Nucleic Acid Concentration Calculator (A260)
Free DNA/RNA concentration calculator from A260 with Beer-Lambert factors (dsDNA 50, ssDNA 33, RNA 40 ug/mL), A260/A280 and A260/A230 purity interpretation, dilution factor and ng/uL to nM molarity conversion.
How it works
Concentration is calculated from absorbance at 260 nm by the Beer-Lambert law with the standard 1 cm path-length conversion factors: an A260 of 1.0 corresponds to 50 ug/mL for double-stranded DNA, 33 ug/mL for single-stranded DNA and 40 ug/mL for RNA, multiplied by any dilution factor. Purity is assessed from two ratios: A260/A280 (about 1.8 for pure DNA, about 2.0 for pure RNA; lower values indicate protein or phenol carry-over) and A260/A230 (2.0-2.2 expected; lower values indicate guanidine salts, phenol or carbohydrates, which inhibit downstream enzymes). With a fragment length, mass concentration converts to molarity using average residue masses of 660 g/mol per bp (dsDNA), 330 (ssDNA) and 340 (ssRNA). Readings below A260 of 0.02 are flagged as inside instrument noise rather than reported as precise, and for oligos under 60 nt the tool recommends the sequence-specific extinction coefficient from the synthesis report instead of the flat conversion factor. All computation happens in the browser.
Frequently asked questions
What does A260/A280 tell me?
The ratio of absorbance at 260 nm to 280 nm reports protein contamination. Pure DNA reads about 1.8 and pure RNA about 2.0, because nucleic acids absorb maximally near 260 nm while aromatic amino acids absorb near 280 nm. A ratio well below 1.8 usually means protein or phenol carried over from extraction; an unusually high ratio in a DNA prep can indicate RNA contamination.
Why is my A260/A230 low, and does it matter?
Guanidine salts (from column lysis buffers), phenol and carbohydrates all absorb near 230 nm, so carry-over depresses the ratio below the expected 2.0-2.2. It matters mainly for downstream enzymology: these contaminants inhibit PCR, ligation and sequencing reactions more than they distort the concentration estimate itself. Re-precipitating or re-washing the sample usually restores the ratio.
How do I convert ng/uL to nM?
Divide the mass concentration by the molar mass of the fragment and scale: nM = (ng/uL divided by (length x average residue mass)) x 10^6, where the residue mass is 660 g/mol per base pair for double-stranded DNA, 330 for single-stranded DNA and 340 for single-stranded RNA. A 5,386 bp dsDNA plasmid at 100 ng/uL is therefore about 28 nM.
Why should I not use this for short oligos?
The flat conversion factors assume average base composition, which holds for long molecules but fails for short ones where individual bases dominate the extinction coefficient. For primers and other oligos under about 60 nt, use the sequence-specific extinction coefficient printed on the synthesis report, calculated by the nearest-neighbour method.