A = ε × l × c — fill in every field except the one you want; the empty field is solved for.
Molar absorptivity examples:
L mol⁻¹ cm⁻¹
Concentration (c) = 67.524 µM
- Rearrange:
c = A ÷ (ε × l) - Substitute:
(0.42) ÷ ((6,220 L mol⁻¹ cm⁻¹) × (1 cm))=6.7524 × 10⁻⁵ M
Absorbance ↔ transmittance (A = −log₁₀ T)
A = 0.301
How it works
The calculator works in centimetres and mol/L, the units in which molar absorptivity (ε) is normally given, and converts your inputs to match. Absorbance is dimensionless; ε is in L mol⁻¹ cm⁻¹.
The Beer–Lambert law holds best for dilute solutions (absorbance roughly 0.1–1), monochromatic light and no chemical changes on dilution. At high concentration, stray light and interactions between molecules make the calibration curve bend.
Frequently asked questions
- What does the Beer–Lambert law say?
- Absorbance is proportional to both concentration and path length: A = εlc. Doubling the concentration or the cuvette width doubles the absorbance.
- How do absorbance and transmittance relate?
- A = −log₁₀(T), where T is the fraction of light transmitted. 10 %T is an absorbance of 1; 1 %T is an absorbance of 2.
- What is molar absorptivity?
- It measures how strongly a substance absorbs light at a given wavelength. NADH, for example, has ε ≈ 6220 L mol⁻¹ cm⁻¹ at 340 nm — the basis of many enzyme assays.
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