# Molar Extinction Coefficient & Beer-Lambert (A = ε·c·l)

Solve A, ε, c or l from the Beer-Lambert law A = ε·c·l, with M/mM/µM/nM and cm/mm unit support.

> Canonical page: https://elysiatools.com/en/tools/molar-extinction-coefficient

- **Category:** Education

- **Keywords:** molar extinction coefficient, molar absorptivity, beer lambert law, beer-lambert, absorbance, spectrophotometry, epsilon, nadh 340nm, cuvette, concentration from absorbance

## Overview

The Molar Extinction Coefficient & Beer-Lambert Calculator solves for any variable in the Beer-Lambert equation (A = ε·c·l)—absorbance, molar extinction coefficient, concentration, or optical path length—with built-in unit conversion for M, mM, µM, nM, cm, and mm.

## Inputs

- **Solve For** (select): Which quantity to compute; the other three must be provided.
- **Absorbance (A)** (number): Measured absorbance (required unless solving for A).
- **Extinction Coefficient ε (M⁻¹cm⁻¹)** (number): Molar extinction coefficient in M⁻¹cm⁻¹ (required unless solving for ε).
- **Concentration** (number): Concentration in the selected unit (required unless solving for c).
- **Concentration** (select): Unit of the entered (or solved) concentration.
- **Path Length** (number): Optical path length in the selected unit (required unless solving for l).
- **Path Length** (select): Unit of the entered (or solved) path length.
- **Decimal Places** (number)

## When to use

- Determining unknown solute concentrations from spectrophotometric absorbance readings.
- Calculating molar absorptivity (ε) for novel chromophores, proteins, or chemical dyes from standard solutions.
- Predicting expected absorbance values or selecting optimal cuvette path lengths for kinetic assays.

## How it works

- Select the variable you want to compute (Absorbance, Extinction Coefficient, Concentration, or Path Length).
- Enter the known values and specify the appropriate concentration (M, mM, µM, nM) and path length (cm, mm) units.
- Adjust the output decimal precision to generate the final calculated value and underlying formula substitution.

## Use cases

- Enzymatic activity assays monitoring NADH or NADPH consumption and production at 340 nm.
- Nucleic acid and protein concentration determination via UV absorbance at 260 nm and 280 nm.
- Analytical chemistry workflows establishing calibration baselines and characterizing newly synthesized dyes.

## Frequently asked questions

### What is the Beer-Lambert law equation?

The Beer-Lambert law is A = ε·c·l, where A is absorbance (dimensionless), ε is the molar extinction coefficient (M⁻¹cm⁻¹), c is molar concentration (mol/L), and l is the optical path length (cm).

### How does the tool handle concentration and path length unit conversions?

Non-standard units such as mM, µM, nM, and mm are automatically converted to M (mol/L) and cm before performing calculations to match the standard dimension of ε (M⁻¹cm⁻¹).

### What is the standard extinction coefficient for NADH at 340 nm?

The accepted molar extinction coefficient for NADH at 340 nm is 6,220 M⁻¹cm⁻¹ (6.22 × 10³ M⁻¹cm⁻¹).

### Why does absorbance have no units?

Absorbance is defined as the logarithm of the ratio of incident light intensity to transmitted light intensity (log₁₀(I₀/I)), making it a dimensionless value.

### Can I solve for concentration when using a microplate reader with a non-standard path length?

Yes, you can enter custom path lengths directly in millimeters or centimeters to accurately determine sample concentrations.

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