OD260 to Concentration Calculator
Convert a spectrophotometer reading at 260 nm into nucleic acid concentration. Enter the absorbance and dilution factor, choose the nucleic acid type, and optionally add the A280 reading to get the purity ratio.
- ng/µL
- 50
- A260/A280
- —
How it works
Nucleic acids absorb ultraviolet light at 260 nm, and the absorbance is proportional to concentration. One absorbance unit (A260 = 1, 1 cm path) corresponds to about 50 µg/mL of double-stranded DNA, 33 µg/mL of single-stranded DNA, or 40 µg/mL of single-stranded RNA. The concentration is therefore A260 × dilution factor × the type factor, and because 1 µg/mL equals 1 ng/µL the two units share the same number.
Enter the measured absorbance and the dilution factor used, and pick the nucleic acid type. If you also read the absorbance at 280 nm, entering it gives the A260/A280 purity ratio — roughly 1.8 for pure DNA and 2.0 for pure RNA, with lower values indicating protein or phenol contamination.
Examples
- A260 of 1.0 for dsDNA (dilution 1) = 50 µg/mL.
- A260 of 0.5 diluted 100× for RNA = 0.5 × 100 × 40 = 2000 µg/mL.
- A260/A280 of about 1.8 indicates pure DNA.
Frequently asked questions
- How does A260 give concentration?
- Nucleic acids absorb at 260 nm in proportion to concentration. Multiplying the absorbance by the dilution factor and a type-specific factor (50, 33 or 40 µg/mL per OD) gives the concentration.
- Why are the factors 50, 33 and 40?
- They are the empirically established amounts of double-stranded DNA, single-stranded DNA and single-stranded RNA that give an absorbance of 1 at 260 nm in a 1 cm path length.
- What is the A260/A280 ratio?
- It is a purity indicator. Pure DNA has a ratio near 1.8 and pure RNA near 2.0; a markedly lower ratio suggests contamination by protein or phenol.
- Is µg/mL the same as ng/µL?
- Yes, numerically. One microgram per millilitre equals one nanogram per microlitre, so the concentration value is the same in both units.