Use the Beer–Lambert law: A = εlc
Measure the absorbance, A, of the solution at the chosen wavelength
Determine the path length, l, of the cuvette, usually in cm
Determine the concentration, c, of the absorbing species, usually in mol/L
Rearrange the equation to solve for molar absorptivity: ε = A / (lc)
Use a calibration curve of absorbance vs concentration if multiple standards are available
Find the slope of the calibration curve
Divide the slope by the path length, if the slope is not already normalized for path length
Use the wavelength of maximum absorbance, λmax, for the most accurate value
Ensure the solution is dilute enough for Beer–Lambert law to apply
Use the correct units for ε: L·mol⁻¹·cm⁻¹