Beer-Lambert Law Calculator
Absorbance from concentration and path length, the transmittance it corresponds to, and the concentration giving A = 1.
Results
What this tool does
Absorbance is proportional to concentration, which is what makes a spectrophotometer a measuring instrument rather than a detector. Molar absorptivity depends on the substance and the wavelength, so it has to come from a calibration or a reference — the default is the value often quoted for NADH at 340 nm.
Formula
A = ε × l × c ; T = 10⁻ᴬ
Variables
| Symbol | Meaning | Unit |
|---|---|---|
ep | Molar absorptivity | L/(mol·cm) |
l | Path length | cm |
cc | Concentration | mmol/L |
A | Absorbance | — |
TR | Transmittance | % |
CM | Concentration | mol/L |
CA | Concentration | mmol/L |
Worked example
- Molar absorptivity6220 L/(mol·cm)
- Path length1 cm
- Concentration0.05 mmol/L
- Absorbance0.31100
- Transmittance48.8652 %
- Concentration0.00005000 mol/L
- Concentration0.160772 mmol/L
Limitations
- For work that must comply with a standard or be signed off, check the result against the applicable code and have it reviewed by a qualified engineer.
- The formula assumes ideal conditions: no friction losses, no air resistance and no efficiency losses unless you enter them.
Frequently asked questions
Why do readings above about 2 absorbance units stop being trustworthy?
Because absorbance 2 means only one percent of the light gets through, and 3 means one part in a thousand. At that point the detector is measuring a signal barely above its own noise, and any stray light leaking past the sample dominates the reading. Dilute the sample until absorbance falls between roughly 0.1 and 1.