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What is a molar extinction coefficient?

Posted August 26, 2019


The term molar extinction coefficient (ε) is a measure of how strongly a chemical species or substance absorbs light at a particular wavelength. It is an intrinsic property of chemical species that is dependent upon their chemical composition and structure. The SI units of ε are m2/mol, but in practice they are usually taken as M-1cm-1. The molar extinction coefficient is frequently used in spectroscopy to measure the concentration of a chemical in solution.

You can use the Beer-Lambert Law to calculate a chemical species’ ε:

A = εLc


  • A is the amount of light absorbed by the sample for a particular wavelength
  • ε is the molar extinction coefficient
  • L is the distance that the light travels through the solution
  • c is the concentration of the absorbing species per unit volume

Rearrange the Beer-Lambert equation in order to solve for the molar extinction coefficient:

ε = A/Lc

Use the molar extinction coefficient to determine the brightness of a fluorescent molecule, by using the following equation:

Brightness = Extinction Coefficient (ε) x Fluorescence Quantum Yield (Φ)

Use our Extinction Coefficient finder to search for the extinction coefficients of other compounds.

Additional resources

Extinction Coefficient Finder