Beer-Lambert Law Calculator
Find the concentration of a solution from its absorbance, molar extinction coefficient, and path length using the Beer-Lambert law, with a dilution factor to recover the original sample concentration. Widely used in spectrophotometry and analytical chemistry labs.
Last updated: September 2026
Formula below · 2 sources (pubchem.ncbi.nlm.nih.gov, Wikipedia) · Updated Sep 2026
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About this calculator
The Beer-Lambert law relates the attenuation of light passing through a solution to the properties of that solution: A = ε × c × l, where A is absorbance (dimensionless AU), ε is the molar extinction coefficient (L·mol⁻¹·cm⁻¹), c is the molar concentration (mol·L⁻¹), and l is the path length (cm). Rearranged to solve for concentration: c = A / (ε × l). If you diluted the sample before measuring, multiply by the dilution factor to get the original concentration: c_original = A / (ε × l) × DF. Solvent and temperature effects are not applied as generic corrections: ε is measured for a specific compound, wavelength, solvent and temperature, so use the ε value that matches your conditions. The law assumes monochromatic light, dilute solutions (typically A < 1.5), and a homogeneous sample.
How to use
Suppose absorbance A = 0.750 AU, ε = 15,000 L·mol⁻¹·cm⁻¹, path length l = 1 cm and no dilution. c = 0.750 / (15,000 × 1) = 5.0 × 10⁻⁵ mol/L (50 µM). If the sample had been diluted 1:10 before measuring, choose 10× and the original concentration is 5.0 × 10⁻⁴ mol/L (500 µM). Note: the site rounds results of 0.005 and above to two decimals and shows smaller values to three significant figures at most, so for sub-millimolar results read the value in scientific notation where shown.
Frequently asked questions
What is the molar extinction coefficient and how do I find it for my compound?
The molar extinction coefficient (ε), also called molar absorptivity, quantifies how strongly a substance absorbs light at a given wavelength. It is a fixed physical property of the compound measured in L·mol⁻¹·cm⁻¹. You can find it in published literature, spectral databases such as NIST or PubChem, or determine it experimentally by measuring absorbance of a series of known concentrations and applying Beer-Lambert law. Common examples include NADH at 340 nm (ε ≈ 6,220) and DNA at 260 nm (ε ≈ 50 for a 1 mg/mL solution per nucleotide conventions).
Why does Beer-Lambert law break down at high concentrations?
At high concentrations (typically A > 1.5 or concentrations above ~0.01 M), solute molecules are close enough to interact with each other, altering their electronic environment and changing ε. Additionally, stray light in the spectrophotometer and refractive index effects become significant. These deviations cause the absorbance–concentration relationship to curve rather than remain linear. For accurate results, dilute concentrated samples into the linear range before measurement, or construct a multi-point calibration curve.
How does path length affect absorbance measurements in a cuvette?
Path length is the distance light travels through the sample, typically 1 cm for standard cuvettes. Because A = ε × c × l, doubling the path length doubles the absorbance for the same concentration, effectively increasing sensitivity. Micro-cuvettes and fiber-optic probes can have path lengths from 0.01 cm to 10 cm, allowing measurement of very concentrated or very dilute samples respectively. Always enter the actual path length of your cuvette or flow cell into the calculator to get an accurate concentration.