Beer-Lambert Law Calculator
Use this tool to calculate extinction coefficient (ε), concentration (c), or absorbance (A) from:
Where A is absorbance (unitless), ε is molar extinction coefficient (M-1 cm-1), c is concentration (M), and l is path length (cm).
1) Solve for Extinction Coefficient (ε)
2) Solve for Concentration (c)
3) Predict Absorbance (A)
What Is an Extinction Coefficient?
The extinction coefficient tells you how strongly a molecule absorbs light at a specific wavelength. In spectroscopy, this value is essential because it connects what your instrument measures (absorbance) to what you usually want to know (concentration).
In practical terms, if two solutions have the same concentration and path length, the one with the higher extinction coefficient will produce a higher absorbance reading.
The Core Equation: Beer-Lambert Law
The extinction coefficient is typically calculated from the Beer-Lambert equation:
- A = absorbance (no unit)
- ε = molar extinction coefficient (M-1 cm-1)
- c = concentration (M)
- l = path length (cm)
Rearranging gives: ε = A / (c l)
How to Use This Calculator
Calculate extinction coefficient (ε)
- Enter absorbance measured at your wavelength of interest.
- Enter concentration and select unit (M, mM, µM, or nM).
- Enter path length and unit (cm or mm).
- Click Calculate ε.
Calculate concentration from absorbance
If you already know ε for your molecule and wavelength, enter absorbance and path length to back-calculate concentration.
Predict absorbance before running samples
Use known ε, concentration, and path length to estimate absorbance and check whether your reading is likely to stay in the linear range of your spectrophotometer.
Unit Conversions Used by the Tool
| Input Unit | Converted To | Factor |
|---|---|---|
| mM | M | 1 × 10-3 |
| µM | M | 1 × 10-6 |
| nM | M | 1 × 10-9 |
| mm path length | cm | 0.1 |
Common Lab Mistakes (and How to Avoid Them)
- Wrong wavelength: Extinction coefficient is wavelength-specific. Always match wavelength exactly.
- Path length confusion: Microvolume instruments may not use 1 cm by default.
- Out-of-range absorbance: Very high absorbance can become non-linear; dilute your sample when needed.
- Poor blanking: Incorrect blank subtraction directly affects calculated values.
- Unit mismatch: The most common source of 10x to 1000x errors.
Typical Use Cases
Protein quantification (A280)
Proteins containing tryptophan, tyrosine, and disulfide bonds absorb strongly at 280 nm. If you know sequence-based extinction coefficient, this method gives a fast concentration estimate.
Nucleic acids (A260)
DNA and RNA are commonly quantified at 260 nm. While many workflows use fixed conversion factors, Beer-Lambert with extinction coefficients provides a more transparent framework for calculations and reporting.
Final Notes
This extinction coefficient calculator is designed for quick bench-side calculations and teaching the Beer-Lambert relationship. For publication-grade analysis, always document wavelength, solvent/buffer, temperature, path length, and instrument details.
If your data consistently looks off, start by checking blank quality, units, and path length assumptions; those three factors explain most discrepancies.