Beer-Lambert Absorbance Calculator

This Beer-Lambert absorbance calculator works out how strongly a solution absorbs light using the foundational equation of spectroscopy. The Beer-Lambert law states that absorbance A equals the molar absorptivity, often written epsilon, times the concentration of the absorbing species times the path length of the light through the sample, written as A equals epsilon times c times l. You enter the molar absorptivity in litres per mole per centimetre, the concentration in moles per litre and the path length in centimetres, which is typically 1 for a standard cuvette, and the tool returns the absorbance as a dimensionless number along with the percentage transmittance. Transmittance is the fraction of light that passes through and is found from absorbance as ten raised to the power of minus A, then expressed as a percentage. Chemistry students, biochemists, water quality analysts and quality control labs use this relationship every day to turn a spectrophotometer reading into a concentration, or to predict a reading before running a sample. The law is most reliable at low to moderate absorbance, roughly up to an absorbance of 1, where the relationship between concentration and absorbance stays linear. At high concentrations the law can curve away from linearity because of chemical and optical effects, so diluting a strongly absorbing sample often gives more trustworthy results. Keep your units consistent, since molar absorptivity and concentration must share the same volume and length basis as the path length. Choosing the wavelength of maximum absorbance also improves sensitivity and accuracy, because small wavelength errors matter less near a peak. Used within its limits, the law gives a fast and dependable link between concentration and the light a sample absorbs.

A = epsilon c l. Transmittance = 10^(-A). Most reliable for absorbance up to about 1.