About the DNA Concentration Calculator
This calculator converts a UV absorbance reading at 260 nm (A260) into a DNA concentration. Nucleic acids absorb ultraviolet light at 260 nm because of their purine and pyrimidine bases, and that absorbance is directly proportional to how much DNA is present in the light path. The tool applies the Beer-Lambert law together with the empirically established conversion factor for double-stranded DNA, then reports the concentration and, if you supply a second reading, the A260/A280 purity ratio.
The formula
For a standard 1 cm pathlength cuvette:
Concentration (µg/mL) = A260 × dilution factor × conversion factor
The conversion factor is the amount of nucleic acid that gives an absorbance of exactly 1.0 at 260 nm through 1 cm:
- Double-stranded DNA (dsDNA): 50 µg/mL per A260 unit — the default and the value used for genomic DNA, plasmids, and PCR products.
- Single-stranded DNA (ssDNA / oligonucleotides): 33 µg/mL per A260 unit.
- RNA: 40 µg/mL per A260 unit.
Because 1 µg/mL is numerically identical to 1 ng/µL, the same number can be read in either unit. So an A260 of 0.150 measured on a sample diluted 100-fold gives 0.150 × 100 × 50 = 750 µg/mL, i.e. 750 ng/µL of dsDNA.
Why the dilution factor matters
Most spectrophotometers give reliable, linear readings when A260 falls roughly between 0.1 and 1.0. Concentrated stock DNA absorbs far too strongly to read directly, so it is diluted before measurement — for example, 2 µL of sample into 198 µL of buffer is a 100× dilution. The calculator multiplies your reading back up by that factor to recover the true stock concentration. Modern microvolume instruments (such as NanoDrop-style readers) often use a sub-millimeter pathlength and report concentration directly; if you read absorbance on such an instrument, enter a dilution factor of 1 and make sure the instrument has already normalized to a 1 cm-equivalent A260.
The A260/A280 purity ratio
Proteins absorb strongly at 280 nm (from tryptophan and tyrosine), so the ratio of absorbance at 260 nm to 280 nm indicates how clean a DNA prep is. Pure DNA gives an A260/A280 ratio of about 1.8; pure RNA is closer to 2.0. A ratio noticeably below 1.8 usually means residual protein or phenol from the extraction. The ratio is a quality flag only — it does not change the concentration number, but a poor ratio tells you the concentration may include non-DNA absorbers.
Common reference points
- An A260 of 1.0 (1 cm, no dilution) = 50 µg/mL dsDNA = 50 ng/µL.
- A typical genomic DNA prep runs 50–500 ng/µL; plasmid minipreps often 100–1000 ng/µL.
- PCR and qPCR usually need input DNA in the low ng/µL range, so stocks are diluted before use.
- Human genomic DNA: roughly 1 µg ≈ 3 × 10⁵ copies of the genome.