Calculate molar and mg/mL protein concentration from an A280 reading, extinction coefficient, path length, and molecular weight.
| Concentration (M) | – |
| Concentration (mg/mL) | – |
Use this calculator for a Beer–Lambert conversion on a purified protein, not a Bradford, BCA, or other dye-binding assay. You need the blanked A280, the molar extinction coefficient ε in M⁻¹ cm⁻¹, the cuvette path length, and the molecular weight if you also want mg/mL.
Molarity is A280 ÷ (ε × path length in cm). Mass concentration in mg/mL equals that molarity times molecular weight in g/mol, because mol/L × g/mol = g/L, which is numerically the same as mg/mL.
Path length can be entered in centimetres (cm) or inches (in); the calculator converts to centimetres before applying Beer–Lambert. A standard cuvette is 1 cm; a NanoDrop-style pedestal is often 0.1 cm.
This estimate assumes a clear, monodisperse solution and the ε you supply. Nucleic-acid contamination, turbidity, and an ε taken from the wrong buffer or isoform will shift the result.
A280: The absorbance at 280 nm reported by the instrument after blanking.
ε: Molar extinction coefficient at 280 nm, in M⁻¹ cm⁻¹, from ProtParam or the product datasheet.
Path length: Optical path of the cuvette or pedestal.
Molecular weight: Protein molecular weight in g/mol (daltons), used only for the mg/mL result.
Concentration (M): Molar concentration in mol/L.
Concentration (mg/mL): Mass concentration.
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