicious with a gel or a fluorometric assay.
- What does A260/A230 tell me?
- A260/A230 should sit around 2.0–2.2 for clean nucleic acid. A low value points to carryover of guanidine salts, phenol, EDTA, carbohydrates or detergent from the extraction. Because these contaminants also suppress downstream enzymatic reactions, a low 230 ratio is often a stronger warning than a slightly low 280 ratio.
- Can I calculate pmol/µL without knowing the fragment length?
- No. Molar concentration requires a molecular weight, which depends on the number of bases — 660 g/mol per bp for dsDNA, 330 for ssDNA and oligos, and 340 per nt for RNA. Once the length is entered, pmol/µL = (ng/µL × 1000) / molecular weight, which is useful for normalising primers, probes and short fragments before setting up a reaction.