Protein Concentration Calculator

How the Protein Concentration Calculator Works

This calculator determines protein concentration in solution using absorbance measurements from UV-Vis spectrophotometry. It applies the Beer-Lambert law, which states that absorbance is directly proportional to concentration and path length.

The core calculation is:

Concentration (mg/mL) = (Absorbance × Dilution Factor) / (Extinction Coefficient × Path Length)

For most protein assays, the path length is 1 cm (standard cuvette). The extinction coefficient varies by method: 1 for absorbance at 280 nm (approximate for 1 mg/mL BSA), or values specific to your protein's amino acid composition.

How to Use the Calculator

1. Enter your absorbance value — the reading from your spectrophotometer at the relevant wavelength.
2. Input the extinction coefficient — use the standard value for your assay method or your protein's specific coefficient.
3. Set the path length — typically 1 cm for standard cuvettes; adjust if using a different cuvette size.
4. Apply a dilution factor — if your sample was diluted before measurement, enter the factor (e.g., 10 for a 1:10 dilution).
5. Click calculate — the tool returns the original sample concentration in mg/mL.

Example Calculation

A purified protein sample is measured at 280 nm and gives an absorbance of 0.45. The extinction coefficient for this protein is 0.8 (mg/mL)⁻¹ cm⁻¹. The sample was diluted 5-fold before measurement.

Calculation: (0.45 × 5) / (0.8 × 1) = 2.25 / 0.8 = 2.81 mg/mL

The original protein concentration is approximately 2.81 mg/mL.

Understanding Your Results

The calculated concentration represents the protein amount in your original, undiluted sample. Key points to consider:

• Accuracy depends on your extinction coefficient — using an incorrect value will produce proportional errors.
• Absorbance readings above 1.0 may fall outside the linear range of the spectrophotometer; dilute your sample and remeasure for best accuracy.
• Nucleic acid contamination at 280 nm can inflate absorbance readings; check the A260/A280 ratio to assess purity.
• Buffer components that absorb at your measurement wavelength can interfere; always use a blank containing your buffer alone.

Common Mistakes to Avoid

• Forgetting the dilution factor — entering the raw absorbance without accounting for dilution gives the diluted concentration, not the original.
• Using the wrong extinction coefficient — different proteins and assay methods (Bradford, BCA, A280) require different coefficients.
• Ignoring path length — if using a microcuvette or plate reader with a different path length, adjust accordingly.
• Measuring at the wrong wavelength — ensure your absorbance reading matches the wavelength your coefficient is based on.

Practical Use Cases

• Protein purification monitoring — track concentration across purification steps to assess yield and recovery.
• Assay preparation — calculate exact volumes needed to prepare standard curves or reaction mixtures.
• Crystallography and biophysics — determine protein concentration for crystallization screens or biophysical measurements.
• Quality control — verify protein concentration before downstream applications like enzymatic assays or binding studies.

Limitations and Constraints

• Linear range — the Beer-Lambert law applies only within the linear absorbance range of your instrument (typically 0.1–1.0 AU).
• Protein-specific coefficients — the calculator requires a known extinction coefficient; for unknown proteins, use a colorimetric assay (Bradford or BCA) instead.
• Interfering substances — detergents, reducing agents, and certain buffers can interfere with absorbance measurements.
• No correction for scattering — turbid samples may produce artificially high absorbance; centrifuge or filter before measurement.