DNA Concentration Calculator
Calculates the concentration of double-stranded DNA in a solution from its UV absorbance at 260 nm and the sample's dilution factor. Used whenever DNA purity and quantity must be confirmed before PCR, sequencing, or cloning.
About this calculator
Nucleic acids absorb ultraviolet light strongly at 260 nm due to the aromatic rings of their nitrogenous bases. For double-stranded DNA (dsDNA), an absorbance of 1.0 at 260 nm corresponds to approximately 50 µg/mL of DNA in solution — a widely accepted empirical conversion factor established from purified DNA samples. The formula is: DNA concentration (µg/mL) = A₂₆₀ × 50 × dilution factor. The dilution factor accounts for any pre-measurement dilution of your stock sample. This method is fast and non-destructive, and modern instruments (NanoDrop, Qubit) automate the calculation. However, absorbance at 260 nm does not distinguish DNA from RNA or free nucleotides, so the A₂₆₀/A₂₈₀ purity ratio (ideal ~1.8 for dsDNA) is always checked alongside concentration to assess sample quality.
How to use
You dilute your DNA stock 1:10 (dilution factor = 10) and measure an absorbance of 0.35 at 260 nm. Apply the formula: DNA concentration = 0.35 × 50 × 10 = 175 µg/mL in the original stock. If you need to set up a PCR reaction requiring 100 ng of DNA in a 50 µL reaction, that corresponds to 2 ng/µL. From a 175 µg/mL (= 175,000 ng/mL = 175 ng/µL) stock, you would dilute approximately 1:87.5 to reach your working concentration. Also check A₂₆₀/A₂₈₀ to confirm the ratio is near 1.8 before proceeding.
Frequently asked questions
What does the A260/A280 ratio tell you about DNA purity?
The ratio of absorbance at 260 nm to absorbance at 280 nm is a standard indicator of DNA purity. Pure dsDNA gives a ratio of approximately 1.8; values significantly below 1.8 (e.g., 1.5–1.6) suggest protein contamination, since proteins absorb strongly at 280 nm due to aromatic amino acids like tryptophan and tyrosine. Values above 2.0 often indicate RNA contamination or the presence of residual chaotropic salts from extraction kits. A ratio in the range 1.7–1.9 is generally considered acceptable for most downstream molecular biology applications such as PCR, restriction digestion, or library preparation.
Why is the conversion factor 50 µg/mL per A260 unit used for double-stranded DNA?
The factor of 50 µg/mL per A260 unit was empirically determined by measuring the absorbance of solutions with known concentrations of purified, high-molecular-weight dsDNA. It reflects the average molar absorptivity of the four DNA bases in their double-helical, stacked conformation. This factor differs for other nucleic acids: single-stranded DNA uses ~33 µg/mL per A260 unit, and RNA uses ~40 µg/mL per A260 unit, because base stacking geometry differs between these forms. Applying the wrong factor to the wrong nucleic acid type is a common source of concentration error in the lab.
When should I use a fluorometric method like Qubit instead of A260 absorbance for DNA quantification?
Fluorometric quantification with intercalating dyes (e.g., Qubit PicoGreen assay) is far more sensitive and selective than UV absorbance. It is the preferred method when your DNA concentration is very low (below ~5 ng/µL, where A260 readings become unreliable), when your sample contains significant RNA contamination that would inflate A260 readings, or when you need to quantify only double-stranded DNA in a mixture. Next-generation sequencing library preparation protocols almost always require Qubit quantification because library loading concentrations are in the picomolar range. UV absorbance remains useful for quick checks of high-concentration genomic DNA preparations where speed matters over ultimate precision.