Beer-Lambert Law Calculator
Written by Thierno Sadou Diallo, formula verified per our methodology • Last checked on 9/9/2026
The Beer-Lambert law is calculated with A = ε × l × c, where A is the absorbance, ε the molar extinction coefficient, l the optical path length, and c the concentration. With ε = 5000 L·mol⁻¹·cm⁻¹, a 1 cm cuvette, and a concentration of 0.0001 mol/L, the absorbance is 0.5, giving a transmittance of about 31.6%.
Explanation
The Beer-Lambert law relates a solution's absorbance to its molar concentration: it's the basic principle of UV-visible spectrophotometry, a widely used lab technique for quantifying a substance in solution (proteins, DNA, dyes, chemical reagents) without needing to destroy it. Absorbance A is proportional to three quantities: the molar extinction coefficient ε (specific to each molecule and the chosen wavelength, it reflects its ability to absorb light), the optical path length l traveled by the light beam (typically 1 cm for a standard spectrophotometer cuvette), and the substance's concentration c. Transmittance, the complement of absorbance, indicates the proportion of light that actually passes through the solution without being absorbed: an absorbance of 2 (fairly high) lets through only about 1% of the incident light. In practice, the Beer-Lambert law is only reliable within a limited absorbance range (generally below 1 to 2): beyond that, secondary physical effects (light scattering, detector saturation) cause the relationship to deviate from its theoretical linearity, which is why lab protocols often recommend using our dilution calculator to bring an overly concentrated sample back into range rather than trusting a very high absorbance reading.
Example: ε = 5000 L·mol⁻¹·cm⁻¹, a 1 cm cuvette, concentration 0.0001 mol/L
Inputs
ε: 5000 L·mol⁻¹·cm⁻¹. Optical path length: 1 cm. Concentration: 0.0001 mol/L (0.1 mM).
Calculation
Absorbance = 5000 × 1 × 0.0001 = 0.5. Transmittance = 10⁻⁰·⁵ × 100 ≈ 31.6%.
Result
This solution's absorbance is 0.5, corresponding to a transmittance of about 31.6%.
Frequently asked questions
Where can I find a substance's molar extinction coefficient (ε)?
It's specific to each molecule and each observation wavelength, and can be found in published spectroscopic tables (spectrophotometry databases, reagent technical sheets) or determined experimentally by measuring the absorbance of solutions of known concentration.
Why dilute a sample if its absorbance is too high?
The linear relationship between absorbance and concentration described by the Beer-Lambert law stops being reliable beyond a certain absorbance (generally 1 to 2), due to physical effects like light scattering or detector saturation. Diluting the sample brings the measurement back into the linear range, where quantification stays accurate, and the result just needs to be multiplied by the dilution factor.
Do absorbance and transmittance measure the same thing?
They describe the same phenomenon from two complementary angles: transmittance is the proportion of light that passes through the solution (in %), while absorbance is its logarithmic inverse (A = −log₁₀(T ÷ 100)). A transmittance of 100% corresponds to zero absorbance (no light absorbed), and a transmittance approaching 0% corresponds to absorbance approaching infinity.