Beer-Lambert Law Calculator
Calculate absorbance, concentration or molar absorptivity with the Beer-Lambert law, and convert between absorbance and percent transmittance.
What this calculator does
Solve A = εlc for absorbance or concentration, and convert between absorbance, transmittance and percent transmittance with the steps shown.
When you need it
Spectrophotometry uses the Beer-Lambert law to turn a measured absorbance into a concentration, for example in calibration curves, enzyme assays and water analysis.
Formulas
- A = ε · l · c
- A = −log₁₀ T = 2 − log₁₀(%T)
- T = I ÷ I₀
How to use it
- Choose whether to find absorbance, concentration or convert from transmittance.
- Enter ε in L/(mol·cm), the path length in cm and the concentration or absorbance.
- Read the result and the equivalent %T.
Worked example
A solution has ε = 1.20 × 10⁴ L/(mol·cm) at its peak, path length 1.00 cm and concentration 5.0 × 10⁻⁵ M. Find A and %T.
- A = εlc = 1.20 × 10⁴ × 1.00 × 5.0 × 10⁻⁵ = 0.60
- T = 10^(−0.60) = 0.251
- %T = 25.1%
Answer
The absorbance is 0.60 and the transmittance is 25.1%.
Common mistakes
- Using a path length in mm instead of cm.
- Using the law at high concentrations, where it stops being linear.
- Treating %T as T in the logarithm.
Related tools
Frequently asked questions
What does the Beer-Lambert law say?
Absorbance is proportional to the concentration of the absorbing species and to the path length: A = εlc.
How do you convert absorbance to transmittance?
T = 10^(−A). An absorbance of 1 means 10% transmittance.
What are the units of molar absorptivity?
L/(mol·cm).