Beer-Lambert Law Calculator

The Beer-Lambert law describes how a beam of light is attenuated as it passes through an absorbing solution. In its common spectrophotometric form, absorbance A equals molar absorptivity ε multiplied by optical path length l and concentration c. This compact relationship makes absorbance measurements useful for determining an unknown concentration when a substance's absorptivity and the cuvette path length are known. Absorbance is dimensionless and is defined as the base-ten logarithm of incident light intensity divided by transmitted intensity. A sample with absorbance zero transmits all measured light in the ideal model, while absorbance one transmits ten percent. Molar absorptivity depends on the chemical species, solvent, wavelength, temperature, and sometimes chemical environment. It is commonly reported in liters per mole per centimeter when concentration is moles per liter and path length is centimeters. The equation can be rearranged for any variable. Concentration equals absorbance divided by molar absorptivity and path length. Path length equals absorbance divided by molar absorptivity and concentration. Molar absorptivity equals absorbance divided by path length and concentration. The units entered must be compatible: if absorptivity uses centimeters, path length must also use centimeters. Changing one unit without converting the corresponding coefficient produces a numerically incorrect answer. Real instruments and samples may depart from ideal linear behavior. At high concentration, molecular interactions and refractive-index changes can alter absorptivity. Stray light, detector limits, polychromatic radiation, scattering particles, fingerprints on a cuvette, and an incorrect blank can also affect the reading. Many laboratories construct a calibration curve from standards rather than relying only on a literature absorptivity. Measurements often perform best in a moderate absorbance range supported by the instrument's specifications. Use this calculator for planning standards, checking manual calculations, interpreting UV-visible measurements, or exploring how path length and concentration influence absorbance. It applies the ideal single-species equation and does not estimate uncertainty, correct a baseline, or separate overlapping spectra. Record the wavelength and experimental conditions, use a matched blank, and report significant figures consistent with the least precise measured input.