Convert absorbance into concentration for precise lab analysis. Choose ε units and path length easily. Export results, compare inputs, and reduce calculation errors today.
Beer–Lambert law relates absorbance to concentration:
To solve for concentration:
The Beer–Lambert law links how strongly a sample absorbs light to how much absorbing species is present. In routine UV–Vis work, absorbance is treated as dimensionless optical density. This calculator solves concentration using measured absorbance, molar absorptivity, and optical path length so you can move from instrument output to actionable chemistry.
Molar absorptivity (ε) describes how efficiently a species absorbs at a specific wavelength and chemical form. Values can range broadly, often from about 102 to 105 L·mol⁻¹·cm⁻¹ depending on the transition and environment. Always use ε that matches your wavelength, solvent, temperature, and analyte state to avoid systematic bias.
Path length (l) is the distance light travels through the sample. A standard cuvette is commonly 1.0 cm, while microvolume cells may be 1–2 mm. Because concentration scales as 1/l, using 0.1 cm instead of 1.0 cm increases the computed concentration tenfold for the same absorbance. This tool converts mm, cm, and m to match your ε basis.
Many instruments display transmittance rather than absorbance. The relationships are A = −log10(T) and A = log10(I₀/I), where T = I/I₀. Useful reference points: A = 0.300 corresponds to about 50.1% transmittance, A = 1.000 corresponds to 10% transmittance, and A = 2.000 corresponds to 1% transmittance.
For reliable results, zero the instrument with a blank, measure the sample at the chosen wavelength, then record A (or T/%T). Enter ε and the path length used by the cell. The calculator returns molar concentration and derived transmittance values for quick sanity checks. If you provide molar mass, it also reports mass concentration in g/L and g/m³.
Beer–Lambert behavior is most linear at moderate absorbance. Many labs target A ≈ 0.1–1.2 for quantitation, because very low A approaches noise and very high A amplifies stray-light errors. If A is too high, dilute the sample and remeasure. If A is too low, increase path length, increase concentration, or choose a stronger band.
Deviations often come from scattering (turbidity), fluorescence, chemical equilibria, and instrument stray light. A quick diagnostic is consistency: if you enter both A and %T (or I₀ and I), the calculator compares them and flags noticeable disagreement. Also confirm units: ε per cm must pair with l in cm, while ε per m must pair with l in meters.
Concentration is commonly reported as mol/L, while some engineering contexts use mol/m³ (1 mol/L = 1000 mol/m³). Document the wavelength, ε source, path length, blank composition, and any dilution factors. When mass concentration is needed, multiply molar concentration by molar mass (g/mol) to obtain g/L, then scale to g/m³ if required.
Transmittance is the fraction of light that passes through a sample (T = I/I₀). Absorbance is a logarithmic measure of attenuation: A = −log10(T). Absorbance increases linearly with concentration under ideal conditions.
Use L·mol⁻¹·cm⁻¹ if your ε reference is tabulated per centimeter, which is common in lab spectroscopy. Use L·mol⁻¹·m⁻¹ if your ε is defined per meter. Match ε and path-length units.
High absorbance can suffer from stray light and nonlinearity. Many methods recommend staying roughly below A ≈ 1–1.5 for best quantitation. Consider dilution, a shorter path length, or a different wavelength.
Not directly. Beer–Lambert concentration requires ε and path length. If ε is unknown, determine it using standards of known concentration to build a calibration curve, then infer ε or use the curve for future samples.
Absorption depends on electronic and vibrational transitions that are wavelength-specific. ε can also shift with solvent polarity, pH, temperature, and complex formation. Always use ε measured under conditions that match your experiment.
Measure the diluted sample and compute concentration using this calculator. Then multiply the result by the dilution factor to recover the original concentration. Example: a 10× dilution means the original concentration is 10 times higher.
Mass concentration converts molar concentration into grams per volume. The calculator multiplies mol/L by molar mass (g/mol) to give g/L, and also provides g/m³ for convenience. This is useful for formulations and process reporting.
Example values for a 1 cm cuvette and a strong absorber.
| Input Type | Given | Value | Computed |
|---|---|---|---|
| Absorbance | A | 0.750 | c = 0.00000500 mol/L (5.00×10⁻⁶) |
| Molar absorptivity | ε | 15000 L·mol⁻¹·cm⁻¹ | |
| Path length | l | 1.0 cm | |
| Derived transmittance | %T | 17.8% | A = −log10(0.178) ≈ 0.750 |
Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.