How the Tauc Plot Calculator Works: Step-by-Step

Load your spectrum

Drop a file into the workspace or pick one of the bundled sample datasets.

The parser accepts TXT, DAT and CSV exports straight from the instrument software (Shimadzu, PerkinElmer, Cary, Jasco, Ocean Optics). Delimiters, header rows and column roles are detected automatically, and a preview table lets you confirm what was read before you continue.

Two-column data: axis value and intensity
Header rows, comments and blank lines are skipped
Use “Inspect preview” to verify the parsed table
Use “Change file” to swap datasets at any time

Declare the input mode

Tell the engine what the columns mean and how the sample was measured.

Choose whether the horizontal axis is wavelength (nm), photon energy (eV) or 1/λ — conversions are handled for you. Then declare whether the vertical column is absorbance, transmittance (%), or already an absorption coefficient.

Wavelength is converted to energy with E = hc / λ
Transmittance % is converted to absorbance with A = −log₁₀(T/100)
Enter film thickness (cm) to obtain α = 2.302585 · A / d
Leave thickness empty to work with the absorbance proxy

Select the transition type

The exponent n encodes which electronic transition you are probing.

Direct allowed transitions use n = 2, indirect allowed n = 1/2, direct forbidden n = 3/2 and indirect forbidden n = 2/3. Choosing the wrong exponent changes the shape of the curve and therefore the extracted Eg — check the material examples on the methodology page if you are unsure.

Direct allowed (n = 2): ZnO, GaAs, CdTe, MAPbI₃
Indirect allowed (n = 1/2): Si, Ge, anatase TiO₂, GaP
Direct forbidden (n = 3/2): Cu₂O, SnO
Indirect forbidden (n = 2/3): SnO₂, rare-earth oxides

Fit the linear region

Extrapolate the steepest linear onset of the Tauc curve to y = 0.

The engine proposes an initial fit window automatically. Adjust the handles so the window covers only the straight onset — exclude the Urbach tail below the gap and the detector-saturation plateau above it. The regression updates live with slope, intercept and R².

Fit window is drag-adjustable on the plot
Slope m, intercept b and R² are reported for every fit
Eg is the x-intercept: Eg = −b / m
Re-fit as many times as needed; nothing is destructive

Read the result

The band gap appears in electronvolts alongside its regression statistics.

A good fit shows a visibly straight segment with R² close to 1 and an intercept inside the measured energy range. If the extrapolation lands far outside the data, the fit window is almost certainly in the wrong region — move it and refit.

Eg is reported in eV with tabular figures
R² and fit parameters are shown for the current window
Results update instantly on every adjustment
No data leaves your browser at any point

Export and report

Download the fitted plot or copy the numbers into your manuscript.

Export the Tauc plot as a high-resolution PNG for slides or a vector SVG for publication figures. The export includes the data curve, the fitted line, the dashed extrapolation to the intercept and the annotated band gap.

PNG for documents and presentations
SVG for vector editing and print
Axes, fit line and Eg annotation preserved
Sample datasets included for reproducing the demo

How the Tauc Plot Calculator works