Beer-Lambert Law Calculator
Choose the quantity you want to find, enter the other three, and the calculator solves A = ε × c × l. Molar absorptivity is in L/(mol·cm), concentration in mol/L, and path length in cm, with unit selectors that convert for you.
Last updated: September 26, 2026
Frequently asked questions
What is the Beer-Lambert law?
It states that absorbance is proportional to the concentration of the absorbing species and the path length: A = ε × c × l, where ε is the molar absorptivity.
What units should I use?
Use ε in L/(mol·cm), concentration in mol/L, and path length in cm. Then absorbance is dimensionless. The unit selectors convert mmol/L, µmol/L, mm, and m for you.
Why is my concentration not reliable?
You need the correct molar absorptivity for your substance, solvent, and wavelength, or a calibration curve. High absorbance, scattering, stray light, and overlapping absorbers also break the linear relationship.
What path length is typical?
Many standard cuvettes have a 1 cm path length, but check your own cuvette or plate. Microplate readings depend on the liquid volume.
How is this different from the Protein or DNA Concentration calculators?
Those apply specific conversion factors for proteins or nucleic acids. This one solves the general A = ε·c·l relationship for any absorbing species when you know ε.
What is percent transmittance?
Transmittance is the fraction of light that passes through, T = 10^(−A). The calculator shows it as a percentage when solving for absorbance.