Protein Concentration Calculator
Use the Beer–Lambert law to estimate protein molar and mass concentrations from absorbance.
On this page
CalculatorHow to useFormula & methodWorked exampleConnecting light absorption to proteinPreparing the measurementReading the unitsFrequently asked questionsRelated calculatorsSourcesHow to use this calculator
- Obtain the molar extinction coefficient and molecular weight for the protein and wavelength in use.
- Enter blank-corrected absorbance, represented path length, and the pre-measurement dilution factor.
- Read both the molar and mass concentration estimates.
Formula and method
For the assumptions behind these results, read the explanation below. You can also explore DNA Concentration and Protein Molecular Weight.
A worked example
For absorbance 0.5, coefficient 43,824 L/(mol·cm), a 1 cm path, and molecular weight 66,430 g/mol, the undiluted concentration is approximately 11.41 µmol/L or 0.758 mg/mL.
Connecting light absorption to protein
The Beer–Lambert law states that absorbance equals molar extinction coefficient multiplied by path length and molar concentration. Solving for concentration gives the first output. Multiplying by molecular weight converts a number-of-molecules measure into a mass measure.
An extinction coefficient is specific to a substance, wavelength, and definition. This tool expects a molar coefficient in liters per mole per centimeter. A mass-based coefficient or a published absorbance for a one-percent solution cannot be entered unchanged in that field.
Preparing the measurement
Use a blank that matches the solvent and relevant buffer components. Absorbing contaminants, scattering from aggregates, or an unsuitable wavelength can affect the reading. The mathematical result assumes the selected coefficient correctly describes the measured protein solution.
When a microvolume instrument reports a one-centimeter-equivalent absorbance, use 1 cm as the represented path length. Do not correct an already normalized reading a second time. If the sample was diluted, the dilution factor converts the measured solution back to its original concentration.
Reading the units
A concentration in moles per liter counts chemical amount. A concentration in milligrams per milliliter measures mass per liquid volume. Two proteins at the same molar concentration can have different mass concentrations because their molecular weights differ.
The equality of grams per liter and milligrams per milliliter makes the final multiplication straightforward. Choose the molecular-weight unit beside its field. The calculator converts the value for the equation; for example, 66.43 kDa corresponds numerically to 66,430 g/mol.
Frequently asked questions
Can I use this for a Bradford assay?
Not directly. A Bradford assay normally uses a standard calibration curve. This form applies a specified extinction coefficient to absorbance.
Are the default values a universal protein standard?
No. They are a worked numerical example. Replace them with appropriate properties for the actual protein.
What does dilution factor one mean?
The measured preparation was not diluted relative to the sample whose concentration you want.
Sources and further reading
- University at Buffalo, NESG — Protein Concentration ↗
States the Beer–Lambert equation, molar coefficient units, dilution correction, and the need to handle normalized path length consistently.
- Harvard Medical School — Measuring Concentration ↗
Explains protein absorbance, extinction coefficients, blank correction, linear measurement range, and conversion between molar and mass coefficients.
- National Institute of Standards and Technology — SI conversion factors ↗
Reference for physical unit conversions. The calculator converts measurements before applying its formula.
Our explanations are written for BiologyCalc. Read how we build and check our tools. These calculations are educational estimates where the underlying model requires assumptions.