Beer–Lambert Law Calculator
Calculate absorbance or concentration and convert absorbance to percent transmittance.
01 Set up your calculation
- Choose absorbance or concentration as the unknown.
- Enter the coefficient at the measurement wavelength and the optical path length.
- Supply the remaining quantity and read absorbance, molar concentration, and transmittance.
02 The relationship
For the assumptions behind these results, read the explanation below. You can also explore Molarity and Solution Dilution.
Follow the numbers
A concentration of 0.00005 mol/L, coefficient 10,000 L/(mol·cm), and path 1 cm give A = 0.5. Transmittance is 100 × 10⁻⁰·⁵ ≈ 31.6228%.
03 Match the coefficient to the measurement
Molar absorption coefficient depends on the substance, wavelength, solvent, and relevant chemical form. It must be compatible with concentration in mol/L and path length in centimeters for this form of the equation.
Absorbance is logarithmic and dimensionless. It is not the percentage of light absorbed: absorbance 1 corresponds to 10% transmitted light, while absorbance 2 corresponds to 1% transmitted light.
04 Check the measurement’s linear range
The relationship assumes a suitable homogeneous sample and optical measurement. Scattering, stray light, chemical equilibria that change with concentration, and large spectral bandwidth can cause departures from linearity. Use an appropriate blank and a validated concentration range.
If an instrument already normalizes a microvolume reading to a 1 cm path, use that normalized path rather than applying the physical small path again. The concentration result describes the measured sample; account separately for any dilution of the original material.
Questions from the notebook
Can absorbance be negative?
A blank-corrected instrument can report a negative number, but this positive-absorption model does not interpret it as a negative concentration.
Does the calculator apply a dilution factor?
No. Multiply the measured-sample concentration by the known dilution factor to recover the original concentration.
Can I use a mass absorption coefficient?
Not in the molar-coefficient field. Convert to a compatible molar coefficient or use a mass-based formulation.
References & reading
- UCLA — Illustrated Glossary of Organic Chemistry: Beer’s Law ↗
States absorbance = molar absorptivity × optical path length × concentration.
- Chemistry LibreTexts — The Beer–Lambert Law ↗
Explains absorbance, transmittance, coefficient units, and the conditions of the optical model.
- NIST — Guide to SI conversion factors ↗
Supports physical unit conversions. Input values are converted to the formula’s base units before calculation.
Our explanations are written for ChemistryCalc. Read how we build and check our tools. These calculations are educational estimates where the underlying model requires assumptions.