Calculate DNA and RNA concentration from spectrophotometer A260 readings. Determine nucleic acid concentration in ng/µL, assess purity with A260/A280 and A260/A230 ratios, and get step-by-step molecular biology solutions.
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Enter the spectrophotometer A260 absorbance value
For A260/A280 purity ratio calculation
For A260/A230 purity ratio calculation
If sample was diluted, multiply by this factor
Standard cuvette path length (default: 1 cm)
Nucleic Acid Concentration
0
ng/µL
Nucleic Acid Type
dsDNA
Conversion factor used
A260/A280 Ratio
—
N/A
A260/A230 Ratio
—
N/A
📝 Step-by-Step Solution
Real-World DNA Concentration Examples
🧬 dsDNA Quantification from Genomic DNA Prep
Problem: A genomic DNA preparation gives an A260 reading of 0.45. The sample was diluted 1:5 (dilution factor = 5). What is the dsDNA concentration? The A280 reading is 0.25 and A230 is 0.20.
The low A260/A230 ratio (1.09) is concerning and suggests the sample may need additional purification. Low A260/A280 (1.62) indicates protein contamination.
Where A₂₆₀ is the absorbance reading at 260 nm, the Conversion Factor depends on nucleic acid type, Dilution Factor accounts for any sample dilution, and Path Length corrects for cuvette path length in cm.
dsDNA: [DNA] (ng/µL) = A₂₆₀ × 50 × DF × (1 / L)
Double-stranded DNA — conversion factor: 50 µg/mL per A₂₆₀ unit
ssDNA: [ssDNA] (ng/µL) = A₂₆₀ × 33 × DF × (1 / L)
Single-stranded DNA — conversion factor: 33 µg/mL per A₂₆₀ unit
RNA: [RNA] (ng/µL) = A₂₆₀ × 40 × DF × (1 / L)
RNA — conversion factor: 40 µg/mL per A₂₆₀ unit
Where DF = Dilution Factor and L = Path Length in cm.
Purity Ratio Reference
Ratio
Pure Range
Acceptable Range
Common Contaminants
A260/A280 (DNA)
1.8–2.0
1.7–2.1
Proteins (lower), RNA (higher)
A260/A280 (RNA)
2.0–2.2
1.9–2.3
Proteins (lower), DNA (lower)
A260/A230
2.0–2.4
1.8–2.4
EDTA, carbohydrates, guanidine, phenol
Key Concepts
📌 Why A260?
Nucleic acids absorb UV light maximally at 260 nm due to the aromatic rings in purine and pyrimidine bases. Beer-Lambert law relates absorbance to concentration — a higher A260 means more nucleic acid in solution.
📌 Conversion Factors
An A260 of 1.0 corresponds to ~50 µg/mL for dsDNA, ~33 µg/mL for ssDNA, and ~40 µg/mL for RNA. These factors are empirically determined and assume a 1 cm path length.
📌 A260/A280 Ratio
This ratio assesses protein contamination. Pure DNA has a ratio of 1.8–2.0, pure RNA has 2.0–2.2. Lower values indicate protein or phenol contamination; higher values may indicate RNA contamination in DNA samples.
📌 A260/A230 Ratio
This secondary measure detects contaminants that absorb at 230 nm, such as EDTA, carbohydrates, phenol, and guanidine isothiocyanate. Pure nucleic acid samples typically have values above 2.0.
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Multi-Type Support
Calculate concentrations for dsDNA (×50), ssDNA (×33), and RNA (×40) all in one tool. Select the appropriate conversion factor with a single click.
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Purity Assessment
Automatically compute A260/A280 and A260/A230 purity ratios. Color-coded results tell you at a glance whether your sample is pure, acceptable, or contaminated.
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Path Length & Dilution
Correct for non-standard cuvette path lengths and sample dilution factors. Get accurate concentration values regardless of your measurement setup.
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Step-by-Step Solutions
Every calculation comes with a detailed step-by-step breakdown showing the formula, substitution, intermediate values, and final result for educational purposes.
⚠️ Important Note: Spectrophotometric quantification assumes pure nucleic acid samples. Contaminants such as proteins, phenol, guanidine, EDTA, and carbohydrates can significantly affect A260 readings and purity ratios. Always validate sample purity through A260/A280 and A260/A230 ratios before proceeding with downstream applications. For precise quantification of low-concentration samples, consider using fluorometric assays (e.g., Qubit) as a complementary method.
Frequently Asked Questions
How does the DNA concentration calculator work?
The DNA concentration calculator uses the Beer-Lambert law (A = ε × c × l) to determine nucleic acid concentration from spectrophotometer readings. At 260 nm, the absorbance (A260) is proportional to the concentration of nucleic acids in solution. For dsDNA, an A260 of 1.0 corresponds to approximately 50 µg/mL; for ssDNA, 33 µg/mL; and for RNA, 40 µg/mL. The calculator multiplies your A260 reading by the appropriate conversion factor, your dilution factor, and adjusts for path length to give the concentration in ng/µL. It also computes purity ratios (A260/A280 and A260/A230) to assess sample quality.
What do A260/A280 and A260/A230 ratios mean for DNA purity?
The A260/A280 ratio measures protein and phenol contamination. For pure DNA, this ratio should be between 1.8 and 2.0. Values below 1.8 indicate protein contamination, while values above 2.0 may suggest RNA contamination. For pure RNA, the expected A260/A280 ratio is 2.0–2.2. The A260/A230 ratio is a secondary measure of purity that detects contaminants absorbing at 230 nm, such as EDTA, carbohydrates, guanidine, and phenol. Pure nucleic acid samples typically have A260/A230 values above 2.0 (ideal: 2.0–2.4). Low A260/A230 ratios (<1.8) indicate significant contamination that may interfere with downstream applications like PCR, qPCR, or NGS library preparation.
Why do dsDNA, ssDNA, and RNA have different conversion factors?
The conversion factors differ because of the hypochromic effect and differences in base stacking between nucleic acid forms. In dsDNA, the two complementary strands are tightly bound with extensive base stacking, which reduces UV absorbance per nucleotide — hence a higher concentration per A260 unit (50 µg/mL). RNA is typically single-stranded but has more ordered secondary structure than ssDNA, giving an intermediate factor (40 µg/mL). ssDNA has the least base stacking and greatest UV absorbance per nucleotide, so less material is needed to achieve the same A260 reading (33 µg/mL). These factors are standardized in molecular biology and widely accepted for routine quantification.
How do I account for dilution in my DNA measurement?
If your nucleic acid sample was diluted before measurement, you must multiply the calculated concentration by the dilution factor. For example, if you diluted 5 µL of your DNA sample with 45 µL of buffer (a 1:10 dilution), your dilution factor is 10. Enter this in the "Dilution Factor" field. The calculator multiplies the raw concentration by this factor: [Actual] = [Measured] × Dilution Factor. If no dilution was used, keep the dilution factor at 1. Remember that very concentrated samples may need dilution to bring the A260 reading into the accurate range (typically 0.1–1.0 for most spectrophotometers).
What if my cuvette has a non-standard path length?
The Beer-Lambert law is linear with path length — absorbance is proportional to the distance the light travels through the sample. Our calculator defaults to 1 cm (standard cuvette). If you use a 0.5 cm path length cuvette (common for NanoDrop and other micro-volume instruments), the absorbance will be half of what a 1 cm cuvette would give for the same sample. The calculator corrects for this by multiplying by (1 / path length). For example, with a 0.5 cm cuvette: corrected absorbance = measured A260 × (1 / 0.5) = measured A260 × 2. Always enter the actual path length of your measurement device for accurate results.
Can I use this calculator for RNA concentration as well?
Yes! Select the RNA (×40) option in the nucleic acid type selector to calculate RNA concentration. The conversion factor automatically changes from 50 (dsDNA) to 40 (RNA). The purity thresholds also adjust — RNA purity is assessed against the 2.0–2.2 range for A260/A280 instead of the 1.8–2.0 range used for DNA. For best results with RNA, ensure your spectrophotometer blank uses the same buffer as your RNA sample, and measure at 260 nm, 280 nm, and 230 nm for complete purity assessment. Degraded RNA may show lower A260/A280 ratios.
What are common sources of error in spectrophotometric DNA quantification?
Common sources of error include: 1) Contaminants — proteins, phenol, guanidine, EDTA, and carbohydrates all absorb UV light and inflate A260 readings. 2) Improper blanking — if the blank solution differs from your sample buffer, readings will be inaccurate. 3) Out-of-range readings — A260 values above 1.5 (or below 0.05) deviate from linearity on many instruments. 4) pH effects — the A260/A280 ratio is pH-dependent; acidic solutions decrease the ratio. 5) Bubble formation — air bubbles in the cuvette scatter light and increase apparent absorbance. 6) Incomplete mixing — concentrated samples that aren't fully mixed give variable readings. For critical applications, always complement spectrophotometric quantification with fluorometric methods.