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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Absorbance A

Molar absorptivity ε (L/mol/cm)

Concentration c (mol/L)

Path length l (cm)

Solve for

Value

Result

The formula

The Beer-Lambert law is A = epsilon times c times l, where A is absorbance (no units), epsilon is the molar absorptivity in litres per mole per centimetre, c is the concentration in moles per litre, and l is the path length in centimetres. Rearrange to solve for any one variable.

Worked example

With molar absorptivity 1000, concentration 0.001 mol/L and a 1 cm cell, the absorbance is A = 1000 times 0.001 times 1 = 1. Solve for A with those values to confirm.

Frequently asked questions

What is the Beer-Lambert law?

Absorbance is proportional to concentration and path length: A = epsilon c l. It lets you find an unknown concentration from a measured absorbance.

What is molar absorptivity?

A constant, epsilon, that measures how strongly a substance absorbs light at a given wavelength, in litres per mole per centimetre.

When does the law break down?

At high concentrations and with scattering or chemical changes, the linear relationship can fail. It works best for dilute solutions.

Who this calculator is for

This calculator is for chemistry students, teachers and lab workers.

What this calculator assumes

You enter values in the units shown.
Standard conditions unless stated.
Results are rounded for display.

Formula and sources

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Molarity Calculator: concentration in mol/L.
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