Beer–Lambert Law Calculator
Beer–Lambert law calculator (A = εlc): absorbance, concentration, path length or molar absorptivity, plus transmittance.
A = ε · l · c
How to use this calculator
- Choose the quantity you want under “Solve for”.
- Type the known values and pick their units: any combination works.
- The result updates as you type. Open “Show the steps” to see the substitution.
The Beer–Lambert law A = ε·l·c states that absorbance is proportional to the concentration of the absorbing species and the path length of the light through the sample.
It is the basis of UV-Vis spectrophotometry. It is most reliable for absorbances between about 0.1 and 1; at higher values, stray light and molecular interactions make the response non-linear.
Formula
A = ε · l · c
| Symbol | Quantity | Units |
|---|---|---|
| ε | Molar absorptivity | L/(mol·cm), m²/mol |
| l | Path length | m, km, cm, mm, µm, nm, pm, Å, in, ft, yd, mi, nmi, au, ly |
| c | Concentration | mol/L, mmol/L, µmol/L, nmol/L, mol/m³ |
| A | Absorbance | — |
Worked example
Given: ε = 6 220 L/(mol·cm) · l = 1 cm · c = 0.1 mmol/L
A = ε · l · c
Result: A = 0.622
Frequently asked questions
How do absorbance and transmittance relate?
A = −log₁₀(T). An absorbance of 1 means 10 % of the light passes; 2 means 1 %.
What path length do standard cuvettes have?
Most cuvettes have a 1 cm path length, which is why ε is usually given in L·mol⁻¹·cm⁻¹.