Calculate optical band gap from UV–Vis data, in your browser.
Our free Tauc plot calculator turns a spectrophotometer export into a defensible optical band gap: upload TXT, CSV or absorbance data, pick the Tauc exponent, fit the linear region of the (αhν)ⁿ vs hν plot and extrapolate to the intercept — with R², slope and Eg reported instantly.
Everything the Tauc analysis needs — file parsing, unit conversion, regression and plot export — and nothing that gets between you and the result.
Parsing, conversion, regression and plotting all execute locally. Your spectra never touch a server, so there is no upload queue, no account and no waiting.
Direct allowed (n = 2), indirect allowed (n = 1/2), direct forbidden (n = 3/2) and indirect forbidden (n = 2/3) — each with its own selection rules and reference materials.
Delimiter, header row and column roles are detected automatically for TXT, DAT and CSV exports from Shimadzu, PerkinElmer, Cary, Jasco and Ocean Optics instruments.
Let the engine propose the steepest linear onset, or drag the fit window yourself. Every regression reports slope, intercept, R² and the extrapolated Eg = −b/m.
Work with an absorbance proxy, or enter film thickness to obtain the Beer–Lambert absorption coefficient in cm⁻¹ for quantitative comparison.
Download the fitted Tauc plot as a high-resolution PNG or a vector SVG, with the fitted line, extrapolation and intercept annotation preserved.
The four steps the calculator walks you through — the full derivation of each step is on the step-by-step guide.
Drop a TXT, DAT or CSV file, or start from one of the bundled experimental sample datasets.
Declare wavelength or energy axis, absorbance or transmittance, and optionally the sample thickness.
Pick the exponent n that matches the electronic transition of your material, then fit the linear onset.
Copy the band gap with its regression statistics, or export the fitted plot as PNG or SVG.
The value of n decides which transition you are probing — and therefore which band gap you extract.
| Electronic transition | Exponent (n) | Canonical materials |
|---|---|---|
| Direct allowed | 2 | ZnO, GaAs, CdTe, MAPbI₃ |
| Indirect allowed | 1/2 | Si, Ge, Anatase TiO₂, GaP |
| Direct forbidden | 3/2 | Cu₂O, SnO |
| Indirect forbidden | 2/3 | SnO₂, rare-earth oxides |
Educational pages covering the calculation, the equation, the transition models and worked examples — each linking straight back to the calculator.
A step-by-step walkthrough: energy conversion, exponent choice, linear fitting and intercept reading.
(αhν)ⁿ = B(hν − Eg) broken down term by term, with the axes, the exponent and the extrapolation.
Which exponent fits which material, and what changes in the plot when you switch transition models.
From absorbance or transmittance spectra to an optical band gap — conversions, pitfalls and checks.
Worked case studies with direct and indirect materials, showing good fits and the classic mistakes.
Dozens of answered questions: transmittance, %T, R², Urbach tails, thin films and file formats.
A Tauc plot is a method commonly used to estimate the optical band gap of a semiconductor from optical absorption data. It plots (αhν)ⁿ against photon energy hν, and the linear onset of the curve is extrapolated to the energy axis to give the band gap Eg.
Convert wavelength to photon energy, select the appropriate Tauc transition, plot (αhν)ⁿ against hν, fit the appropriate linear region, and extrapolate the fitted line to the energy axis. The intercept gives the estimated optical band gap.
The general Tauc relationship is (αhν)ⁿ = B(hν − Eg), where α is the absorption coefficient, hν is photon energy, Eg is the optical band gap, and n depends on the type of electronic transition.
Common values are: direct allowed n = 2, indirect allowed n = 1/2, direct forbidden n = 3/2, and indirect forbidden n = 2/3.
The exponent should be selected according to the electronic transition model appropriate for the material. It should not simply be chosen because it produces the highest R².
Yes. Absorbance data can be used for an absorbance-based Tauc analysis. If sample thickness and appropriate experimental assumptions are known, absorbance can also be converted to an absorption coefficient.
Yes. Transmittance can first be converted to absorbance using A = −log10(T), when T is expressed as a fraction rather than a percentage.
The x-axis is photon energy, hν, usually expressed in electron volts (eV).
The linear portion of the Tauc plot is fitted and extrapolated to the x-axis. The x-axis intercept corresponds to the estimated optical band gap, calculated from the fit as Eg = −b/m.
No. A high R² alone does not guarantee a physically meaningful Tauc fit. The fitting region, number of points, slope, transition model, and optical spectrum should also be considered.
Possible reasons include an inappropriate transition model, incorrect preprocessing, multiple optical transitions, scattering, baseline effects, insufficient data quality, or selection of a non-linear region.
Not necessarily. Tauc analysis estimates an optical transition energy under the assumptions of the selected model. The result should be interpreted together with the material's optical spectrum and other characterization.
No sign-up, no credits, no data leaving your machine. Open the workspace and use a bundled sample dataset if you want to test it first.