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A260 DNA / RNA Concentration & Purity Calculator

Convert a NanoDrop or spectrophotometer A260 reading into ng/µL of dsDNA, ssDNA, RNA or oligonucleotide, judge purity from the A260/A280 and A260/A230 ratios, and work out the volume needed for a given mass of nucleic acid.

Nucleic acid

A260 reading

A280 reading (0 if unknown)

A230 reading (0 if unknown)

Dilution factor of the measured sample

Mass you need for the next step

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How it works

Concentration (ng/µL) = A260 × conversion factor (dsDNA 50, ssDNA 33, RNA 40) × dilution factor · purity = A260 ÷ A280 (≈ 1.8 DNA, ≈ 2.0 RNA) and A260 ÷ A230 (2.0–2.2)

Nucleic acids absorb ultraviolet light most strongly at 260 nm because of their aromatic bases, and at a 1 cm path an absorbance of 1.0 corresponds to about 50 µg/mL of double-stranded DNA, 40 µg/mL of RNA or 33 µg/mL of single-stranded DNA — the differences come from base stacking, which lowers absorbance in the double helix. Multiplying the reading by that factor and any dilution gives concentration directly. Proteins absorb at 280 nm and common contaminants such as phenol, guanidine and EDTA absorb near 230 nm, so the ratios of A260 to those wavelengths are a quick purity check: a low 260/280 flags protein, a low 260/230 flags salts or organics, either of which can inhibit enzymes downstream. Absorbance cannot distinguish DNA from RNA or intact from degraded material, so a gel or a fluorometric assay is still needed when that matters.

Worked example

A dsDNA prep reading A260 = 0.250 undiluted on a NanoDrop is 0.250 × 50 = 12.5 ng/µL. With A280 = 0.135 the 260/280 ratio is 1.85 (clean) and with A230 = 0.120 the 260/230 is 2.08 (clean). To take 500 ng into a ligation you would pipette 40 µL.

Frequently asked questions

Why does my NanoDrop say 1.8 is pure but my RNA reads 2.0?

RNA has a higher proportion of uracil and single-stranded structure, so pure RNA sits near 2.0 while pure DNA sits near 1.8. A DNA prep reading 2.0 often contains RNA.

Is the A260 concentration accurate for very dilute samples?

Below about 2–5 ng/µL (A260 < 0.05–0.1 at a 1 cm equivalent path) the reading is dominated by noise and buffer background. Use a fluorescent dye assay (PicoGreen, Qubit) for low concentrations.

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Standard laboratory formulas and textbook recipes (Sambrook, Bio-Rad, Thermo Fisher protocol conventions). Follow your own kit or instrument manual where it differs, and validate with controls — this tool does not replace lab training or safety procedures.