What is the Beer-Lambert Law?

The Beer-Lambert Law states that A = ε × c × l, where A is absorbance (dimensionless), ε (epsilon) is the molar absorptivity (L·mol⁻¹·cm⁻¹), c is the molar concentration (mol/L), and l is the path length (cm). It describes how strongly a chemical solution absorbs light at a specific wavelength.

What are the units for molar absorptivity (ε)?

Molar absorptivity (ε) has units of L·mol⁻¹·cm⁻¹ (also written M⁻¹·cm⁻¹). It is a property of the substance at a specific wavelength and temperature. Values typically range from near zero to over 100,000 L·mol⁻¹·cm⁻¹ for strongly absorbing compounds.

What is absorbance and what range is reliable?

Absorbance (A) is defined as A = log₁₀(I₀/I), where I₀ is incident light intensity and I is transmitted intensity. It is dimensionless. Most spectrophotometers give reliable readings between A = 0.1 and A = 1.5. Above A ≈ 2, stray light and detector noise make measurements unreliable.

What path length should I use?

Standard cuvettes have a path length of 1 cm. Some microvolume instruments use shorter path lengths (e.g., 0.1 mm). If using the standard 1 cm cuvette, enter 1 in the path length field.

What are the assumptions of the Beer-Lambert Law?

The law assumes: (1) monochromatic incident light, (2) the solution is homogeneous, (3) no scattering or fluorescence, (4) the absorbing species does not interact with each other (valid at low concentrations, typically below ~0.01 M). Deviations occur at high concentration or in highly scattering solutions.

What is transmittance and how does it relate to absorbance?

Transmittance (T) is the fraction of light that passes through the sample: T = I/I₀. Absorbance relates to transmittance as A = −log₁₀(T). A transmittance of 10% equals an absorbance of 1.0; 1% transmittance equals A = 2.0.

Beer-Lambert Law Calculator

Solve for absorbance, concentration, path length, or molar absorptivity using A = εcl. Supports all four variables.

Beer-Lambert Law Inputs

A = ε × c × l

Solve for

Absorbance (A) — dimensionless

Molar Absorptivity (ε) L·mol⁻¹·cm⁻¹

Concentration (c) mol/L

Path Length (l) cm

Example presets (click to load)

Select what to solve for, enter known values, and click Calculate

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Summary

The Beer-Lambert Law Calculator applies the equation A = ε × c × l to solve for any one of the four variables: absorbance (A), molar absorptivity (ε), concentration (c), or path length (l). It is the fundamental relationship in spectrophotometry, linking the attenuation of light through a solution to its chemical concentration. All calculations run entirely in the browser — no data is sent to any server.

How it works

Select which variable you want to solve for: Absorbance (A), Molar Absorptivity (ε), Concentration (c), or Path Length (l).
Enter values for the remaining three variables in their respective fields.
Click Calculate (or press Enter) to see the result instantly.
Review the full equation with your values substituted in the Equation Applied section.
Use the Copy button to copy the result to your clipboard.
Click Reset to clear all inputs and start a new calculation.

Use cases

Determine the concentration of a solution from absorbance measurements in a lab.
Calculate the expected absorbance before running a spectrophotometer experiment.
Find the molar absorptivity of a compound from known concentration and path length data.
Verify cuvette path length from absorbance and concentration measurements.
Prepare standard curves for protein or DNA quantification assays.
Cross-check spectrophotometry results from published literature.
Teach Beer-Lambert Law concepts in chemistry or biochemistry courses.
Estimate detection limits based on minimum measurable absorbance.

Frequently Asked Questions

Last updated: 2026-09-30 · Reviewed by Nham Vu