Beer's Law Path Length Calculator

About this calculator

The Beer-Lambert law relates how much light a sample absorbs to the concentration of the absorbing substance and the distance the light travels through it: A = ε · c · l, where A is absorbance, ε is the molar absorptivity (extinction coefficient, M⁻¹cm⁻¹, specific to the substance and wavelength), c is molar concentration, and l is the optical path length. Solving for path length rearranges this to l = A / (ε · c).

This is standard practice in UV-Vis spectrophotometry: analytical chemists use it to determine the path length of a non-standard cuvette or flow cell, or to check that a measured absorbance is consistent with an assay’s expected concentration and known molar absorptivity. The resulting transmittance, T = 10^(−A), is the fraction of incident light that makes it through the sample. This calculator solves for path length from your absorbance, molar absorptivity and concentration, and also reports the transmittance.