Beer–Lambert Law Calculator

How it is calculated

The Beer-Lambert law gives absorbance as A = ε · b · c, where ε is the molar absorptivity in L/(mol·cm), b is the path length in cm (1 cm for a standard cuvette) and c is the concentration in mol/L. Transmittance follows from T = 10−A × 100, in percent. All inputs must be zero or positive. Absorbance has no unit and is shown with four decimals. The law holds for dilute solutions and a single absorbing species; at high concentration the relationship stops being linear. Absorbance of 1 means 10% of the light gets through, 2 means 1%.

Common mistakes

• Entering concentration in mmol/L or µmol/L instead of mol/L. A value off by 1,000 gives an absorbance off by 1,000 too.
• Entering the path length in mm or metres. It must be in cm, and a typical cuvette is 1.
• Treating absorbance and transmittance as the same scale. Absorbance is logarithmic, so doubling it does not halve the transmittance; it squares the fraction.
• Trusting a high result. Real instruments lose linearity at large absorbance, so dilute the sample if the value is far above about 1 to 2.