DNA/RNA Concentration Calculator (A260)
Written by Thierno Sadou Diallo, formula verified per our methodology • Last checked on 9/8/2026
A nucleic acid's concentration is calculated with A260 × conversion factor × dilution factor. For double-stranded DNA (factor 50) with an absorbance of 0.5 measured without dilution, the concentration is 25 ng/µL.
Explanation
Measuring DNA or RNA concentration by UV spectrophotometry relies on the same principle as the Beer-Lambert law: absorbance at 260 nm (the wavelength at which nucleic bases strongly absorb UV light) is proportional to the sample's concentration. In lab practice, this relationship is simplified into three standard conversion factors, specific to each type of nucleic acid: one unit of absorbance at 260 nm corresponds to about 50 µg/mL for double-stranded DNA, 40 µg/mL for RNA, and 33 µg/mL for single-stranded DNA, always over a reference optical path of 1 cm (the standard format for NanoDrop-type instruments, which automatically adjust a shorter-path measurement to this reference). If the sample was diluted before measurement, simply multiply the result by the dilution factor to recover the original sample's concentration. This method remains fast and inexpensive, but it does not distinguish DNA or RNA from other substances that also absorb around 260 nm (proteins, residual phenol, salts): the A260/A280 ratio (a classic purity indicator, not calculated here) helps detect likely contamination if the concentration measurement seems inconsistent with the extraction protocol used.
Example: double-stranded DNA, A260 = 0.5, no dilution
Inputs
A260 absorbance: 0.5. Type: double-stranded DNA (dsDNA). Dilution factor: 1.
Calculation
Concentration = 0.5 × 50 × 1 = 25 ng/µL.
Result
This sample's estimated concentration is 25 ng/µL.
Frequently asked questions
Why does the conversion factor differ between DNA, RNA, and single-stranded DNA?
Each type of molecule has a slightly different structure and base composition, which changes its UV light absorption capacity at 260 nm per unit of mass. These three factors (50, 40, and 33) are standard empirical values established for pure samples of each category, widely used in molecular biology protocols.
What if my sample was measured over an optical path other than 1 cm?
Most modern benchtop spectrophotometers (NanoDrop-type) automatically normalize their reading to an equivalent 1 cm optical path before displaying the absorbance: the reading can therefore be used directly in this calculator. On an older instrument with a different cuvette path, the measured value must first be converted to a 1 cm path before using it here.
Does this calculator check the sample's purity?
No: it only calculates concentration from the absorbance at 260 nm, assuming a relatively pure sample. In the lab, purity is checked separately with the A260/A280 ratio (around 1.8 for pure DNA, 2.0 for pure RNA); a noticeably lower ratio suggests contamination by proteins or other residues, which would also skew the concentration calculated here.