DNA and Protein Concentration Calculator
Quantifying nucleic acid and protein concentrations by ultraviolet spectrophotometry relies upon the Beer-Lambert Law (A = ε·c·l). Ring resonance of purine and pyrimidine aromatic nitrogenous bases exhibits peak light absorption at 260 nm, whereas aromatic amino acids (tryptophan, tyrosine) absorb at 280 nm. This tool calculates sample concentrations, A260/A280 and A260/A230 purity ratios, and total yield.
Type of biomolecule being quantified
UV absorbance reading at 260 nm (nucleic acid absorption peak)
UV absorbance reading at 280 nm (protein aromatic amino acid peak)
UV absorbance reading at 230 nm (organic salts/phenol contamination peak)
Fold dilution applied prior to measurement (e.g. 50 for 1:50 dilution, 1 for neat NanoDrop)
Total stock sample volume in microlitres (µL) to determine total yield
Calculated biomolecule concentration in µg/mL (numerically identical to ng/µL)
How to Use This Calculator
- Select the sample biomolecule type (dsDNA, ssDNA, RNA, or Protein).
- Enter spectrophotometer absorbance readings at 260 nm, 280 nm, and 230 nm.
- Input any dilution factor applied (enter 1 if measured neat on a microvolume instrument like a NanoDrop).
- Review concentration in µg/mL (ng/µL), purity ratios, assessment, and total stock yield.
Formula & Calculation Methodology
Under 1 cm pathlength: 1.0 A260 = 50 µg/mL for dsDNA, 33 µg/mL for ssDNA, and 40 µg/mL for RNA. For proteins, the Warburg-Christian equation accounts for nucleic acid background absorbance: Protein (mg/mL) = 1.55·A280 - 0.76·A260.
Worked Example
Purified genomic dsDNA with A260 = 1.20, A280 = 0.65, A230 = 0.55 neat in a 50 µL volume
Frequently Asked Questions
What causes an A260/A280 ratio significantly lower than 1.8 for DNA?↓
A low ratio (< 1.7) indicates contamination by residual proteins (which absorb strongly at 280 nm via tryptophan/tyrosine) or residual phenol carryover from organic extraction protocols.
Why is the expected A260/A280 ratio higher for RNA (2.0) than DNA (1.8)?↓
RNA contains uracil rather than thymine. The 260/280 absorbance ratio of pure uracil bases (4.00) is significantly higher than that of thymine bases (1.47), elevating pure RNA's composite optical ratio to ~2.0.
What contaminants cause an abnormally low A260/A230 ratio?↓
An A260/A230 ratio below 1.8 typically indicates contamination with chaotropic salts (guanidine isothiocyanate, guanidine hydrochloride from spin columns), EDTA, TRIzol reagents, or polysaccharides.
What limitation applies to spectrophotometric absorbance at very high concentrations?↓
The Beer-Lambert relationship is strictly linear only up to approximately 1.5 to 2.0 absorbance units. Beyond 2.0 AU, detector stray light causes severe negative deviations, requiring dilution for accurate reading.
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Source Citation & Disclaimer
Informational tool only; not licensed tax, legal, or financial advice.