Protein Concentration Calculator (A280 to mg/mL)
Protein Concentration Calculator
This calculator estimates protein concentration from spectrophotometer absorbance, using Beer-Lambert law and your sample setup (dilution, pathlength, and molecular weight). It is designed for quick A280 workflows and for logging multiple calculations into a sortable, exportable results table.
What the calculator computes
- Stock concentration (mg/mL) - concentration of the original sample before dilution.
- Diluted concentration (mg/mL) - concentration in the cuvette/well that was actually measured.
- Stock molar concentration (µM) - useful when comparing proteins by molar amount.
- Total protein mass (mg) - estimated mass in the measured sample volume at the diluted concentration.
Beer-Lambert law and the concentration formula
Beer-Lambert law relates absorbance to molar concentration:
\[ A = \varepsilon \cdot b \cdot c \]
Where:
- \(A\) - absorbance (unitless, commonly A280 for proteins)
- \(\varepsilon\) - extinction coefficient \((M^{-1}\cdot cm^{-1})\)
- \(b\) - pathlength (cm)
- \(c\) - molar concentration (mol/L)
If you measured a diluted sample and want the original (stock) concentration, apply the dilution factor \(n\):
\[ c_{stock} = \frac{A \cdot n}{\varepsilon \cdot b} \]
Convert molar concentration to mass concentration using the molecular weight \(m\) (g/mol). A convenient identity is that \(g/L = mg/mL\), so:
\[ C_{stock\,(mg/mL)} = c_{stock} \cdot m \]
The diluted concentration is:
\[ C_{diluted\,(mg/mL)} = \frac{C_{stock\,(mg/mL)}}{n} \]
The molar concentration in micromolar is:
\[ c_{stock\,( \mu M)} = c_{stock} \cdot 10^6 \]
Total protein mass in the measured sample
If the measured sample volume is \(V\) (mL), then the estimated mass of protein in that measured sample is:
\[ Mass_{sample\,(mg)} = C_{diluted\,(mg/mL)} \cdot V \]
Practical measurement notes
- Low absorbance - very small absorbance values can be dominated by instrument noise, so results may be less reliable.
- High absorbance - very high absorbance can indicate saturation or non-linearity; consider diluting the sample and re-measuring.
- Use the correct \(\varepsilon\) - extinction coefficient is protein-specific and may vary by sequence, tag, and buffer conditions. If you have a datasheet value, use the custom mode.
Example calculation
Given:
- \(A = 0.80\)
- \(n = 1\)
- \(b = 1.00\) cm
- \(\varepsilon = 43824\) \(M^{-1}\cdot cm^{-1}\)
- \(m = 66463\) g/mol
\[ c_{stock} = \frac{0.80 \cdot 1}{43824 \cdot 1.00} \] \[ C_{stock} = c_{stock} \cdot 66463 \]
The calculator outputs the final values rounded to no more than two decimals for readability.