numvana

Beer-Lambert Law Absorbance Calculator

Solve for absorbance, concentration, or path length via the Beer-Lambert law A = ε × c × l, and see the corresponding percent transmittance.

Absorbance (A)
1
Concentration
0.001 mol/L
Path length
1 cm
Transmittance
10%

How it works

The Beer-Lambert law relates how much light a solution absorbs to what's dissolved in it: A = ε × c × l, where ε is the solute's molar absorptivity at a given wavelength (a property of the specific substance and wavelength, which you supply — the same way this site's molarity calculator takes molar mass as an input rather than looking it up), c is the molar concentration, and l is the path length the light travels through the sample. Given ε and any two of {absorbance, concentration, path length}, this calculator rearranges that identity to solve for whichever one you leave as the target.

Percent transmittance — the fraction of light that makes it through the sample — follows from absorbance's own logarithmic definition, A = −log₁₀(T): T% = 10^(−A) × 100. An absorbance of 0 means 100% transmittance (nothing absorbed); an absorbance of 1 means 10% transmittance, since each whole unit of absorbance corresponds to another factor of 10 in light blocked.

The default values (ε = 1000 L·mol⁻¹·cm⁻¹, c = 0.001 mol/L, l = 1 cm) give a clean absorbance of exactly 1.0 and transmittance of exactly 10%.

FAQ

Where do I find a substance's molar absorptivity?

It's a published constant specific to the compound and the wavelength of light used — found in a reference table, a lab manual, or the instrument's own documentation for that assay. This calculator takes it as a direct input rather than looking it up internally, since it varies by wavelength and would otherwise be a hardcoded fact that could go stale or apply to the wrong compound.

Why might my measured absorbance not match this calculator's prediction?

The Beer-Lambert law assumes a dilute solution, monochromatic light, and no scattering or chemical interactions that change the light-absorbing species' concentration. At high concentrations (roughly above 0.01 M for most compounds), real solutions commonly deviate from this idealized straight-line relationship.

Related calculators