Beer–Lambert Law Calculator
Find a solution's concentration from its absorbance, the molar absorptivity and the path length — A = εlc — or the absorbance a concentration will give.
Turn a spectrophotometer reading into a concentration.
How the beer–lambert law calculator works
Absorbance grows in proportion to how much absorbing substance the light passes through: A = ε × l × c, where ε is the molar absorptivity (L·mol⁻¹·cm⁻¹, a property of the substance at that wavelength), l the cuvette path length (usually 1 cm) and c the concentration. Rearranged, c = A ÷ (ε l). The law is linear only up to about A = 1–1.5; dilute darker samples.
Formula: A = ε l c → c = A ÷ (ε l)
Worked examples
| Inputs | Concentration | Note |
|---|---|---|
| A = 0.45, ε = 6,220, 1 cm (NADH) | 0.000072 mol/L | 72.3 µM |
| A = 0.8, ε = 12,000, 1 cm | 0.000067 mol/L | 66.7 µM |
| A = 0.3, ε = 6,220, 0.5 cm | 0.000096 mol/L | half the path, twice the concentration |
FAQFrequently asked questions
Where do I get ε?
From the literature for the compound at the wavelength used, or from your own calibration curve (the slope of absorbance against concentration for a 1 cm cell).
Why does the law fail at high absorbance?
Stray light, instrument limits and molecular interactions make the response non-linear above about A = 1–1.5. Dilute and re-measure.
What is transmittance?
The fraction of light that gets through: T = 10^(−A). A = 1 means 10% transmitted.
Does the path length have to be 1 cm?
No — enter the cuvette's actual path; 1 cm is just the standard cell.
Where these figures come from
- IUPAC — Standard atomic weights (2021 conventional values) — the molar-mass table
- NIST — CODATA 2018 fundamental physical constants — Avogadro constant, gas constant, speed of light
- NIST Chemistry WebBook — thermochemical data
- CSIRO — Australia's national science agency
Last checked: September 2026. Atomic masses are the IUPAC conventional values; constants are CODATA 2018; equations are the standard textbook forms.