Beer-Lambert Law Calculator

This Beer-Lambert law calculator solves A = epsilon times c times l for whichever quantity you are missing. Choose what to solve for from the dropdown, absorbance, molar absorptivity, concentration or path length, then enter the other three: absorbance A (unitless), molar absorptivity epsilon in litres per mole per centimetre, concentration c in moles per litre, and path length l in centimetres. The calculator rearranges the formula instantly and returns the solved value with its unit, plus a labelled result showing which variable it worked out. This is the everyday tool behind spectrophotometry: most often you measure absorbance in a known cell with a reference molar absorptivity, then solve for the unknown concentration of your sample, whether that is a protein assay, a dye dilution or a water quality test. It works equally well the other way, predicting the absorbance a planned concentration should give, or back-calculating molar absorptivity from calibration standards. The worked example on this page, an absorptivity of 1000, a concentration of 0.001 mol/L and a 1 cm cell, gives an absorbance of 1, showing how the three knowns combine. Keep in mind the law only holds in a straight line for dilute solutions; at high concentrations, or where scattering and chemical interactions occur, the relationship can drift and your solved result becomes less reliable.

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