OD260 Calculators

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OD260 (optical density at 260 nm, also written A260) is the absorbance of a solution at 260 nm wavelength — the peak absorption wavelength of nucleic acids. It is used to quantify DNA and RNA concentrations using the Beer-Lambert law. Standard conversion factors: 1 OD260 unit = 50 μg/mL for double-stranded DNA, 33 μg/mL for single-stranded DNA, and 40 μg/mL for RNA. The A260/A280 ratio (typically ~1.8 for DNA, ~2.0 for RNA) assesses purity. A260 measurements are commonly made with spectrophotometers or microvolume NanoDrop instruments.

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OD260 Conversion Factors

  • dsDNA: 1 OD260 = 50 μg/mL
  • ssDNA: 1 OD260 = 33 μg/mL
  • RNA (ssRNA): 1 OD260 = 40 μg/mL
  • Oligonucleotides (ssDNA): 1 OD260 ≈ 33 μg/mL (approximate; use extinction coefficient for short oligos)

Concentration = OD260 × conversion factor × dilution factor.

Example: 10-fold diluted dsDNA sample with OD260 = 0.42: concentration = 0.42 × 50 × 10 = 210 μg/mL = 0.21 mg/mL.

A260/A280 Purity Ratio

Proteins absorb maximally at 280 nm (Trp, Tyr residues). The ratio A260/A280 assesses nucleic acid purity relative to protein contamination:

  • Pure dsDNA: A260/A280 ≈ 1.8
  • Pure RNA: A260/A280 ≈ 2.0
  • Ratio significantly below target: protein or phenol contamination
  • Ratio above 2.0: RNA contamination in a DNA sample

A260/A230 Ratio

The A260/A230 ratio detects co-purification of chaotropic salts (guanidinium), EDTA, carbohydrates, phenol, or organic solvents. Acceptable range: 2.0–2.2. Values below 1.8 indicate these inhibitory contaminants are present at potentially problematic levels for downstream applications like PCR and sequencing.

Limitations of OD260

OD260 cannot distinguish between DNA and RNA (both absorb at 260 nm). Free nucleotides and short degraded fragments also absorb at 260 nm, inflating the apparent concentration. For critical applications, use fluorometric methods (Qubit, PicoGreen) which are more specific for intact double-stranded DNA or single-stranded RNA.

Glossary

OD260 / A260
Absorbance at 260 nm; used to quantify nucleic acids: 1 OD260 = 50 μg/mL dsDNA, 40 μg/mL RNA, 33 μg/mL ssDNA; measured by spectrophotometer or NanoDrop.
A260/A280 Ratio
Nucleic acid purity metric: pure dsDNA ≈ 1.8; pure RNA ≈ 2.0; significantly lower values indicate protein or phenol contamination; measured simultaneously during OD260 quantification.
A260/A230 Ratio
Purity metric for chaotropic salts, EDTA, phenol, and carbohydrate co-purification; acceptable range 2.0–2.2; low values indicate inhibitory contaminants present in the nucleic acid preparation.

Frequently Asked Questions

Concentration (μg/mL) = OD260 × 50 × dilution factor (for dsDNA). Example: OD260 = 0.35 for an undiluted sample → concentration = 0.35 × 50 = 17.5 μg/mL. For a 5-fold diluted sample with OD260 = 0.35: concentration = 0.35 × 50 × 5 = 87.5 μg/mL. For RNA use factor 40; for ssDNA use 33. On NanoDrop, select the correct nucleic acid type so the software uses the appropriate conversion factor automatically.

A260/A280 ratio assesses protein contamination: pure dsDNA ratio ≈ 1.8; pure RNA ≈ 2.0. Values significantly below these indicate protein, phenol, or other UV-absorbing aromatic contaminants at 280 nm. Values slightly above 2.0 in a DNA sample may indicate RNA contamination. Ratio below 1.6 for DNA: the sample has significant contamination that may inhibit downstream applications like PCR, restriction digestion, or sequencing. However, A260/A280 alone is insufficient — a pure sample could have a ratio of 1.8 but still contain organic contaminants absorbed at other wavelengths.

Qubit (fluorometric quantification) uses dyes that bind specifically to intact double-stranded DNA (PicoGreen/dsDNA assay) or RNA (RiboGreen/RNA assay), giving a fluorescence signal proportional to the target nucleic acid only. It is preferred when: (1) Sample contains both DNA and RNA (Qubit distinguishes; OD260 cannot); (2) Degraded or fragmented nucleic acids are present (Qubit measures intact full-length nucleic acids better); (3) Very low concentrations (<10 ng/μL) need quantification; (4) Inhibitors or contaminants are present that absorb at 260 nm. For clean, pure, high-concentration samples, OD260 (NanoDrop) is faster and requires no reagents.

Beer-Lambert law (OD = ε × c × l) is linear only up to OD ≈ 0.8–1.0. At higher OD, detector saturation, stray light, and inner filter effects cause non-linear responses — the measured OD underestimates the true concentration. NanoDrop instruments automatically adjust path length (from 1 mm to 0.2 mm) to keep OD in the linear range, allowing accurate measurement of high-concentration samples without dilution. For traditional cuvette spectrophotometers, dilute samples to maintain OD260 in the 0.1–0.8 range.