About this tool
The Beer–Lambert Law Calculator is a free, in-browser spectrophotometry tool that relates a sample's absorbance to its concentration via A = ε·l·c. Choose which term to solve for — absorbance A, molar absorptivity ε, path length l, or concentration c — and the value is computed instantly as you type.
By convention absorbance is unitless, molar absorptivity is in M⁻¹·cm⁻¹ (L·mol⁻¹·cm⁻¹), path length is in centimetres and concentration is in mol/L (molar). The tool also reports percent transmittance using %T = 10^(−A) × 100.
Everything runs locally in your browser with no data leaving your device, making it convenient for lab reports, calibration checks and coursework.
Frequently asked questions
What units go with each quantity?
Absorbance A is dimensionless, molar absorptivity ε is in M⁻¹·cm⁻¹ (L·mol⁻¹·cm⁻¹), path length l is in centimetres, and concentration c is in mol/L (molarity).
How is transmittance calculated?
Absorbance and transmittance are linked by A = −log₁₀(T). The tool shows percent transmittance as %T = 10^(−A) × 100, so an absorbance of 1 corresponds to 10% transmittance.
Is the Beer–Lambert law always accurate?
It assumes a dilute, homogeneous, non-scattering sample and monochromatic light. At high concentrations or with chemical association, absorbance can deviate from linearity, so calibrate within a validated range.
What is molar absorptivity?
ε (also called the extinction coefficient) is a wavelength-specific constant for a substance describing how strongly it absorbs light per unit concentration and path length; larger ε means stronger absorption.