💡 Beer, Lambert Law Calculator
Relate absorbance to concentration with A = εlc, enter any three of absorbance, molar absorptivity, path length and concentration to get the fourth.
Concentration (c)
5.0000e-5 M
Beer, Lambert law: A = ε·l·c (absorbance = molar absorptivity × path length × concentration). 🔒 Computed in your browser.
How the beer, lambert law calculator works
The Beer, Lambert law is A = ε·l·c, where A is absorbance, ε the molar absorptivity (M⁻¹cm⁻¹), l the path length (cm), and c the concentration (M). Enter any three and the tool solves for the fourth, most often concentration from a measured absorbance. Rearranged, c = A/(εl), ε = A/(lc) and l = A/(εc); absorbance is unitless, so with ε in M⁻¹cm⁻¹ and l in cm the concentration comes out in mol/L.
The workhorse of UV-Vis spectroscopy: with a known ε and a 1 cm cuvette, absorbance is directly proportional to concentration. Linearity holds at low-to-moderate absorbance (roughly A < 1).
Frequently asked questions
What is the Beer, Lambert law?
A = εlc: absorbance equals molar absorptivity times path length times concentration. It says absorbance is proportional to how much light-absorbing substance the beam passes through.
How do I find concentration from absorbance?
Rearrange to c = A / (ε·l). With ε and the cuvette path length (usually 1 cm) known, divide your measured absorbance to get concentration. The tool does this when you solve for c.
What is molar absorptivity (ε)?
A substance-and-wavelength-specific constant (units M⁻¹cm⁻¹) describing how strongly it absorbs light. Higher ε means more absorbance at the same concentration.
What path length should I use?
The width of the cuvette the light passes through, standard cuvettes are 1 cm.
When does the Beer, Lambert law break down?
At high concentration or absorbance (roughly A > 1), where the linear relationship fails due to interactions and stray light. Dilute the sample to stay in the linear range.
How do I find concentration from a calibration curve?
Plot absorbance against known concentrations; the slope equals ε·l. Divide any measured absorbance by that slope to read off concentration, the graphical form of c = A/(εl).
What does an absorbance of 1 mean?
Absorbance is logarithmic: A = 1 means 10% of the light is transmitted (90% absorbed), and A = 2 means 1% transmitted. Because it is a log scale, doubling absorbance is not the same as doubling the light lost.
How is absorbance related to transmittance?
A = −log₁₀(T), where T is the fraction of light transmitted. So 50% transmittance (T = 0.5) is an absorbance of about 0.30, and 10% transmittance is exactly A = 1.
Why keep the path length at 1 cm?
It is the standard cuvette width, so tabulated ε values (M⁻¹cm⁻¹) apply directly and A equals ε·c numerically. If you use a different path length, scale by it, a 2 cm cell doubles the absorbance at the same concentration.