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Beer-Lambert Law Calculator

Solve the missing Beer-Lambert variable from absorbance, molar absorptivity, concentration, and path length, with explicit concentration and cuvette-path unit handling for real spectrophotometry work.

How It Works

Formula

A=ε⋅c⋅lA = \varepsilon \cdot c \cdot l

c=Aε⋅lc = \frac{A}{\varepsilon \cdot l}

l=Aε⋅cl = \frac{A}{\varepsilon \cdot c}

ε=Ac⋅l\varepsilon = \frac{A}{c \cdot l}

T=10−AT = 10^{-A}

%T=100⋅T\%T = 100 \cdot T

Variables, symbols and units

AA

Absorbance

ε\varepsilon

Molar absorptivity(L/mol/cm)

cc

Concentration(M, mM, uM)

ll

Path length(cm, mm)

TT

Transmittance fraction

%T\%T

Percent transmittance(%)
Calculation method explained

Choose the missing variable, enter the other three Beer-Lambert values, and keep the units explicit. The calculator solves one rearranged form of A = εcl at a time and then derives transmittance from the absorbance used in that case.

The math is normalized before solving: concentration is converted to molar units (M), path length is converted to centimeters, and molar absorptivity is treated as a user-supplied L/mol/cm value. After solving, the answer is returned in the concentration or path-length unit you selected. This page is intentionally narrow: it does not estimate extinction coefficients, build calibration curves, or plan dilutions.

Frequently Asked Questions

What does this equation assume?
Beer-Lambert only stays meaningful when absorbance, molar absorptivity, concentration, and path length all belong to the same assay setup. Match wavelength, solvent or buffer, blanking method, and path-length assumption before trusting the result.
Why does path length matter so much?
Absorbance is proportional to path length in this model. If concentration and molar absorptivity stay fixed, doubling path length doubles A and halving path length halves A.
What does transmittance mean here?
Transmittance is derived from absorbance with T = 10^-A and %T = 100 × T. Higher absorbance means less light transmitted through the sample.
When should I dilute or rerun instead of trusting one absorbance reading?
If the absorbance is very high for your method, the optical read may be less clean or outside the range your assay was calibrated around. Check your protocol or calibration, and use the Solution Dilution calculator if the next job is preparing a diluted rerun.
Does this calculator know compound presets or normal ranges?
No. It does not preload substances, extinction coefficients, calibration tables, or normal ranges. Result quality depends on the values you supply for your own assay.

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