Molar Absorptivity Calculator

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.

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.