The formula
concentration (ng/µL) = A260 × factor × dilution
Standard factors at a 1 cm path length: 50 for double-stranded DNA, 40 for RNA and 33 for single-stranded DNA.
Worked example
dsDNA diluted 1:10 reading A260 = 0.75: 0.75 × 50 × 10 = 375 ng/µL. With A280 = 0.40 the 260/280 ratio is 1.88, in the expected range for DNA.
Purity ratios
- 260/280: about 1.8 for pure DNA and 2.0 for pure RNA. Lower values can point to protein or phenol.
- 260/230: about 2.0 to 2.2. Lower values can point to salt, guanidine or carbohydrate carryover.
Readings below 0.1 or above about 1.5 are less reliable; change the dilution and measure again.
This calculator is checked against independent calculations before every release. How we check our calculators.