DNA concentration calculator.
Turn a spectrophotometer reading into a concentration. Enter the A260 absorbance, your dilution and the nucleic acid type to get ng per microlitre and the purity ratio.
Absorbance
LiveConcentration is A260 times the conversion factor times the dilution. For dsDNA, one A260 unit is 50 ng/uL. Pure DNA has an A260/A280 ratio near 1.8.
Concentration
400 ng/uL
in the original sample
Assumes a 1 cm path length. Purity is a guide, not a guarantee. Calculations run in your browser; nothing you enter is stored.
How it works
Absorbance to concentration
Nucleic acids absorb ultraviolet light most strongly at 260 nm, and that absorbance is proportional to concentration. Multiplying the A260 reading by a conversion factor, and by your dilution, gives the concentration. For double-stranded DNA the factor is 50 ng/uL per A260 unit; for RNA it is 40 and for single-stranded DNA 33.
An A260 of 0.8 on a 10-fold diluted dsDNA sample is 0.8 times 50 times 10, which is 400 ng/uL. The A260/A280 ratio then estimates purity.
Reference
A260/A280 purity.
What the ratio of absorbance at 260 and 280 nm tells you about the sample.
| A260/A280 | Interpretation |
|---|---|
| Below 1.8 | Protein contamination |
| 1.8 to 2.0 | Pure DNA |
| Around 2.0 | Pure RNA |
| Above 2.3 | RNA contamination |
The full guide
The complete guide to DNA quantification.
How absorbance gives concentration, what the purity ratios mean, and where it can mislead.
How absorbance gives concentration
Under the Beer-Lambert law, absorbance is proportional to concentration over a fixed path length. Nucleic acids peak at 260 nm, so the A260 reading, times a known conversion factor and the dilution, gives the concentration. The factor depends on the molecule: 50 for dsDNA, 40 for RNA, 33 for single-stranded DNA or oligos.
Most spectrophotometers assume a 1 cm path; small-volume instruments correct for their own short path automatically.
Reading the purity ratios
The A260/A280 ratio flags protein contamination: pure DNA sits around 1.8 and pure RNA around 2.0. A low ratio suggests protein or phenol. The A260/A230 ratio, where available, flags salts and organic carryover and should be 2.0 to 2.2.
Ratios are guides, not guarantees; a good ratio with a tiny A260 can still be unreliable.
Choosing the right factor
Pick the factor that matches your sample. Using the dsDNA factor of 50 on an RNA sample overestimates the concentration. The calculator lets you switch between dsDNA, RNA and single-stranded DNA so the maths matches your prep.
If you are unsure, dsDNA is the default for genomic DNA, plasmids and PCR products.
Where it can mislead
Absorbance cannot tell DNA from free nucleotides or degraded fragments, so a reading can overstate usable material. Contaminants that absorb at 260 nm inflate the result, and very dilute samples are noisy.
For demanding work, confirm with a fluorescent dye method, which measures intact double-stranded DNA specifically.
The formula
Light to
nanograms.
Concentration is A260 times the factor (50 for dsDNA) times the dilution. Purity is A260 over A280.
DNA copy number ›# Concentration
conc = A260 × factor × dilution
dsDNA 50, RNA 40, ssDNA 33 (ng/uL)
# worked example
0.8 × 50 × 10 = 400 ng/uLQuestions
DNA questions.
How do I calculate DNA concentration from A260?
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Multiply the A260 reading by the conversion factor and by the dilution. For dsDNA the factor is 50 ng/uL, so an A260 of 0.8 diluted 10-fold is 0.8 x 50 x 10 = 400 ng/uL.
What conversion factor should I use?
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50 ng/uL per A260 for double-stranded DNA, 40 for RNA, and 33 for single-stranded DNA or oligos. Pick the one that matches your sample.
What does the A260/A280 ratio mean?
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It estimates purity. Pure DNA is around 1.8 and pure RNA around 2.0. A lower ratio suggests protein or phenol contamination.
Why might the reading be misleading?
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Absorbance counts anything that absorbs at 260 nm, including free nucleotides and contaminants, so it can overstate usable DNA. A fluorescent dye assay is more specific.
Is this DNA concentration calculator free?
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Yes. It is completely free with no sign-up, and every calculation runs locally in your browser, so nothing you enter is stored or sent anywhere.
About the developer
Jean Borg
Jean builds and maintains every calculator on freecalculators.pro from Malta, with a focus on tools that are fast, free and show their working. The DNA concentration calculator uses the Beer-Lambert A260 method and is for lab estimates.