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Beer-Lambert Law Calculator (Absorbance)

Calculate absorbance, transmittance, and back-calculate concentration from percent transmittance using the Beer-Lambert law (A = epsilon * c * l).

Molar absorptivity in L mol-1 cm-1. Found in literature or determined by calibration.
Analyte concentration in mol/L (Molar).
Cuvette path length in cm. Standard = 1 cm.
Enter %T to back-calculate concentration. Ignored when using forward direction.

Results

Absorbance (from concentration)5.0,000 AU
Transmittance (from concentration)0.001 %T
Concentration (from %T input)0.000,200 mol/L

📖What is it?

The Beer-Lambert Law (A = epsilon * c * l) states that absorbance is directly proportional to the concentration of the absorbing species (c), the path length through the solution (l), and the molar absorptivity (epsilon), a substance-specific constant at a given wavelength.

🎯How to use

Forward direction: enter epsilon, concentration, and path length to get absorbance and transmittance. Reverse direction: enter a measured %T value from your spectrophotometer along with epsilon and path length to calculate the unknown concentration.

💡Example scenario

NADH at 340 nm (epsilon = 6,220 L mol-1 cm-1), concentration 0.1 mM (0.0001 mol/L), 1 cm cuvette: A = 6,220 x 0.0001 x 1 = 0.622 AU, %T = 23.9%. Spectrophotometer reads 31.6 %T, back-calculate: c = -log(0.316) / 6220 = 0.500 / 6220 = 0.0000804 mol/L = 80.4 uM.

🏆Pro tip

Beer-Lambert law is linear only in the absorbance range of approximately 0.1 to 1.0 AU. Above 1.0 AU, detector noise and stray light cause deviations. Below 0.1 AU, accuracy is limited. Dilute your sample to fall within this range for reliable results. Turbid samples (suspensions) do not follow Beer-Lambert -- scatter contributes to apparent absorbance.