Beer-Lambert Law Calculator
Calculate absorbance, molar absorptivity, concentration, or path length using A = εcl. Solve any one unknown from three known values. Includes a built-in OD600 to bacterial cell density converter.
Solve for:
OD600 → Cell Density Converter
Approximation based on E. coli standard: 1 OD600 ≈ 8 × 10⁸ cells/mL. Values vary by organism and instrument.
Beer-Lambert Law Explained
A = ε × c × lThe Beer-Lambert law relates the attenuation of light passing through a substance to the properties of that substance. Absorbance (A) is proportional to the product of the molar absorptivity (ε), the concentration (c), and the optical path length (l). The law assumes monochromatic light, dilute solutions (typically A < 1.0), and no chemical interaction between solute molecules.
| Symbol | Variable | Description |
|---|---|---|
| A | Absorbance | The measured optical absorbance (log₁₀(I₀/I)), dimensionless. Valid range: 0–2; ideal: 0.1–1.0. |
| ε | Molar absorptivity | Substance-specific constant (M⁻¹cm⁻¹). e.g. dsDNA at 260 nm: ~6,600 M⁻¹cm⁻¹ per base pair. |
| c | Concentration | Molar concentration of the absorbing species (mol/L or M). |
| l | Path length | Length of the light path through the sample (cm). Standard cuvettes: 1 cm. |
How to Use This Calculator
- 1
Select your unknown
Choose which variable you want to solve for: A, ε, c, or l. The other three fields become your inputs.
- 2
Enter known values
Type the three known values. ε is typically looked up from literature or provided by the instrument manufacturer.
- 3
Click Calculate
The missing value appears instantly. For concentration, the result is in mol/L (M).
- 4
Use the OD600 converter (optional)
For bacterial cultures, enter your OD600 reading to get an estimated cell density in cells/mL.