Nucleic Acid concentration from OD260

Calculate the concentration of DNA or RNA from a A₂₆₀ spectrophotometer reading, and check sample purity with the A₂₆₀/A₂₈₀ ratio. Enter your absorbance, dilution and nucleic acid type below.

How the calculation works

Nucleic acids absorb UV light at 260 nm. By the Beer–Lambert law, absorbance is proportional to concentration, so:

concentration (ng/µL) = A260 × dilution factor × conversion factor

The conversion factor depends on the type of nucleic acid, because each absorbs differently per unit mass:

• Double-stranded DNA: 50 ng/µL per A₂₆₀ unit
• RNA: 40 ng/µL per A₂₆₀ unit
• Single-stranded DNA / oligos: ~33 ng/µL per A₂₆₀ unit

(Note: ng/µL and µg/mL are numerically identical.)

Checking purity

• A₂₆₀/A₂₈₀: ~1.8 indicates pure DNA, ~2.0 pure RNA. A lower ratio suggests protein or phenol contamination.
• A₂₆₀/A₂₃₀: a secondary purity check; values of 2.0–2.2 are expected. Low values point to carbohydrate, phenol or chaotropic-salt carryover.

Tips for an accurate reading

• Keep A₂₆₀ within the instrument’s linear range (roughly 0.1–1.0); dilute and re-read concentrated samples.
• Blank with the same buffer your sample is in.
• Use the correct conversion factor for your molecule — using the dsDNA factor for RNA overestimates concentration.

Frequently asked questions

How do I calculate DNA concentration from A₂₆₀?

Multiply A₂₆₀ by the dilution factor and by 50 (for dsDNA) to get ng/µL. Enter the values above and the tool does it for you.

What conversion factor should I use for RNA?

40 ng/µL per A₂₆₀ unit for RNA; 33 for single-stranded DNA/oligos; 50 for double-stranded DNA.

What A₂₆₀/A₂₈₀ ratio means pure DNA?

About 1.8 for DNA and 2.0 for RNA. Markedly lower values indicate protein or phenol contamination.

My A₂₆₀ is above 1. Is the value reliable?

It may be outside the linear range. Dilute the sample, re-read, and the tool’s dilution factor will scale the result.

Related guide: DNA & RNA quantification →