Nearly every molecular workflow — cloning, qPCR and PCR, library prep, transfection, mRNA manufacturing — depends on knowing how much DNA or RNA you have and how clean it is. Nucleic acid quantification answers both questions. This guide covers how UV absorbance gives concentration, the conversion factors for dsDNA, ssDNA and RNA, what the A260/A280 and A260/A230 purity ratios mean, how UV compares with fluorometric and gel methods, and how to convert a ng/µL reading into the molarity an enzymatic reaction actually needs.
How UV quantification works
Nucleotide bases absorb ultraviolet light with a peak near 260 nm. The more nucleic acid in the light path, the more 260 nm light is absorbed — a direct application of the Beer–Lambert law, where absorbance is proportional to concentration and path length:
A = ε × c × l
Rather than work with the molar extinction coefficient ε for every sequence, the field uses empirical conversion factors: the concentration that gives an absorbance of 1.0 at 260 nm in a 1 cm path. Protein and phenol absorb at 280 nm, and chaotropic salts and organics absorb near 230 nm, so reading those wavelengths alongside 260 nm gives a purity check for free. The spectrum below shows the three diagnostic points.
Conversion factors and the concentration formula
The working equation for UV quantification is simply Beer–Lambert rewritten with the empirical factor and a correction for any dilution you made to bring the reading into range:
concentration (µg/mL) = A260 × factor × dilution factor
Numerically, µg/mL and ng/µL are the same, so the same number reads directly as ng/µL. The factor depends on the structure of the nucleic acid, because single strands stack their bases differently from a duplex (the hyperchromic effect):
| Nucleic acid | Factor (A260 = 1) | Typical use |
|---|---|---|
| Double-stranded DNA | 50 µg/mL | Genomic DNA, plasmids, PCR products |
| Single-stranded DNA | 33 µg/mL | ssDNA, denatured templates, cDNA |
| RNA | 40 µg/mL | Total RNA, mRNA, in vitro transcripts |
| Oligonucleotide | ~30 µg/mL (sequence-dependent) | Primers, probes — prefer the sequence ε |
Figure 2. The conversion factor differs by nucleic acid type — using the dsDNA factor on an RNA sample over-reports by 25%.
Worked example: concentration from absorbance
A plasmid prep reads A260 = 0.25 after a 1:50 dilution. It is double-stranded DNA, so the factor is 50 µg/mL.
concentration = 0.25 × 50 × 50 = 625 ng/µL
Always dilute so the reading lands in the instrument's linear range (roughly A260 of 0.1–1.0 on a cuvette), then multiply the dilution back out. The DNA/RNA calculator applies the right factor and dilution automatically.
Quantify DNA or RNA in one step
Enter A260, sample type and dilution to get concentration, plus ng/µL to nM and copy-number conversions.
Purity ratios: A260/A280 and A260/A230
Concentration alone does not tell you whether a sample is usable. Two absorbance ratios do most of the quality-control work, and they catch different problems.
A260/A280 compares nucleic acid signal to protein and phenol signal. Pure DNA sits around 1.8 and pure RNA around 2.0; values well below those mean protein or phenol carry-over. Critically, the ratio depends on the pH and ionic strength of the diluent — Wilfinger and colleagues showed that the same DNA reads a meaningfully different A260/A280 in water versus a slightly alkaline buffer, so quantify in low-EDTA Tris (around pH 8) rather than pure water for reproducible numbers.
A260/A230 compares nucleic acid signal to the 230 nm absorbance of chaotropic salts, phenol, EDTA and carbohydrates. A clean prep reads 2.0–2.2. A low A260/A230 is the classic fingerprint of incomplete column washing or guanidinium carry-over, and it often matters more than A260/A280 because those contaminants inhibit ligases, polymerases and transfection.
| Ratio | Ideal | If low, suspect |
|---|---|---|
| A260/A280 (DNA) | ~1.8 | Protein, phenol, low pH diluent |
| A260/A280 (RNA) | ~2.0 | Protein, phenol, DNA carry-over (raises it) |
| A260/A230 | 2.0–2.2 | Guanidine, phenol, EDTA, carbohydrate |
UV vs fluorometric vs gel
UV absorbance is fast and consumable-free, but it has one structural weakness: it counts everything that absorbs at 260 nm. Free nucleotides, degraded RNA in a DNA prep, and some contaminants all inflate the reading. Fluorometric assays solve that by using a dye that lights up only when bound to the target.
- UV spectrophotometry (NanoDrop-style microvolume or cuvette): measures total nucleic acid over a wide range, no standards, 1–2 µL. Best for a quick concentration and purity check on reasonably pure, concentrated samples.
- Fluorometry (Qubit, PicoGreen): a dye binds the specific target — dsDNA, ssDNA, or RNA — so it ignores free nucleotides and the wrong nucleic acid. Far more accurate at low concentration (down to pg/µL), but needs a standard curve and assay tubes. Best before sequencing or transfection, where the true dsDNA mass matters.
- Gel electrophoresis with a mass ladder: gives integrity and a rough concentration in one image. Best for confirming size and degradation, not for precise numbers.
| Method | Specificity | Low-end sensitivity | Best for |
|---|---|---|---|
| UV (A260) | Low (all nucleic acid + contaminants) | ~2 ng/µL | Quick concentration + purity |
| Fluorometric | High (target only) | ~0.01 ng/µL | Accurate input for NGS, transfection |
| Gel + ladder | Medium (size-resolved) | ~1–5 ng/band | Integrity and rough quantity |
Converting ng/µL to molarity (nM)
A spectrophotometer gives mass per volume, but ligations, qPCR standards and library pooling are specified in molarity. The bridge is molecular weight. For double-stranded DNA, a good approximation is base pairs × 650 g/mol; for ssDNA use bases × 330, and for RNA bases × 320. Then:
nM = ( ng/µL × 106 ) ÷ molecular weight (g/mol)
Worked example: ng/µL to nM
A 500 bp PCR product reads 20 ng/µL. Its molecular weight is about 500 × 650 = 325,000 g/mol.
nM = (20 × 106) ÷ 325,000 = 61.5 nM
The shorter the fragment, the more molecules per nanogram — a key reason equimolar library pooling needs length, not just mass. Working in moles throughout? The companion guide on how to calculate molarity covers the same mass-to-moles logic for any solute, and the molarity calculator handles dilutions to a target nM.
Practical tips and troubleshooting
- Blank with the same buffer you eluted in. Blanking on water when the sample is in TE shifts the 230 nm baseline and corrupts A260/A230.
- Stay in the linear range. Above A260 ≈ 1.0 on a cuvette, dilute; microvolume instruments extend higher but still saturate — verify with a dilution if a value looks too high.
- A low A260/A280 may be the diluent, not the sample. Re-read in Tris pH 8 before re-purifying.
- Use fluorometry when the wrong species would mislead UV — a DNA prep with carry-over RNA, or any sub-ng/µL sample headed for sequencing.
- For proteins, the same instrument reads A280 — concentration there needs the protein's extinction coefficient, which the protein MW & extinction coefficient tool computes from sequence.
Together these steps turn a raw absorbance into a number you can trust for the next reaction. For plasmid-scale and therapeutic DNA, the same ratios gate release — see plasmid DNA manufacturing for where A260/A230 becomes a specification, not just a guideline.
Frequently Asked Questions
How do you calculate DNA concentration from A260?
Multiply A260 by the conversion factor and the dilution factor: concentration = A260 × factor × dilution. The factor is 50 µg/mL for dsDNA, 33 for ssDNA, and 40 for RNA (A260 = 1, 1 cm path). An A260 of 0.25 on a 1:50 dsDNA dilution gives 0.25 × 50 × 50 = 625 ng/µL.
What is a good A260/A280 ratio?
About 1.8 for pure DNA and 2.0 for pure RNA. Lower values indicate protein or phenol contamination. Because the ratio shifts with pH and ionic strength, read in slightly alkaline Tris rather than water for reproducibility.
What does the A260/A230 ratio tell you?
It flags organic and chaotropic carry-over — guanidinium, phenol, EDTA, carbohydrate — that absorbs near 230 nm. A clean sample reads 2.0–2.2; a low value usually means insufficient washing and often inhibits downstream enzymes.
What is the difference between Nanodrop and Qubit?
A NanoDrop-style UV reader counts all species absorbing at 260 nm over a wide range with no consumables; a Qubit fluorometer uses a target-specific dye, so it is more specific and far more accurate at low concentration, at the cost of standards. Use UV for a quick check, fluorometry when accuracy or specificity matters.
How do you convert ng/µL of DNA to nM?
Divide by molecular weight. For dsDNA, MW ≈ base pairs × 650 g/mol, then nM = (ng/µL × 106) / MW. A 500 bp fragment (MW ≈ 325,000) at 20 ng/µL is 61.5 nM.