Beer-Lambert Law Calculator (ε)
where A = absorbance, l = path length (cm), c = concentration (mol/L)
What is the molar extinction coefficient?
The molar extinction coefficient (also called molar absorptivity) tells you how strongly a molecule absorbs light at a specific wavelength. It is represented by the Greek letter ε and typically reported in L·mol-1·cm-1.
In UV-Vis spectrophotometry, this value is central because it links what your instrument measures (absorbance) to what you want to know (concentration or molecular behavior).
The equation behind the calculator
This calculator uses the Beer-Lambert law:
A = εlc
- A = absorbance (unitless)
- ε = molar extinction coefficient (L·mol-1·cm-1)
- l = optical path length (cm)
- c = concentration (mol/L)
Rearranging gives: ε = A / (l × c)
How to use this calculator
Step 1: Enter absorbance or transmittance
If you measured absorbance directly, enter A. If you only have transmittance (%), leave absorbance blank and enter transmittance. The tool converts it automatically.
Step 2: Enter path length
Most cuvettes are 1 cm, but microvolume and special cuvettes can differ. Use the correct path length to avoid major errors.
Step 3: Enter concentration
Use molarity (mol/L). If your concentration is in mM, convert first (1 mM = 0.001 mol/L).
Step 4: Calculate
Click the calculate button to get ε in both standard and scientific notation.
Worked example
Suppose:
- Absorbance (A) = 0.65
- Path length (l) = 1.00 cm
- Concentration (c) = 2.5 × 10-5 mol/L
Then:
ε = 0.65 / (1.00 × 2.5 × 10-5) = 26,000 L·mol-1·cm-1
That means your compound absorbs quite strongly at the wavelength used.
Tips for better spectrophotometry results
- Measure a proper blank before samples.
- Use clean cuvettes with matching optical quality.
- Avoid bubbles and fingerprints in the light path.
- Stay in a linear absorbance range (commonly about 0.1 to 1.0).
- Confirm wavelength accuracy for your instrument.
Common mistakes
- Unit mismatch: using mM directly instead of converting to mol/L.
- Wrong path length: assuming all cuvettes are 1 cm.
- Out-of-range absorbance: very high absorbance can reduce reliability.
- Ignoring wavelength: ε is wavelength-specific.
FAQ
Is molar extinction coefficient constant?
It is constant only for a specific compound under specific conditions (wavelength, solvent, pH, temperature). Change conditions and ε can change.
Can I calculate concentration if I know ε?
Yes. Rearrange Beer-Lambert law to c = A / (εl).
Why does this calculator allow transmittance input?
Some instruments report %T. Since absorbance and transmittance are directly related, the calculator can convert %T to A for you.
Final note
This molar extinction coefficient calculator is a fast way to perform routine UV-Vis calculations. For publication-quality work, always report wavelength, solvent conditions, path length, and the method used to prepare standards.