What is the Beer-Lambert law?

A = ε × c × l. Absorbance is proportional to concentration and path length, with proportionality constant ε (molar absorptivity).

What are typical units?

A is dimensionless. ε is L/(mol·cm), c is mol/L, l is cm. With these units the formula gives a consistent dimensionless absorbance.

Why does Beer-Lambert fail at high concentrations?

At high concentrations, solute molecules interact (changing ε), the solution may become opaque, and stray light becomes significant. The law is reliable for absorbance below about 1.

What is a colorimeter or spectrophotometer used for?

To measure absorbance at a chosen wavelength. By comparing the sample absorbance to a calibration curve (A vs c), you can determine the concentration of an unknown solution.

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NSW · HSCChemistry

Beer-Lambert law calculator

A = ε c l, solve for any of the four variables.

Inputs

Solve forAbsorbance ADimensionless, typically 0-2Molar absorptivity ε (L/(mol·cm))Concentration c (mol/L)Path length l (cm)Standard cuvette = 1.0 cm

Result

Concentration c

5.000e-5mol/L

A = ε c l. Beer-Lambert law assumes a dilute solution and monochromatic light.

How this calculator works

It rearranges A = ε × c × l for whichever variable you need. The default path length is 1 cm, the standard cuvette width.

Common questions

What is the Beer-Lambert law?: A = ε × c × l. Absorbance is proportional to concentration and path length, with proportionality constant ε (molar absorptivity).
What are typical units?: A is dimensionless. ε is L/(mol·cm), c is mol/L, l is cm. With these units the formula gives a consistent dimensionless absorbance.
Why does Beer-Lambert fail at high concentrations?: At high concentrations, solute molecules interact (changing ε), the solution may become opaque, and stray light becomes significant. The law is reliable for absorbance below about 1.
What is a colorimeter or spectrophotometer used for?: To measure absorbance at a chosen wavelength. By comparing the sample absorbance to a calibration curve (A vs c), you can determine the concentration of an unknown solution.