What is Beer-Lambert Law?

Beer-Lambert Law states that the absorbance (A) of a solution equals the product of molar absorptivity (ε), path length (b), and molar concentration (c): A = εbc. It describes how light is attenuated as it passes through a sample.

What units does molar absorptivity use?

Molar absorptivity (ε) is typically expressed in L·mol⁻¹·cm⁻¹ (also written M⁻¹cm⁻¹). When ε is in those units, path length must be in cm and concentration in mol/L (M).

Why does Beer's Law break down at high concentrations?

At high concentrations, solute molecules interact with each other and alter each other's charge distribution, changing ε. Absorbance values above ~1 also reduce the signal-to-noise ratio of detectors. Beer's Law is typically linear only when A < 1.

How do I convert percent transmittance (%T) to absorbance?

Use A = 2 − log₁₀(%T). For example, 50 %T → A = 2 − log₁₀(50) ≈ 0.301. You can enter that absorbance value here to find concentration.

What is a standard path length for a cuvette?

Most disposable and quartz cuvettes have a 1 cm path length, which conveniently makes A = ε × c. Micro-cuvettes and flow cells may differ — always check and enter the correct path length.

Beer-Lambert Law Concentration Calculator

Name: Beer-Lambert Law Concentration Calculator
Availability: InStock
Author: Nham Vu

Solve for any variable in A = εbc — absorbance, molar absorptivity, path length, or concentration.

Beer-Lambert Law: A = ε · b · c

Select the variable to solve for, then enter the three known values.

Solve for

Absorbance (A) — dimensionless

Molar absorptivity (ε) L·mol⁻¹·cm⁻¹

Path length (b) cm

Concentration (c) mol/L (M)

Formula reference

Absorbance A = ε · b · c

Concentration c = A / (ε · b)

Molar absorptivity ε = A / (b · c)

Path length b = A / (ε · c)

Summary

Solve for any variable in A = εbc — absorbance, molar absorptivity, path length, or concentration.

How it works

Select which variable you want to solve for from the dropdown.
Enter the three known values with their units.
Click Calculate — the missing value appears instantly.
Use the Reset button to clear all fields for a new calculation.

Use cases

Determine the molar concentration of a solution from a spectrophotometer reading.
Find the theoretical absorbance expected at a known concentration.
Back-calculate molar absorptivity (ε) from a standard curve measurement.
Verify that path length assumptions hold for non-standard cuvettes.
Convert transmittance readings to absorbance before applying Beer's Law.
Check linearity range: confirm A < 1 for reliable Beer's Law predictions.

Frequently Asked Questions

Last updated: 2026-07-01 · Reviewed by Nham Vu