DNA Concentration Calculator (A260)

A plasmid prep. A 1:10 dilution of a plasmid reads A260 = 0.5, A280 = 0.27 and A230 = 0.24 in a 1 cm cuvette. Concentration = 0.5 × 50 µg/mL × 10 = 250 ng/µL. Purity: 0.5 ÷ 0.27 = 1.85 and 0.5 ÷ 0.24 = 2.08, both in the usual range. The 3,000 bp plasmid weighs 3,000 × 617.90 + 36.03 = 1,853,733 g/mol, so 250 ng/µL is 134.9 nM, or 8.122 × 10¹⁰ copies per µL.

ng to pmol. 100 ng of that plasmid is 100 × 1,000 ÷ 1,853,733 = 0.05395 pmol.

The common mistake: using the DNA factor for RNA. An RNA sample reading A260 = 1.2 is 1.2 × 40 = 48 ng/µL. Using the double-stranded DNA factor gives 1.2 × 50 = 60 ng/µL, 25% too high, and every reverse-transcription reaction set up from it gets 20% less RNA than planned. Forgetting the dilution factor is the other classic slip: the plasmid above would read as 25 ng/µL instead of 250.

Nucleic acids absorb ultraviolet light most strongly near 260 nm because of their aromatic bases, while proteins peak near 280 nm from tryptophan and tyrosine. In the early 1940s Otto Warburg and Walter Christian used the ratio of the two wavelengths to estimate protein in the presence of nucleic acid, and the same pair of readings, turned round, became the everyday purity check for DNA. In 1995 Jay Glasel showed in BioTechniques that the 260/280 ratio is a weak test for protein contamination, which is why the ratios here are labelled as guidelines.

The conversion factors of 50, 40 and 33 µg/mL per absorbance unit are the standard values in laboratory manuals such as Sambrook and Russell’s Molecular Cloning and in spectrophotometer makers’ technical notes. They are averages: base composition changes the true value slightly, and short oligos are better quantified with a sequence-specific extinction coefficient.

The molecular weights on this page are calculated from the chemical formulas of the nucleotides with IUPAC standard atomic weights: a deoxyadenosine residue (C₁₀H₁₂N₅O₅P) is 313.21 g/mol, deoxycytidine 289.18, deoxyguanosine 329.21 and thymidine 304.20, so an average base pair is 617.90 g/mol.

This calculator turns UV absorbance into a nucleic acid concentration for double-stranded DNA, single-stranded DNA, RNA or a factor of your own, with any dilution and path length, and reads the 260/280 and 260/230 purity ratios against the usual guidelines. Give it the length and it adds the molecular weight, built from the nucleotide formulas, the molar concentration and the number of copies per µL; it also converts between nanograms and picomoles, including the picomoles of ends that ligation calculations use.

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Related tools: Primer Tm Calculator, Dilution Calculator, and Molarity Calculator.