Protein Concentration Calculator

Protein concentration is commonly estimated by ultraviolet absorbance or by a colorimetric assay compared with known standards. A280 measurement uses the absorption of aromatic amino acids, especially tryptophan and tyrosine. When a mass extinction coefficient is available in milliliters per milligram per centimeter, the Beer-Lambert relationship gives concentration in milligrams per milliliter by dividing blank-corrected absorbance by coefficient and optical path length. The result is then multiplied by the sample dilution factor. Bradford, bicinchoninic acid, and Lowry assays produce color whose absorbance changes with protein amount. These methods require a standard curve measured under the same conditions as the unknown samples. This calculator uses a linear equation in which absorbance equals slope times concentration plus intercept. It rearranges that equation by subtracting the intercept, dividing by slope, and applying the dilution factor. Enter slope and intercept from your regression in units that make the resulting concentration milligrams per milliliter. If standards were prepared in another concentration unit, convert the result or calibration values consistently. Reliable quantification depends on experimental quality rather than arithmetic alone. Blank the instrument with the same buffer and reagents, use clean compatible cuvettes or plates, and measure within the assay's validated range. Turbidity, nucleic acids, detergents, reducing agents, chelators, lipids, and strongly absorbing buffer components can bias results. Protein-to-protein response also differs in colorimetric methods. A standard similar to the unknown generally improves comparability, while replicate standards and samples reveal pipetting or instrument variation. Microplate path length is usually not one centimeter unless the reader corrects it, so use the instrument's effective or corrected path length for A280. The extinction coefficient must match the protein, wavelength, and coefficient convention; a molar coefficient cannot be entered directly into a field expecting a mass coefficient. Review residuals and avoid extrapolating far beyond a standard curve. Dilute concentrated samples and remeasure rather than trusting absorbance outside the linear range. This tool performs the specified calculation but does not validate sample preparation, calibration quality, assay compatibility, or instrument performance. Record raw readings, blanks, standards, dilution steps, and units so the reported concentration remains traceable.