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Molar Absorptivity Calculator

Enter absorbance, concentration, and path length to find molar absorptivity.

How it is calculated

Starting from the Beer-Lambert law A = ε × c × l, the tool solves for the molar absorptivity: ε = A ÷ (c × l). Absorbance A has no unit, concentration c is in moles per litre (mol/L) and the path length l is in centimetres, so ε comes out in L·mol-1·cm-1. Concentration and path length must both be greater than zero. Absorbance is not checked: zero gives ε = 0 and a negative reading gives a negative ε, which usually signals a blanking problem rather than a real result. The value is rounded to two decimals. ε belongs to one substance at one wavelength and solvent, so use an absorbance read at that wavelength. The law holds best for dilute solutions, with absorbance roughly below 1.

Worked example

A dye solution at 0.0001 mol/L (1.0 × 10-4 M) is measured in a standard 1 cm cuvette and reads A = 0.60. Multiply concentration by path length: 0.0001 × 1 = 0.0001. Divide absorbance by that product: 0.60 ÷ 0.0001 = 6,000.00 L·mol⁻¹·cm⁻¹. The tool also echoes the absorbance as 0.60 and the path length as 1.00 cm. If the same reading and concentration came from a 0.5 cm cell, ε would be 12,000, because the light passes through half as much solution.

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Common mistakes

  • Entering concentration in mg/mL, µM or mM without converting. The field is mol/L, so 0.1 mM must be typed as 0.0001.
  • Forgetting the cuvette width. Many cells are 1 cm but some are 0.5 cm or 0.2 cm, and the path length goes straight into the divisor.
  • Using a reading that is too high. Absorbance well above about 1 often falls outside the straight-line range of the law, so dilute and re-measure.
  • Not subtracting the blank (solvent) reading. A background offset changes A and therefore ε, and a negative result points to this.
  • Applying ε from a different wavelength or solvent. The value is specific to the wavelength and conditions of the measurement, not a universal property of the substance.

About this calculator

The Beer-Lambert law states A = ε·c·l, where A is absorbance, c is molar concentration, and l is the path length of light through the sample (typically a 1 cm cuvette). Solving for ε gives the molar absorptivity (also called the molar extinction coefficient), measured in L·mol⁻¹·cm⁻¹ — a constant specific to a given substance at a given wavelength.

Molar absorptivity values are used to identify compounds and calculate unknown concentrations from a measured absorbance reading in UV-Vis spectrophotometry. It's a routine calculation for analytical and biochemistry labs — for example, quantifying DNA or protein concentration from A260/A280 readings — and for pharmaceutical quality-control testing.

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Comments (1)

  • Editorial team

    A single reading gives a rough epsilon. With A = 0.5, c = 0.001 mol/L and a 1 cm cell the tool returns 500.00, but absorbance readings above roughly 1 drift from the Beer-Lambert line, and a mix-up between mM and M shifts the answer a thousandfold. A slope fitted through several dilutions is sturdier than any single point. Which compound and wavelength are you measuring?

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