The Absorbance Calculator converts between absorbance, transmittance, and percent transmittance using the Beer-Lambert law. Calculate optical density from intensity measurements or transmittance readings for spectrophotometry, colorimetry, and analytical chemistry applications.
1
AU
10
%
0.1
0.00011905
mol/L
23,094.6882
L/mol·cm
2.749368
cm
1
1
AU
10
%
0.1
0.00011905
mol/L
23,094.6882
L/mol·cm
2.749368
cm
1
The calculator for absorbance converts between absorbance (optical density), transmittance, and percent transmittance — the three interrelated quantities at the heart of every spectrophotometric measurement. Whether you are reading a UV-Vis spectrum, performing a Bradford protein assay, or calibrating a colorimetric method, this tool provides instant conversion between all measurement formats.
Absorbance is defined through the Beer-Lambert law, which relates the intensity of transmitted light to sample concentration and path length:
A = ε × c × l = −log₁₀(T) = log₁₀(I₀/I)
where ε is the molar absorptivity (L·mol⁻¹·cm⁻¹), c is the molar concentration (mol/L), l is the path length (cm), T is transmittance (I/I₀), and I₀ and I are the incident and transmitted light intensities. The logarithmic relationship means absorbance is additive — two absorbing species contribute independently to total absorbance at a given wavelength. The Beer-Lambert law calculator extends this to concentration determination from absorbance measurements.
The three measurement formats are mathematically equivalent:
An absorbance of 1 corresponds to 10% transmittance — 90% of the light is absorbed. An absorbance of 2 corresponds to 1% transmittance. Most spectrophotometers reliably measure absorbance between 0.1 and 1.5; readings above 2 approach the instrument's detection limit. Use this online calculator to convert any measured value to the format required by your analysis. The molar absorptivity calculator computes ε from known concentration and absorbance data.
The logarithmic relationship between absorbance and transmittance is not arbitrary — it arises directly from the physics of light absorption. Each successive layer of absorbing molecules removes the same fraction of remaining light, not the same absolute amount. This multiplicative attenuation produces exponential decay of intensity with path length or concentration, and taking the logarithm converts this exponential relationship into the linear Beer-Lambert form that makes concentration determination straightforward. Absorbance is therefore directly proportional to concentration at a fixed path length — a crucial property for quantitative analysis.
Accurate absorbance measurements require careful experimental technique. The blank correction (measuring the solvent and cuvette without analyte) removes background absorbance from the optical path, including cuvette wall absorption and solvent scattering. Cuvette material matters: standard glass cuvettes are suitable for visible wavelengths (340–900 nm); quartz cuvettes are required for UV measurements (200–340 nm). The linear range of Beer-Lambert behavior typically extends to absorbance values of 1.0–1.5 — above this, stray light and detector saturation cause negative deviations from linearity. The spectroscopy calculators category includes wavelength, wavenumber, and energy conversion tools for complete spectroscopic analysis.
Absorbance is defined as the negative base-10 logarithm of the transmittance:
$$A = -\log_{10}(T) = -\log_{10}\left(\frac{I}{I_0}\right)$$
where I₀ is the incident light intensity (before the sample), I is the transmitted light intensity (after the sample), and T = I/I₀ is the transmittance (fractional). When transmittance is given as a percentage:
$$A = -\log_{10}\left(\frac{T\%}{100}\right) = 2 - \log_{10}(T\%)$$
The percentage of light absorbed by the sample is simply:
$$\% \text{Absorbed} = (1 - T) \times 100 = \left(1 - \frac{I}{I_0}\right) \times 100$$
Note that absorbance and percent absorbed are not linearly related — a sample absorbing 90% of light has A = 1, while one absorbing 99% has A = 2, and one absorbing 99.9% has A = 3. This logarithmic relationship is what makes absorbance (rather than percent absorption) proportional to concentration in Beer's law.
Absorbance values provide a direct measure of sample opacity at the measurement wavelength. Values between 0.1 and 1.0 AU are considered ideal for quantitative work. An absorbance of 0 means no absorption (100% transmittance), while an absorbance of 3 means only 0.1% of light is transmitted. The logarithmic scale means each unit of absorbance represents a tenfold decrease in transmitted light. The percent absorbed output gives a more intuitive sense of how much light the sample removes, while the absorbance value is the quantity used in Beer's law calculations for concentration determination.
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When a sample transmits only 500 out of 5000 intensity counts, the transmittance is 10% and the absorbance is 1.0 AU. The sample absorbs 90% of the incident light at this wavelength.
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A sample with 25% transmittance has an absorbance of 0.602 AU, within the optimal measurement range. Three-quarters of the incident light is absorbed.
Absorbance (A) is the quantitative measurement defined as A = -log₁₀(I/I₀), a dimensionless number. Absorption is the physical process by which matter takes up photon energy. A sample 'absorbs' light; the 'absorbance' is the numerical value we measure. The unit AU (absorbance unit) is sometimes used for clarity.
In principle, absorbance should not be negative (it would mean more light exits than enters). However, negative readings can occur due to instrument baseline errors, fluorescence, scattering differences between reference and sample, or when the sample has lower absorption than the reference blank.
An absorbance of 2 means only 1% (10⁻² = 0.01) of the incident light passes through the sample. In other words, 99% of the light is absorbed. This is near the upper limit of reliable measurement for most spectrophotometers.
The logarithmic scale makes absorbance directly proportional to concentration (Beer's law: A = εlc). If we used percent absorption instead, the relationship with concentration would be exponential, making calculations and graph interpretation much more complex.
Most benchtop UV-Vis spectrophotometers can reliably measure up to A = 3 or 4 (0.1% or 0.01% transmittance). Beyond this, stray light and detector limitations cause significant errors. High-performance instruments may extend to A = 5 or 6 with stray light correction.
By using a reference (blank) cuvette containing only the solvent in the reference beam. The spectrophotometer measures the difference in absorbance between the sample and reference, automatically subtracting the cuvette and solvent contributions.
For solutions that only absorb (no scattering), absorbance and optical density are identical. For turbid samples, optical density includes both absorption and scattering contributions: OD = A + S, where S represents scattering losses. In microbiology, OD₆₀₀ for cell cultures is primarily scattering.
Yes, absorbances are additive. If multiple species absorb at the same wavelength, the total absorbance is the sum: A_total = A₁ + A₂ + A₃ + ... This additivity property is the basis for multicomponent spectroscopic analysis.
A blank contains everything except the analyte of interest (solvent, cuvette, reagents). Measuring the blank sets the baseline (A = 0) by accounting for all non-analyte absorption. Without proper blanking, the measured absorbance would include contributions from the solvent and container.
Use Beer's law: c = A/(εl), where ε is the molar absorptivity and l is the path length. If ε is unknown, prepare calibration standards, plot absorbance vs. concentration, and use the linear regression equation to calculate the unknown concentration from its absorbance.
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