Beer-Lambert Law Calculator

This Beer-Lambert law calculator relates the absorbance of a solution to its concentration, the standard tool behind spectrophotometry. The Beer-Lambert law states that absorbance equals the molar absorptivity times the concentration times the path length, A equals epsilon c l. Choose which quantity you want and enter the other three; the calculator solves for it. Most often you measure absorbance and a known path length and absorptivity to find an unknown concentration, the basis of colorimetric assays in chemistry and biology. The law holds for dilute solutions. The formula, a worked example and the assumptions are below.

Absorbance A

Molar absorptivity \u03B5 (L/mol/cm)

Concentration c (mol/L)

Path length l (cm)

Solve for

Value

Result

The formula

The Beer-Lambert law is A = epsilon times c times l, where A is absorbance (no units), epsilon is the molar absorptivity in litres per mole per centimetre, c is the concentration in moles per litre, and l is the path length in centimetres. Rearrange to solve for any one variable.

Worked example

With molar absorptivity 1000, concentration 0.001 mol/L and a 1 cm cell, the absorbance is A = 1000 times 0.001 times 1 = 1. Solve for A with those values to confirm.

Frequently asked questions

What is the Beer-Lambert law?

Absorbance is proportional to concentration and path length: A = epsilon c l. It lets you find an unknown concentration from a measured absorbance.

What is molar absorptivity?

A constant, epsilon, that measures how strongly a substance absorbs light at a given wavelength, in litres per mole per centimetre.

When does the law break down?

At high concentrations and with scattering or chemical changes, the linear relationship can fail. It works best for dilute solutions.

Who this calculator is for

This calculator is for chemistry students, teachers and lab workers.

What this calculator assumes

You enter values in the units shown.
Standard conditions unless stated.
Results are rounded for display.

Formula and sources

Related calculators

Molarity Calculator: concentration in mol/L.
pH Calculator: acidity from H+ concentration.
Ideal Gas Law Calculator: PV = nRT.

If you've found a bug, or would like to contact us, or learn more about James Graham and Calculate.co.nz.

Calculate.co.nz is partnered with Interest.co.nz for New Zealand's highest quality calculators and financial analysis.

All calculators and tools are provided for educational and indicative purposes only and do not constitute financial advice.

Calculate.co.nz is proudly part of the Realtor.co.nz group, New Zealand's leading property transaction literacy platform, helping Kiwis understand the home buying and selling process from start to finish. Whether you're a first home buyer navigating your first property purchase, an investor evaluating your next acquisition, or a homeowner planning to sell, Realtor.co.nz provides clear, independent, and trustworthy guidance on every step of the New Zealand property transaction journey.

Calculate.co.nz is also partnered with Health Based Building and Premium Homes to promote informed choices that lead to better long-term outcomes for Kiwi households.

Calculate.co.nz is hosted in Auckland via SiteHost new Zealand.

All content on this website, including calculators, tools, source code, and design, is protected under the Copyright Act 1994 (New Zealand). No part of this site may be reproduced, copied, distributed, stored, or used in any form without prior written permission from the owner.

About & trust: Why Calculate is NZ's most comprehensive · By the Numbers · How we compare · Editorial standards · How we keep data current · NZ finance glossary · Research & data · Financial literacy NZ · About · Privacy policy · Terms of use

Reviewed and maintained. Last reviewed 2026-06-21 and checked on a twice-monthly cycle against IRD, RBNZ and Stats NZ. How we keep data current.