DNA/RNA Concentration & Purity Calculator (A260)

Understanding the DNA/RNA concentration calculator — concept and principle

Before PCR, cloning, sequencing or library preparation you need to know how much nucleic acid is in a tube and whether it is clean. UV absorbance at 260 nm gives the concentration, and the ratios A260/A280 and A260/A230 flag protein, phenol, salt or other contaminants. This calculator turns your readings into ng/µL and both ratios.

Absorbance at 260 nm gives the amount; the 280 and 230 readings check for impurities.

How it is calculated

Concentration (ng/µL) = A260 × factor × dilution factor, for a 1 cm path length. The factor is 50 for double-stranded DNA, 40 for RNA, 33 for single-stranded DNA and about 20 as a rough value for short oligonucleotides.
A260/A280 and A260/A230 are simple quotients of the readings.
The purity line shows "OK" when A260/A280 is within 0.05 of its target (1.8 for DNA, 2.0 for RNA) and A260/A230 is at least 1.8; otherwise it names the low ratio (protein or salt carry-over).

The principle

The Beer–Lambert law states that absorbance is proportional to concentration and path length: A = ε × c × l. The aromatic bases of DNA and RNA absorb strongly near 260 nm. Molecular biology uses empirical mass factors instead of a molar ε: a 1 cm solution of dsDNA at 50 µg/mL has an A260 of about 1.0. RNA and ssDNA absorb more per microgram because their bases are not stacked in a double helix, so their factors are smaller. Proteins absorb most near 280 nm (tryptophan and tyrosine), and many extraction reagents such as guanidine salts and phenol absorb near 230 nm, which is why these wavelengths serve as contamination checks.

Worked example

Inputs: A260 = 0.5, A280 = 0.27, A230 = 0.25, dsDNA (factor 50), dilution factor 1.
Concentration = 0.5 × 50 × 1 = 25 ng/µL.
A260/A280 = 0.5 ÷ 0.27 = 1.85.
A260/A230 = 0.5 ÷ 0.25 = 2.0.
Both checks pass for dsDNA (1.85 ≥ 1.75 and 2.0 ≥ 1.8), so the purity line reads OK.

Each point is the calculator re-run with only “A260” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

A260	Concentration (ng/µL)
0.125	6.25
0.25	12.5
0.375	18.75
0.5	25
0.625	31.25
0.75	37.5
1	50

How to read the result

Where does your result fall?

Pure DNA typically gives A260/A280 near 1.8 and pure RNA near 2.0; noticeably lower values suggest protein or phenol carry-over. A260/A230 for clean samples is commonly in the range of about 2.0–2.2; lower values point to salts, carbohydrates or extraction reagents. With the RNA type selected, the same 1.85 would be flagged as 260/280 low, because RNA should be near 2.0.

Common mistakes

Entering a 1:10 dilution as 0.1 instead of 10.
Choosing the dsDNA factor for an RNA sample.
Blanking with water when the sample is in TE or another buffer.

Keep in mind: A260 cannot distinguish DNA from RNA or free nucleotides, so contaminating RNA inflates a DNA reading. For sensitive work, confirm with a dye-based fluorometric assay or gel analysis.

Frequently asked questions

Why is my A260/A280 above 2.0 for a DNA sample?

Contaminating RNA raises the ratio, as can measuring in a slightly alkaline buffer. An RNase treatment or a fluorometric DNA assay can confirm.

How accurate is the oligo factor of 20?

Only roughly. Short oligos vary with base composition, so use the extinction coefficient calculated from the actual sequence for accurate work.

Sources: Sambrook, J. and Russell, D. W., Molecular Cloning: A Laboratory Manual (Cold Spring Harbor Laboratory Press), appendix on quantitation of nucleic acids · Thermo Fisher Scientific, NanoDrop technical notes on 260/280 and 260/230 ratios

Checked 8 Nov 2026. Standard textbook formulas in SI units, computed in your browser. For learning and estimates — real designs must follow your local codes and be checked by a qualified engineer.

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