What is the Beer-Lambert law?
The Beer-Lambert law relates how much light a solution absorbs to its concentration and the distance the light travels through it. It’s the basis for how spectrophotometers measure concentration in chemistry and biology labs.
How this calculator works
The law is expressed as A = ε × c × l, where A is absorbance (unitless), ε is the molar absorptivity of the substance at a given wavelength, c is the molar concentration, and l is the path length of light through the sample (usually the width of a cuvette, in centimeters).
The calculator also converts absorbance into percent transmittance, the fraction of light that passes through the sample, using T = 10^(−A) × 100.
- Molar absorptivity (ε) is specific to a substance and wavelength — look it up for your compound and wavelength of interest.
- Higher absorbance means less light gets through and generally corresponds to a more concentrated or more strongly absorbing solution.
This tool is for general chemistry education purposes.
Last reviewed August 2026