Beer-Lambert Law Calculator
Calculate absorbance, transmittance, concentration or path length using the Beer-Lambert law A = εcl. Enter any three values to find the fourth.
How to use this tool
- Enter absorbance a, molar absorptivity ε, concentration c and path length l in the fields above.
- Results update instantly as you type — or click Calculate.
- Read your absorbance a and the full breakdown beneath it.
The Beer-Lambert law relates light absorption to solution concentration: A = εcl, where A is absorbance, ε is molar absorptivity (L/mol·cm), c is concentration (mol/L), and l is path length (cm). Transmittance T = 10⁻ᴬ.
Formula
Beer-Lambert law: A = ε c l
Transmittance: T = 10−A
Percent transmittance: %T = T × 100
How it works
The Beer-Lambert law relates the absorbance A of a solution to its molar absorptivity ε (L mol−1 cm−1), concentration c (mol/L), and path length l (cm). Given any three of the four quantities, the calculator solves for the fourth. The law assumes monochromatic light, a homogeneous solution, and concentrations low enough to avoid analyte-analyte interactions; deviations occur at high absorbance or with stray light.
Worked example
Worked example
- Inputs: ε = 1000 L/mol·cm, c = 0.001 mol/L, l = 1.0 cm; absorbance is unknown (entered as 0).
- Solve for A: A = ε × c × l = 1000 × 0.001 × 1.0 = 1.0.
- Transmittance: T = 10^−1.0 = 0.1.
- Percent transmittance: %T = 0.1 × 100 = 10.0%.
Absorbance A = 1.0; transmittance T = 0.1; percent transmittance = 10.0%.
Key terms
- Absorbance (A)
- A dimensionless logarithmic measure of how much light is absorbed by a sample; A = log₁₀(I₀/I).
- Molar absorptivity (ε)
- An intrinsic property of a substance describing how strongly it absorbs light at a given wavelength, in units of L mol⁻¹ cm⁻¹.
- Transmittance (T)
- The fraction of incident light that passes through the sample, ranging from 0 (complete absorption) to 1 (no absorption).
- Path length (l)
- The distance the light beam travels through the absorbing medium, typically the inner width of a cuvette in cm.
- Concentration (c)
- The amount of absorbing species per unit volume, expressed in mol/L (molarity) in the Beer-Lambert context.
Frequently asked questions
- What is molar absorptivity?
- Also called the molar extinction coefficient (ε), it is a measure of how strongly a substance absorbs light at a given wavelength. Units: L/(mol·cm).
- When does Beer-Lambert law break down?
- At high concentrations (above ~0.01 mol/L) due to molecular interactions, or when the light source is not monochromatic.