Protein Concentration Calculator
Calculate protein concentration from absorbance, molar extinction coefficient, optical path length, and molecular weight using the Beer–Lambert law.
The Protein Concentration Calculator is designed for laboratory and biochemical research where protein concentration is estimated from spectrophotometric absorbance measurements. It uses the Beer–Lambert relationship to calculate molar concentration and then converts that value to mass concentration using the supplied protein molecular weight.
Enter the measured absorbance, molar extinction coefficient, optical path length, and molecular weight of the protein. The calculator reports the resulting concentration in mol/L, mg/mL, and µg/mL.
Calculate Protein Concentration
Protein Concentration Calculator
Calculate protein concentration using absorbance and the Beer–Lambert law.
Calculation Result
How the Protein Concentration Calculator works
Therefore, c = A ÷ (ε × l), where A is absorbance, ε is molar extinction coefficient, c is concentration in mol/L, and l is path length in cm.
Step 1: Calculate molar concentration
The calculator determines protein concentration in mol/L by dividing absorbance by the product of molar extinction coefficient and optical path length.
c = A/(εl)
Step 2: Convert to mass concentration
The molar concentration is multiplied by molecular weight to obtain mass concentration in g/L. Numerically, g/L is equivalent to mg/mL.
mg/mL = c × molecular weight
Absorbance
Enter the measured absorbance value after appropriate blank correction according to your experimental method.
Extinction coefficient
Enter the molar extinction coefficient in L·mol⁻¹·cm⁻¹ for the protein and wavelength being analyzed.
Molecular weight
Enter the protein molecular weight in g/mol to convert molar concentration into mass concentration.
How to use the Protein Concentration Calculator
- Measure the protein sample absorbance using the appropriate spectrophotometric method.
- Enter the absorbance value into the calculator.
- Enter the protein’s molar extinction coefficient in L·mol⁻¹·cm⁻¹.
- Enter the optical path length in centimetres. A standard cuvette commonly uses a 1 cm path length, but the actual experimental path should be used.
- Enter the molecular weight of the protein in g/mol.
- Click Calculate to obtain molar and mass concentration.
- Review the result together with wavelength, blank correction, dilution factor, and experimental conditions before reporting the concentration.
Understanding protein concentration results
Molar concentration
Molar concentration expresses the amount of protein in mol/L. This form is useful when comparing protein quantities on a molar basis or performing calculations involving molecular interactions and stoichiometry.
Mass concentration
Mass concentration expresses protein amount as mg/mL or µg/mL. These units are commonly convenient for laboratory sample preparation, protein assays, and reporting sample concentrations.
Protein concentration example using Beer–Lambert law
Consider a protein sample with an absorbance of 0.85, molar extinction coefficient of 50,000 L·mol⁻¹·cm⁻¹, path length of 1 cm, and molecular weight of 65,000 g/mol.
First calculate molar concentration:
c = 0.85 ÷ (50,000 × 1)
c = 1.70 × 10⁻⁵ mol/L
Then convert to mass concentration:
1.70 × 10⁻⁵ × 65,000 = 1.105 mg/mL
Therefore, the calculated protein concentration is 1.105 mg/mL, equivalent to 1,105 µg/mL.
Important checks for protein concentration measurements
Blank correction
Follow the measurement method used in your laboratory and account for background absorbance where appropriate.
Wavelength
Extinction coefficient values are wavelength-dependent. Use a value appropriate for the measurement wavelength and protein.
Dilution factor
If the measured sample was diluted before measurement, apply the appropriate dilution correction when estimating the concentration of the original sample.
Common protein concentration calculation errors
- Using the wrong extinction coefficient: the coefficient should correspond to the protein, wavelength, and relevant experimental conditions.
- Ignoring path length: non-standard cuvette or microvolume instruments may use path lengths different from 1 cm.
- Forgetting dilution: the concentration calculated from a diluted sample may need dilution correction to obtain the original concentration.
- Mixing units: keep ε, concentration, path length, and molecular weight in compatible units.
- Using uncorrected absorbance: background or buffer absorbance can affect the calculated protein concentration.
- Assuming every protein behaves identically: protein composition and experimental conditions influence absorbance and extinction coefficient values.
Protein Concentration Calculator FAQ
What formula does the Protein Concentration Calculator use?
It uses the Beer–Lambert equation A = εcl. Molar concentration is calculated as c = A/(εl), and the result is converted to mass concentration using the supplied molecular weight.
What units should I use for the extinction coefficient?
Enter molar extinction coefficient in L·mol⁻¹·cm⁻¹ when concentration is calculated in mol/L and path length is entered in cm.
Why do I need protein molecular weight?
Molecular weight is required to convert molar concentration into mass concentration such as mg/mL or µg/mL.
Can I use a path length other than 1 cm?
Yes. Enter the actual optical path length used for your measurement. The Beer–Lambert equation accounts for path length directly.
Should I correct for sample dilution?
Yes, when the measured sample was diluted and the concentration of the original sample is required. Apply the dilution factor appropriate to your experimental preparation.
Can this calculator replace a protein assay?
No. It is a calculation tool based on supplied spectrophotometric inputs. Whether direct absorbance measurement is appropriate depends on the protein, wavelength, sample composition, instrument, and validated experimental method.
Related ResearchUtility tools
Continue your molecular biology and laboratory calculations with these related ResearchUtility tools:
Use protein concentration calculations as part of your research workflow
Record the absorbance, wavelength, extinction coefficient, path length, molecular weight, dilution information, and calculated concentration so the result can be reviewed and reproduced.
For quantitative laboratory work, interpret calculated values alongside appropriate blanks, controls, calibration procedures, instrument specifications, and experimental validation.