DNA Concentration (A260) Calculator
Enter an A260 absorbance reading (and dilution factor) to find nucleic acid concentration, using the standard conversion factors for double-stranded DNA, single-stranded DNA, or RNA. Two 3D diagrams compare a dilute sample to a concentrated one, and charts show the linear relationship between absorbance and concentration for each nucleic acid type.
Reviewed by the ToolNestr Editorial Team — July 2026
Dilute vs. concentrated sample
1. Dilute sample
Fewer nucleic acid molecules — less UV light absorbed, a lower A260 reading.
2. Concentrated sample
Many more nucleic acid molecules — much more UV light absorbed, a higher A260 reading.
Absorbance-to-concentration charts
How it works
The core idea in one line: nucleic acids absorb UV light at 260nm in direct proportion to how much of them is present, so a single absorbance reading — scaled by a nucleic-acid-specific conversion factor — is enough to estimate concentration in seconds, without running a single gel.
Concentration (ng/µL) = A260 × dilution factor × conversion factor
conversion factor: dsDNA=50, ssDNA=33, RNA=40 ng/µL per A260 unit
A260/A280 ratio
protein contamination check — pure DNA ≈1.8, pure RNA ≈2.0
A260/A230 ratio
organic/chaotropic salt contamination check — good samples ≈2.0-2.2
The Beer-Lambert law describes how a solution's absorbance scales linearly with the concentration of whatever's absorbing the light — for nucleic acids, that means A260 absorbance scales directly with how much DNA or RNA is dissolved in the sample. Because double-stranded DNA, single-stranded DNA, and RNA each absorb slightly differently at 260nm (due to how their strands stack and pair), each nucleic acid type has its own standard conversion factor for translating absorbance into a concentration in nanograms per microlitre. Two extra readings — A280 and A230 — flag common contaminants: proteins absorb strongly at 280nm, while leftover extraction chemicals absorb around 230nm, so comparing each to A260 gives a fast purity check alongside the concentration itself.
Worked example 1 — a diluted dsDNA sample
Given: A dsDNA sample was diluted 1:10 before measuring, giving A260 = 0.75.
Multiplying the dilution factor back in is essential — forgetting it is one of the most common errors when reporting nucleic acid concentration from a spectrophotometer reading.
Worked example 2 — an RNA sample with a purity check
Given: An undiluted RNA sample reads A260 = 1.2 and A280 = 0.60.
A ratio noticeably below 2.0 for RNA (or below 1.8 for DNA) would signal protein contamination requiring further sample cleanup before use in sensitive downstream applications.
Conversion factors and expected purity ratios by nucleic acid type
Each nucleic acid type has its own standard conversion factor and expected pure-sample ratio.
| Type | ng/µL per A260 unit | Expected A260/A280 |
|---|---|---|
| dsDNA ★ | 50 | ≈1.8 |
| ssDNA | 33 | ≈1.8 |
| RNA | 40 | ≈2.0 |
★ Reference row (worked example 1). Selecting the wrong nucleic acid type is a common source of concentration errors — always confirm whether the sample is DNA or RNA before applying a conversion factor.
Where DNA/RNA concentration measurement actually matters
🧬 PCR and sequencing sample prep
Every PCR reaction and sequencing run requires a precisely known amount of template DNA — spectrophotometric quantification is the fast, routine first check before more expensive downstream steps.
🔬 Quality control in molecular labs
The A260/A280 and A260/A230 purity ratios are standard quality checkpoints reported alongside concentration in essentially every DNA/RNA extraction protocol, flagging contaminated samples before they're used further.
💉 Clinical and diagnostic genomics
Genetic testing labs quantify and check the purity of extracted patient DNA/RNA samples before running diagnostic assays, since contaminated or under-concentrated samples can produce unreliable results.
🧪 RNA-seq and gene expression studies
RNA integrity and concentration checks (including A260 measurements) are a mandatory first step before RNA sequencing, since low-quality or degraded RNA input can compromise an entire sequencing run.
Common misconceptions
"A260 absorbance directly gives you the concentration in ng/µL without any conversion."
A260 is just a raw absorbance reading — it must be multiplied by the appropriate conversion factor (which differs for dsDNA, ssDNA, and RNA) and by the dilution factor to arrive at an actual concentration value.
"A260/A280 near 1.8 always means the sample is 100% pure with no other issues."
A260/A280 only screens for protein contamination specifically — it says nothing about degradation, salt contamination (which A260/A230 checks instead), or whether the sample is genuinely intact, undegraded nucleic acid.
"Using the same conversion factor for DNA and RNA gives an acceptable approximation."
The dsDNA (50) and RNA (40) factors differ by 25%, which is far too large an error to ignore for quantitative work — using the wrong factor will systematically over- or under-estimate concentration.
"A higher A260/A230 ratio is always better, with no upper limit to worry about."
While a low A260/A230 does indicate contamination, ratios noticeably above the typical 2.0-2.2 range can also signal measurement or blank-reference issues — the expected range itself is the target, not simply "as high as possible."
Formula sources & further reading
The formulas here are standard, traceable to:
- • OpenStax, Biology 2e — Chapter 17, "Biotechnology and Genomics" (free, peer-reviewed). openstax.org
- • Thermo Fisher Scientific (NanoDrop) — standard nucleic acid quantification conversion factors and purity ratio guidelines.
- • Sambrook & Russell, Molecular Cloning: A Laboratory Manual — Nucleic acid quantification chapter.
Conversion factors: dsDNA=50, ssDNA=33, RNA=40 ng/µL per A260 unit (standard spectrophotometric convention). Results are rounded for display.
How to use this calculator
Enter A260 and dilution factor
Type the absorbance reading and how much the sample was diluted (use 1 if undiluted).
Select the nucleic acid type
Choose dsDNA, ssDNA, or RNA to apply the correct conversion factor.
Check purity (optional)
Enter A280 and A230 to see the standard purity ratios.
Related tools
Frequently asked questions
What is A260?
A260 is the absorbance of a sample at a wavelength of 260 nanometres, the wavelength nucleic acids absorb most strongly due to their purine and pyrimidine bases — it's the standard measurement for estimating DNA or RNA concentration.
Why do dsDNA, ssDNA, and RNA use different conversion factors?
The exact absorbance-to-concentration relationship depends on subtle structural differences — double-stranded DNA has some UV absorbance quenched by base stacking, so its factor (50 ng/µL per A260 unit) differs from single-stranded DNA (33) and RNA (40), which stack less and absorb slightly more per unit mass.
What is the A260/A280 ratio used for?
A260/A280 checks for protein contamination, since proteins absorb strongly at 280 nm. Pure DNA typically reads about 1.8, and pure RNA about 2.0 — a noticeably lower ratio suggests protein contamination in the sample.
What is the A260/A230 ratio used for?
A260/A230 checks for contamination by organic compounds and chaotropic salts (like phenol or guanidinium, often left over from extraction), which absorb around 230 nm. A good ratio is typically 2.0-2.2 — a lower ratio suggests carryover contamination from the extraction process.
Why does the dilution factor matter?
Absorbance readings are only accurate within a linear range (roughly 0.1–1.0 for most spectrophotometers) — samples are often diluted before measuring, so the dilution factor must be multiplied back in to get the original, undiluted sample's true concentration.