Use the Beer-Lambert law to find the concentration of a solution from its absorbance, or any other unknown in A = εlc. This free absorbance calculator also converts between absorbance and % transmittance, for UV-Vis spectrophotometry and colorimetry.
Beer-Lambert law formulas
A is absorbance (no units), ε is the molar absorptivity (molar extinction coefficient) in L mol⁻¹ cm⁻¹, l is the path length in cm and c is the concentration in mol/L.
Worked example: NADH
An NADH solution in a 1 cm cuvette reads A = 0.62 at 340 nm, where ε = 6,220 L mol⁻¹ cm⁻¹. c = 0.62 / (6,220 × 1) = 9.97 × 10⁻⁵ M = 99.7 µM. The sample transmits 10^(−0.62) = 24% of the light.
Absorbance and transmittance
| Absorbance | % Transmittance | Light absorbed |
|---|---|---|
| 0 | 100% | 0% |
| 0.3 | 50% | 50% |
| 0.5 | 31.6% | 68.4% |
| 1 | 10% | 90% |
| 2 | 1% | 99% |
| 3 | 0.1% | 99.9% |
Frequently asked questions
How do I calculate concentration from absorbance?
Rearrange the Beer-Lambert law: c = A / (εl). Enter the absorbance, molar absorptivity and path length, and leave the concentration box empty.
What are the units of molar absorptivity?
L mol⁻¹ cm⁻¹ (also written M⁻¹ cm⁻¹), so that A = εlc has no units when c is in mol/L and l is in cm.
How do I convert absorbance to transmittance?
Use %T = 10^(2 − A). An absorbance of 1 means 10% of the light gets through.
What absorbance range is accurate?
Most spectrophotometers are linear between about 0.1 and 1.0 to 1.5. Dilute samples that read higher.
Can I use a calibration curve instead of ε?
Yes. The slope of an absorbance vs concentration standard curve equals ε × l, so enter the slope as ε and 1 as the path length.
Related tools
Free educational tool by HawkInc. Results are calculated in your browser and rounded for display; check critical work by hand or with a second method.