Protein Concentration Calculator

Our Protein Concentration Calculator estimates protein quantity based on ultraviolet absorbance at 280 nm. Here's how to use it:

Step 1: Enter the absorbance of your protein sample at 280 nm (A280).

Step 2: Enter the Extinction Coefficient (ε). If unknown, you can use the general average of 1.0 (for a 1 mg/mL solution) for a rough estimate, or calculate it based on amino acid composition.

Step 3: Enter the path length of your cuvette (usually 1 cm).

Step 4: Enter any dilution factor used.

Step 5: Click "Calculate" to get the protein concentration in mg/mL.

This calculator uses the Beer-Lambert Law:

A = ε × c × l

Where: - A = Absorbance at 280 nm - ε = Extinction coefficient (mL/(mg·cm) or M⁻¹cm⁻¹) - c = Concentration - l = Path length (cm)

Rearranging for concentration: c = A / (ε × l)

We then multiply by the Dilution Factor to get the concentration of the original sample. Common estimation: For a mixture of proteins, an A280 of 1.0 is roughly equivalent to 1 mg/mL (assuming ε ≈ 1.0).

Protein concentration is a fundamental value in biochemistry and molecular biology. Knowing how much protein you have in a sample is essential before running assays, preparing gels, or adding reagents in any reaction — using too little gives weak results, and too much can saturate or interfere with measurements.

This calculator uses spectrophotometric quantification at 280 nm, which exploits the fact that aromatic amino acids (primarily tryptophan and tyrosine) absorb UV light at this wavelength. The technique is fast, requires no additional reagents, and is non-destructive to the sample. However, it depends on five key input values:

Absorbance (A280): Measured with a spectrophotometer. Captures how much UV light the sample absorbs at 280 nm, where most proteins show their peak absorption.

Extinction Coefficient (ε): A protein-specific constant that quantifies how strongly the molecule absorbs light. It varies based on the amino acid composition and must be looked up or calculated for each protein.

Path Length: The inner diameter of the cuvette used — almost always 1 cm in standard lab setups.

Molecular Mass: The atomic mass of the protein in g/mol, needed to convert from molar units to mg/mL.

Dilution Factor: Applied when the original sample was diluted before measurement. Multiply absorbance by this number to recover the original concentration.

Key Reference Values: Some commonly used extinction coefficients and molecular masses — BSA: ε = 43,824 M⁻¹cm⁻¹, MW = 66,463 g/mol. IgG: ε = 210,000 M⁻¹cm⁻¹, MW = 150,000 g/mol. Lysozyme: ε = 37,901 M⁻¹cm⁻¹, MW = 14,000 g/mol. Streptavidin: ε = 176,000 M⁻¹cm⁻¹, MW = 55,000 g/mol. These values are validated for measurements at 280 nm.