Beer–Lambert law calculator
Choose the unknown and enter the other three values. Readings can be entered as absorbance or as percent transmittance, and the concentration in mol/L, mmol/L or µmol/L. The result comes with the matching transmittance.
Runs in your browser: nothing you enter is sent anywhere.
How to use it
- Choose the unknown: absorbance, concentration, molar absorptivity or path length.
- Enter the reading as absorbance or as percent transmittance.
- Enter the other values and pick the concentration unit.
- Read the result and the transmittance.
- A = ε × l × c
- c = A ÷ (ε × l), ε = A ÷ (l × c), l = A ÷ (ε × c)
- T = 10⁻ᴬ, shown in percent; A = −log₁₀(T ÷ 100) when T is in percent.
Worked example
NADH absorbs at 340 nm with ε = 6220 L/(mol·cm). A 100 µmol/L solution in a 1 cm cuvette: A = 6220 × 1 × 0.0001 = 0.622, so 23.88% of the light gets through.
The other way: a sample reads A = 0.5, so c = 0.5 ÷ (6220 × 1) = 0.0000804 mol/L = 80.3859 µmol/L.
Tips
- Zero with a blank of the solvent so that A measures only the substance.
- Keep A between about 0.1 and 1 for the most reliable readings.
- ε depends on wavelength. Use the value for the wavelength you measure at.
Frequently asked questions
What is the Beer–Lambert law?
Absorbance is proportional to concentration and path length, A = ε × l × c. ε is the molar absorptivity in L/(mol·cm), l the path length in cm and c the concentration in mol/L. A has no unit.
How are absorbance and transmittance related?
A = −log₁₀(T), with T as a fraction. 100% transmittance is A = 0, 10% is A = 1 and 1% is A = 2. The calculator converts either way.
Why is ε given in L/(mol·cm)?
So that A = εlc has no units when l is in centimetres and c in mol/L. Cuvettes are usually 1 cm long, which is why that is the default path length.
When does the Beer–Lambert law fail?
At high concentrations (often above about 0.01 mol/L), when absorbance is above about 1.5–2, with scattering or fluorescent samples, and when the light is not monochromatic. Dilute the sample to stay in the linear range.