Solve A = εlc for any variable, or convert between absorbance and transmittance.
Select what you want to solve for, fill in the known values, and the result updates instantly. The path length defaults to 1 cm (standard cuvette). The transmittance converter at the bottom works independently — enter %T to get absorbance, or vice versa.
The Beer-Lambert law relates the absorbance of light to the properties of the material it passes through. The equation is A = εlc, where A is absorbance (unitless), ε is the molar absorptivity or extinction coefficient (L mol⁻¹ cm⁻¹), l is the path length through the sample (cm), and c is the molar concentration (mol/L).
Worked example: NADH has an extinction coefficient of 6,220 L mol⁻¹ cm⁻¹ at 340 nm. If you measure an absorbance of 0.311 in a 1 cm cuvette: c = A / (ε × l) = 0.311 / (6220 × 1) = 5.0 × 10⁻⁵ M = 50 µM.
Absorbance and transmittance are related by A = −log₁₀(T), where T is the fractional transmittance (0 to 1). A %T of 35% corresponds to T = 0.35, so A = −log₁₀(0.35) = 0.456.
The law is most accurate at low absorbance values (A < 1.0). At higher absorbances, deviations occur due to stray light, detector nonlinearity, and intermolecular interactions. If your sample reads above A ≈ 1.5, consider diluting before measurement.