DNA Concentration (A260) Calculator
Convert a spectrophotometer A260 reading into nucleic-acid concentration using the Beer-Lambert factor, with total yield and the A260/A280 purity ratio.
How it works
concentration (ng/µL) = A260 × extinction factor × dilution factor · total (µg) = concentration × volume ÷ 1000 · purity = A260 ÷ A280
Nucleic acids absorb ultraviolet light at 260 nm, and over the usable range absorbance is proportional to concentration (Beer-Lambert law). An A260 of 1.0 corresponds to about 50 ng/µL of double-stranded DNA, 33 ng/µL of single-stranded DNA or 40 ng/µL of RNA, so multiplying the reading by that factor and the dilution factor gives the stock concentration. The A260/A280 ratio flags contamination: pure DNA sits near 1.8 and pure RNA near 2.0, while protein or phenol carryover pulls the ratio down.
Worked example
A double-stranded DNA sample reads A260 = 0.25 after a 50× dilution. Concentration = 0.25 × 50 × 50 = 625 ng/µL. Across a 100 µL stock that is 62.5 µg. If A280 = 0.13, the A260/A280 ratio is 1.92 — clean DNA.
Sources: Beer-Lambert law — Wikipedia
Frequently asked questions
Why the factor of 50 for DNA?
It is the empirical extinction factor: a 1 cm path-length sample of double-stranded DNA at 50 ng/µL gives an A260 of about 1.0. Single-stranded DNA and RNA absorb slightly more per mass, so they use 33 and 40 respectively.
What is a good A260/A280 ratio?
Around 1.8 for pure DNA and 2.0 for pure RNA. A ratio well below 1.8 suggests protein or phenol contamination; a very high ratio can indicate RNA in a DNA prep.
Do I need the dilution factor?
Only if you diluted the sample before reading it. If you measured the neat stock, set the dilution factor to 1. Otherwise enter total volume ÷ sample volume, e.g. 100 µL ÷ 2 µL = 50.
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Educational estimate, not medical or laboratory advice — verify formulas, units and assumptions for your specific protocol.