Beer–Lambert law calculator
A = εlc. Leave the unknown blank.
Work it out
Fill in all but one. 4 boxes are empty, so there is more than one answer.
A worked example
A sample reads A = 0.542 in a 1 cm cell, and ε is 15,600 M⁻¹cm⁻¹ at that wavelength.
34.7 µM
- 1c = A / (ε l) = 0.542 / (15600 × 1) = 3.47 × 10⁻⁵ M.
- 2That is 34.7 µM.
What this assumes
- The relationship is linear only in dilute solution — usually below about 0.01 M. Concentrated samples deviate because the absorbing species interact and the refractive index changes.
- ε depends on the wavelength, the solvent and often the pH, so it must be the value for the conditions you measured in.
- Readings above roughly A = 2 are unreliable; scattering, fluorescence and stray light all break the law.
The terms used here
- Absorbance
- log₁₀ of the ratio of light in to light out. It has no units.
- Molar absorptivity (ε)
- How strongly one mole per litre absorbs at a given wavelength. A property of the substance, not the sample.
- Path length
- How far the light travels through the sample. A standard cuvette is 1 cm.
The other calculators
Molar mass calculator
Enter a formula and get its molar mass, broken down by element.
Molarity calculator
Relate concentration, volume and moles. Leave the unknown blank.
Dilution calculator (C₁V₁ = C₂V₂)
C₁V₁ = C₂V₂. Leave the unknown blank.
Percent yield calculator
Actual over theoretical, as a percentage. Leave the unknown blank.
pH, pOH and [H⁺] calculator
Convert between pH, pOH, [H⁺] and [OH⁻]. Leave the unknown blank.
Buffer pH calculator (Henderson–Hasselbalch)
pH = pKa + log([A⁻]/[HA]). Leave the unknown blank.
This calculator runs in your browser. Nothing you type is sent anywhere, and no account is needed. If you want to go further than arithmetic — look a structure up, see what a reaction has been recorded as giving, or follow a mechanism — the library is open too.