DNA Concentration (A260) Calculator
Nucleic acid quantification by UV absorbance at 260 nm is the most common method in molecular biology. This calculator applies the Beer-Lambert relationship: concentration (ng/uL) = A260 x extinction coefficient x dilution factor. The standard extinction coefficients are 50 ng/uL per A260 unit for double-stranded DNA (dsDNA), 33 ng/uL per unit for single-stranded DNA (ssDNA), and 40 ng/uL per unit for RNA. Enter your absorbance reading, select the nucleic acid type, and provide the dilution factor (1 if undiluted) to get the concentration in ng/uL and ug/mL.
A260 DNA concentration formula
Concentration (ng/uL) = A260 x extinction coefficient x dilution factor
dsDNA: coefficient = 50 ng/uL per A260 unit
ssDNA: coefficient = 33 ng/uL per A260 unit
RNA: coefficient = 40 ng/uL per A260 unit
Note that 1 ng/uL = 1 ug/mL. The dilution factor accounts for any pre-measurement dilution of the stock sample.
Purity assessment with A260/A280
- Pure dsDNA: A260/A280 ratio approximately 1.8.
- Pure RNA: A260/A280 ratio approximately 2.0.
- Ratio below 1.7 may indicate protein contamination or residual phenol.
- Ratio significantly above 2.0 may indicate RNA contamination of a DNA sample.
- Always use blank-corrected readings for accuracy.
Frequently asked questions
Why is A260 used to measure DNA?
DNA and RNA absorb ultraviolet light strongly at 260 nm due to the aromatic bases (adenine, guanine, cytosine, thymine/uracil). The absorbance at 260 nm is proportional to nucleic acid concentration, allowing quick quantification using a spectrophotometer.
What is the extinction coefficient for dsDNA?
For double-stranded DNA, an A260 of 1.0 corresponds to approximately 50 ng/uL. For single-stranded DNA it is approximately 33 ng/uL, and for RNA it is approximately 40 ng/uL. These factors are built into this calculator.
What is the A260/A280 ratio and why does it matter?
The A260/A280 ratio indicates purity. A ratio of approximately 1.8 indicates pure DNA; approximately 2.0 indicates pure RNA. Lower ratios suggest protein contamination (proteins absorb at 280 nm). Values below 1.7 indicate the sample may not be suitable for sensitive downstream applications.
What does the dilution factor do?
If you dilute your sample before reading (e.g., 1 in 10), you must multiply the calculated concentration by the dilution factor to get the concentration in the original undiluted sample. Enter 10 for a 1:10 dilution.
What are the limitations of A260 measurement?
A260 cannot distinguish between DNA, RNA, and free nucleotides, all of which absorb at 260 nm. Contamination with these can overestimate DNA concentration. Fluorometric methods (e.g., Qubit) are more specific when high accuracy is needed.
Official sources
- NCBI Bookshelf: Nucleic Acid Quantification methods.
- NIH Office of Science Policy: NIH guidelines for nucleic acid research.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.