Conc = (Absorbance − Intercept) / Slope × Dilution Factor
Common methods: Bradford assay (Coomassie Blue binding, read at 595 nm, range 1-25 µg/mL), BCA assay (bicinchoninic acid, read at 562 nm, range 20-2000 µg/mL), and UV absorbance at 280 nm (uses aromatic amino acids, A280 of 1 ≈ 1 mg/mL for typical proteins). All colorimetric assays require a standard curve using BSA or IgG standards. Plot absorbance vs concentration, find the linear region, and determine slope and intercept. Sample concentration = (absorbance − intercept) / slope × dilution factor. Run samples in duplicate or triplicate. The Bradford assay is most popular for its speed and sensitivity but is incompatible with detergents >0.1%. BCA is more tolerant of detergents.