Molar Absorptivity Calculator (Beer–Lambert)
Calculate the molar absorptivity (extinction coefficient) of a species from its absorbance, concentration and path length using the Beer–Lambert law.
How it works
A = ε · c · l → ε = A / ( c · l ) · transmittance T = 10^(−A)
The Beer–Lambert law says absorbance rises in direct proportion to how concentrated the absorbing species is and how far the light travels through it. Rearranging it gives the molar absorptivity ε — an intrinsic property of the substance at a given wavelength that measures how strongly it absorbs light. A large ε means even a dilute solution absorbs strongly, which is why intensely coloured dyes and complexes are detectable at very low concentrations.
Worked example
A solution of 1.0 × 10⁻⁴ mol/L in a 1.00 cm cuvette gives an absorbance of 0.50. Then ε = 0.50 / (1.0 × 10⁻⁴ × 1.00) = 5,000 L·mol⁻¹·cm⁻¹, and the transmittance is 10^(−0.50) = 31.6 %.
Sources: Beer–Lambert law — Wikipedia
Frequently asked questions
What are the units of molar absorptivity?
L·mol⁻¹·cm⁻¹ (sometimes written M⁻¹·cm⁻¹). Those units make absorbance dimensionless when multiplied by concentration in mol/L and path length in cm.
Does molar absorptivity depend on concentration?
No. ε is a constant for a given substance, wavelength and solvent. Absorbance changes with concentration, but ε itself does not — that is what makes it useful for identifying and quantifying compounds.
When does the Beer–Lambert law break down?
At high concentrations, where molecules interact and shift the absorption, and when stray light or a non-monochromatic beam is present. Absorbance readings much above about 1.5–2 often fall outside the linear range.
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This calculator is for educational purposes. Double-check important results before acting on them.