Beer-Lambert Law Calculator
Enter the molar absorptivity, concentration, and path length to get the absorbance of a solution — and see why absorbance rises in step with how much light-absorbing material the beam passes through.
Absorbance from three inputs
Enter the molar absorptivity, the concentration, and the path length and the calculator returns the absorbance A = ε·c·l, a dimensionless number.
Match your units
Molar absorptivity in L/(mol·cm), concentration in mol/L, and path length in cm cancel cleanly to a unitless absorbance — keep them consistent.
What is the Beer-Lambert law?
Absorbance from concentration and path length
The Beer-Lambert law is the rule that lets the beer lambert law calculator turn how much light a solution absorbs into the amount of dissolved substance it contains. Absorbance measures, on a logarithmic scale, how much of a light beam a solution takes up: it grows in proportion to three things — how strongly each molecule absorbs at the chosen wavelength (the molar absorptivity ε), how many molecules are in the path (the concentration c), and how far the light travels through the sample (the path length l). Multiply the three together and you get a dimensionless absorbance. It is the number behind every UV-Vis spectrophotometer reading, from measuring DNA purity to tracking a reaction's progress.
Enter a molar absorptivity, a concentration, and a path length to get the absorbance of the solution instantly.
Absorbance is the molar absorptivity multiplied by the concentration and the path length — a simple product of the three inputs.
A = ε × c × lEach factor is to the first power, so absorbance scales linearly: double the concentration and the absorbance doubles, halve the path length and the absorbance halves. The molar absorptivity is a property of the substance at a given wavelength, so it stays fixed while you vary the concentration or the cell size. Use L/(mol·cm), mol/L, and cm and the units cancel to leave a pure, dimensionless number.
Suppose a solution has a molar absorptivity of 18,400 L/(mol·cm), a concentration of 0.0001 mol/L, and is measured in a standard 1 cm cuvette.
Multiply ε by the concentration
18,400 × 0.0001 = 1.84 — absorptivity scaled by how concentrated the solution is.
Multiply by the path length
1.84 × 1 = 1.84 — the 1 cm path leaves the value unchanged.
Read the absorbance
A = 1.84, a dimensionless number. About 1.4 % of the light makes it through the sample (T = 10⁻¹·⁸⁴).
Absorbance answers one question: how much of the light is taken up by the sample. Because the three inputs multiply, the result is fully linear — an absorbance of 1.84 would fall to 0.92 if you halved the concentration, and rise to 3.68 if you doubled it. This linearity is exactly what makes the law so useful: measure the absorbance of an unknown sample, divide by the known ε and path length, and you have its concentration directly. Absorbance also ties to transmittance, the fraction of light that passes through, by A = −log₁₀(T): an absorbance of 1 means 10 % of the light gets through, 2 means 1 %, and 3 means just 0.1 %. That logarithmic link is why high-absorbance samples are usually diluted before measurement — beyond about A = 1 the transmitted signal becomes too faint to read reliably.
The formula is exact, but the linear law only holds under the right conditions.
Linear only at low concentrations
The Beer-Lambert law is linear only while the solution is dilute — at high concentrations, molecular interactions, refractive-index changes, and stray light bend the absorbance-versus-concentration line away from straight, so a calculated value may overstate the true absorbance. Remember that absorbance itself is dimensionless: the L/(mol·cm), mol/L, and cm units cancel. Absorbance also relates to transmittance by A = −log₁₀(T), so an absorbance of 2 means just 1 % of the light is transmitted.