What does A260 measure?

A260 is the UV absorbance at 260 nm used to quantify nucleic acids via the Beer–Lambert law.

Which factor should I use for dsDNA, ssDNA and RNA?

Common factors for a 1 cm path are 50 µg/mL per A260 for dsDNA, 33 for ssDNA and 40 for RNA.

How is molarity calculated from ng/µL?

nM = (ng/µL × 1e6) / (MW), where MW = 660 × bp for dsDNA, 330 × nt for ssDNA, or 340 × nt for RNA.

Do I need to enter A280 or A230?

No. A280 and A230 are optional and are only used to compute purity ratios (260/280 and 260/230).

Can I use microvolume instruments?

Yes. Enter the instrument path length (e.g., 0.1 cm). The calculator normalizes A260 to 1 cm for concentration.

DNA Concentration Calculator

Calculate DNA (and other nucleic acid) concentration from A260 absorbance using the Beer–Lambert law. Ideal for molecular biology labs, sequencing prep, cloning, and qPCR workflows. Get precise mass concentration (ng/µL), molarity (nM), copies per µL, and purity ratios with full WCAG-accessible controls.

Interactive calculator

Nucleic acid type *

dsDNA (×50)

ssDNA (×33)

RNA (×40)

Custom factor

Measured A260 *

Path length (cm) *

Dilution factor *

Fragment length (optional)

bp for dsDNA; nt for ssDNA/RNA

A280 (optional)

A230 (optional)

Results

Normalized A260 (1 cm)

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Mass concentration

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Molar concentration (requires length)

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Copies per µL (requires length)

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Purity ratios

260/280 —

260/230 —

Data source and methodology

Authoritative sources:

Thermo Scientific, NanoDrop One/OneC Spectrophotometers User Guide (2017). PDF
Sambrook J., Russell D. W. Molecular Cloning: A Laboratory Manual, 3rd ed. Cold Spring Harbor Laboratory Press (2001).
IDT (Integrated DNA Technologies), “Calculating Molarity, Mass, and Copy Number.” Tool and documentation

Tutti i calcoli si basano rigorosamente sulle formule e sui dati forniti da questa fonte.

The formula explained

Beer–Lambert law: A = \varepsilon \, c \, l

Mass concentration from A260 (normalized to 1 cm): C_{\text{mass}}(\mu g/mL) = F \times \frac{A_{260}}{l} \times D

F is the factor: 50 (dsDNA), 33 (ssDNA), 40 (RNA) µg/mL per A260; l in cm; D is the dilution factor.

Convert to ng/µL: 1\,\mu g/mL = 1\,ng/\mu L

Molarity: MW_{\text{dsDNA}} \approx 660 \times \text{bp},\quad MW_{\text{ssDNA}} \approx 330 \times \text{nt},\quad MW_{\text{RNA}} \approx 340 \times \text{nt}

C_{\text{nM}} = \dfrac{C_{\text{ng/}\mu L} \times 10^{6}}{MW\;(\text{g/mol})}

Copies per µL: \text{copies}/\mu L = C_{\text{nM}} \times 6.022\times 10^{8}

Glossary of variables

A260: Absorbance at 260 nm.
Path length (l): Optical path in cm used during measurement.
Dilution factor (D): Fold dilution prior to reading (≥1).
Factor (F): µg/mL per A260 at 1 cm; dsDNA 50, ssDNA 33, RNA 40.
Fragment length: Size of the nucleic acid (bp for dsDNA, nt for ssDNA/RNA).
Mass concentration: Reported as ng/µL and µg/mL (same numeric value).
Molar concentration: Reported as nM, requires fragment length and molecular weight model.
Purity ratios: 260/280 and 260/230, indicative of protein and reagent contamination respectively.

How it works: a step‑by‑step example

Scenario: dsDNA plasmid, A260 = 0.20, path length = 1.0 cm, dilution = 10×, fragment length = 3000 bp.

Normalize A260 to 1 cm: A260/l = 0.20 / 1.0 = 0.20.
Mass concentration (µg/mL): F × A260/l × D = 50 × 0.20 × 10 = 100 µg/mL = 100 ng/µL.
MW ≈ 660 × 3000 = 1,980,000 g/mol.
nM = (100 ng/µL × 10^6) / 1,980,000 ≈ 50.51 nM.
Copies/µL = 50.51 × 6.022×10^8 ≈ 3.04×10^10 copies/µL.

Frequently asked questions (FAQ)

What purity ratios should I expect?

Typical pure values: dsDNA 260/280 ≈ 1.8, RNA ≈ 2.0. 260/230 should be ~2.0–2.4. Lower values suggest contaminants (proteins, salts, phenol, guanidinium).

Does path length matter with microvolume instruments?

Yes. Enter the instrument’s path length (often 0.05–0.2 cm). The calculator normalizes A260 to 1 cm before applying the factor.

How accurate are the universal factors (50/33/40)?

They are widely accepted approximations for average base composition. For oligos or unusual compositions, use a custom factor or sequence-specific ε for higher accuracy.

Why do nM and copies/µL require length?

Molarity depends on molecular weight, which scales with length. Without length, only mass concentration can be computed from A260.

Can I enter negative absorbance or extremely high values?

No. Absorbance must be ≥ 0. Values above instrument linearity (often ~2–3 A) can be inaccurate; dilute samples to stay within range.

Will this work for dsRNA or modified nucleic acids?

Use the closest model (RNA for dsRNA) or a custom factor. For heavily modified bases, sequence-specific extinction coefficients yield better results.

Strumento sviluppato da Ugo Candido. Contenuti verificati da CalcDomain Editorial Team.
Ultima revisione per l'accuratezza in data: September 15, 2025.