Molar Extinction Coefficient Calculator
The molar extinction coefficient (also called molar absorptivity, symbol epsilon) is the intrinsic ability of a chemical species to absorb light at a given wavelength. It is determined from the Beer-Lambert law: A = epsilon x b x c, where A is the measured absorbance (dimensionless), b is the path length in centimetres, and c is the molar concentration in mol/L. Rearranging gives epsilon = A / (b x c) in units of L mol(-1) cm(-1). This calculator solves for epsilon from measured absorbance, and also solves for concentration given epsilon, allowing you to use a known extinction coefficient to quantify an unknown sample.
Beer-Lambert law formula
A = epsilon x b x c
epsilon = A / (b x c)
%T = 100 x 10^(-A)
A = absorbance (unitless), epsilon = molar extinction coefficient (L mol(-1) cm(-1)), b = path length (cm), c = concentration (mol/L). Transmittance %T = 100 x 10^(-A). IUPAC recommends the terms molar absorptivity and molar absorption coefficient as alternatives to molar extinction coefficient.
UV-Vis spectroscopy in organic chemistry
- UV-Vis spectroscopy detects conjugated systems, aromatic rings, and heteroatom-containing chromophores.
- The wavelength of maximum absorption (lambda max) is characteristic of the chromophore.
- Higher epsilon indicates a more strongly absorbing chromophore (more allowed electronic transition).
- Beer-Lambert law allows quantification of unknown concentrations once epsilon is determined.
- Woodward-Fieser rules predict lambda max and approximate epsilon for conjugated dienes and enones.
Molar extinction coefficient: frequently asked questions
What is the Beer-Lambert law?
The Beer-Lambert law states that the absorbance (A) of a solution is proportional to the concentration (c) of the absorbing species and the path length (b) of the light through the solution: A = epsilon x b x c. This relationship holds for dilute solutions and monochromatic light.
What are the units of molar extinction coefficient?
The molar extinction coefficient (epsilon) has units of L mol(-1) cm(-1), also written as M(-1) cm(-1). If concentration is in mol/L and path length is in cm, then epsilon = A / (b x c) gives the result in the correct units.
What is a typical epsilon value for organic chromophores?
Values range enormously. Simple n to pi* transitions (ketones, enones): epsilon 10 to 100. Conjugated dienes (pi to pi*): epsilon 10,000 to 30,000. Aromatic rings: epsilon 200 to 10,000. Porphyrins (Soret band): epsilon up to 500,000. DNA bases at 260 nm: epsilon approximately 10,000 per nucleotide.
What path length should I use?
Standard UV-Vis cuvettes have a 1 cm path length. Microcuvettes and flow cells may use shorter paths (0.1 cm or 0.01 cm). Fiber optic probes range from 0.2 mm to 10 cm. Always match the path length entered here to the actual cuvette used.
When does Beer-Lambert law break down?
The law deviates at high concentrations (above approximately 0.01 M for many compounds) due to solute-solute interactions and inner filter effects. It also deviates for very broad-band or polychromatic light, or when photodecomposition occurs during measurement.
Official sources
- IUPAC: Quantities, Units and Symbols in Physical Chemistry (Green Book).
- NIST: NIST UV-Vis spectral data.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.