Beer-Lambert Absorbance Calculator
The Beer-Lambert law is the foundation of quantitative spectrophotometry: absorbance rises in direct proportion to how strongly a species absorbs (molar absorptivity), how much of it is present (concentration), and how far the light travels through the sample (path length). This calculator returns absorbance from those three inputs, along with the transmittance as a fraction and a percentage. Use it to predict a reading, check an instrument, or work back to concentration from a known absorptivity and path length.
Beer-Lambert formula
A = e * c * l
Transmittance T = 10^(-A)
Transmittance % = 100 * T
(rearrange for c) c = A / (e * l)
Here e is molar absorptivity in L per mol per cm, c is concentration in mol per L, and l is path length in cm. The product is dimensionless, as absorbance must be. Transmittance is the antilog of negative absorbance.
Using the calculator
- A standard cuvette has a 1 cm path length, the most common default.
- Keep absorbance below roughly 1 for the linear regime; dilute the sample if it reads higher.
- Molar absorptivity is wavelength-specific: use the value at your measurement wavelength.
- To find unknown concentration, divide a measured absorbance by e times l.
- An absorbance of 1 transmits 10 percent of light; an absorbance of 2 transmits 1 percent.
Beer-Lambert law: frequently asked questions
What is the Beer-Lambert law?
The Beer-Lambert law states that absorbance A is proportional to the molar absorptivity (extinction coefficient) epsilon, the concentration c of the absorbing species, and the path length l of the light through the sample: A = epsilon times c times l. It underpins quantitative spectrophotometry.
How is transmittance related to absorbance?
Transmittance T is the fraction of light that passes through the sample, and absorbance is its negative base-10 logarithm: A = -log10(T). This calculator reports transmittance as a fraction and as a percentage. An absorbance of 1 means 10 percent of the light is transmitted; an absorbance of 2 means 1 percent.
What units does molar absorptivity use?
Molar absorptivity epsilon is usually given in litres per mole per centimetre (L per mol per cm). When path length is in centimetres and concentration in moles per litre, the product epsilon times c times l is dimensionless, as absorbance must be.
Why does the law break down at high concentration?
Beer-Lambert linearity assumes dilute, non-interacting absorbers. At high concentration, molecular interactions, refractive-index changes and stray light cause deviations, so absorbance no longer rises linearly with concentration. Most laboratories keep absorbance below about 1 for reliable quantification.
Official sources
- NIST: spectrophotometry measurement standards.
- IUPAC Gold Book: terminology for absorbance and the Beer-Lambert law.
Reviewed by the CalculatorHub team, edited by James Graham, 19 June 2026. See our methodology.