DNA Concentration Calculator

A DNA concentration calculator converts a UV absorbance reading at 260 nanometers, called A260, into DNA concentration in nanograms per microliter (ng/µL). It also reports the total amount of DNA in a sample, in micrograms (µg).

What is A260 absorbance?

A260 is the amount of ultraviolet light a solution absorbs at a wavelength of 260 nanometers. DNA absorbs UV light most strongly near this wavelength because of its nucleotide bases. A spectrophotometer measures A260 after a blank reading, using the same buffer as the sample, removes background absorbance. The more DNA in the solution, the higher the A260 reading.

DNA concentration formula

Double-stranded DNA absorbs UV light in proportion to its concentration, a relationship known as the Beer-Lambert law. In a standard 1-centimeter light path, a solution with an A260 reading of 1.0 contains about 50 ng/µL of double-stranded DNA. This number, the conversion factor, turns any A260 reading into a concentration:

DNA concentration (ng/µL) = A260 × 50 × dilution factor

The dilution factor accounts for any dilution made before the sample was measured. A sample measured without dilution has a dilution factor of 1.

How to calculate total DNA yield

Once the concentration is known, the total mass of DNA in the sample can be found from its volume:

Total DNA (µg) = concentration (ng/µL) × volume (µL) ÷ 1000

How to calculate DNA concentration: step by step

Mix the sample thoroughly before measuring. DNA can stick to tube walls or settle unevenly.
Dilute the sample if it is highly concentrated, aiming for an A260 reading between 0.1 and 1.0. Spectrophotometer readings are least reliable outside that range.
Blank the spectrophotometer with the same buffer used for dilution, then measure the sample at 260 nm.
Enter the A260 reading, sample volume, and dilution factor into the calculator to get the concentration and total yield.

Example calculations

Example 1: undiluted sample

An A260 reading of 0.5, no dilution (dilution factor 1), and a sample volume of 100 µL.

Concentration = 0.5 × 50 × 1 = 25 ng/µL
Total DNA = 25 × 100 ÷ 1000 = 2.5 µg

Example 2: diluted sample

A 50 µL sample was diluted 10-fold before measurement, giving an A260 reading of 0.75.

Concentration = 0.75 × 50 × 10 = 375 ng/µL
Total DNA = 375 × 50 ÷ 1000 = 18.75 µg

Calculating the dilution factor

Dilution factor = total volume after dilution ÷ volume of the original sample.

Adding 5 µL of DNA solution to 45 µL of buffer gives a total volume of 50 µL, so the dilution factor is 50 ÷ 5 = 10. A sample that was not diluted has a dilution factor of 1.

Typical DNA concentration ranges

Concentration depends on the sample type and extraction method. Genomic DNA extractions commonly fall between about 10 and 100 ng/µL. Plasmid DNA preparations often yield higher concentrations, sometimes several hundred ng/µL. A result far outside the expected range for a given method can point to a problem with the extraction or the measurement itself.

Limits of UV absorbance measurement

Measuring A260 is fast, but it is not the most sensitive or specific method for DNA. Proteins, RNA, and some buffer components also absorb light near 260 nm and can inflate the reading. Scientists often check the A260/A280 ratio alongside concentration to screen for protein contamination; a ratio near 1.8 usually indicates a pure DNA sample. Fluorescent dye-based methods, such as Qubit assays, bind only double-stranded DNA and give a more accurate reading for dilute or impure samples, though they require extra reagents and a fluorometer.

Frequently asked questions

How do you calculate DNA concentration from A260?

Multiply the A260 reading by 50, the standard conversion factor for double-stranded DNA, then multiply by the dilution factor. If the sample was not diluted, use a dilution factor of 1.

What is a good A260/A280 ratio?

A ratio near 1.8 usually indicates a pure DNA sample. A lower ratio suggests protein contamination. A ratio above 2.0 may indicate RNA contamination.

Can this calculator be used for RNA?

Not directly, because RNA uses a conversion factor of 40 ng/µL instead of 50. To estimate RNA concentration, multiply the calculator's result by 0.8 (40 ÷ 50), or use A260 × 40 × dilution factor directly.

Why should the A260 reading fall between 0.1 and 1.0?

Absorbance stays roughly proportional to concentration, as described by the Beer-Lambert law, mainly within this range. Outside it, readings become less reliable, so a sample that is too concentrated or too dilute should be diluted or concentrated before measuring again.

What is the difference between concentration and purity?

Concentration measures how much DNA is present, in ng/µL. Purity measures how clean the sample is, judged mainly from absorbance ratios such as A260/A280. A sample can have a high concentration and still be impure, or a low concentration and be very pure.

How do I convert ng/µL to µg/mL?

The two units are numerically equal. 1 ng/µL equals 1 µg/mL, because the mass unit and the volume unit each scale by a factor of 1000.

References

Sambrook, J., & Russell, D. W. (2001). Molecular Cloning: A Laboratory Manual (3rd ed.). Cold Spring Harbor Laboratory Press.
Gallagher, S. R., & Desjardins, P. R. (2006). Quantitation of DNA and RNA with absorption and fluorescence spectroscopy. Current Protocols in Molecular Biology, 76(1), A-3D.
Manchester, K. L. (1995). Value of A260/A280 ratios for measurement of purity of nucleic acids. BioTechniques, 19(2), 208–210.

DNA Concentration Calculator | A260 to ng/μL Converter