UV Vis Spectroscopy Band gap Calculator

UV Vis Spectroscopy Band Gap Calculator

Absorption Peak Wavelength (nm)



Calculation Tutorial: STEP1: Open the absorption graph of the material, which is obtained from the UV Vis spectroscopy. Raw HTML W a v e l e n g t h I n t e n s i t y 200 220 240 260 280 300 0.0 0.2 0.4 0.6 0.8 1.0 STEP2: Now zoom on the area for which you want to calculate the band gap and note down the wavelength (λ) at which absorption peak is shown. Raw HTML W a v e l e n g t h I n t e n s i t y 200 220 240 260 280 300 0.0 0.2 0.4 0.6 0.8 1.0 220 STEP3: Now enter the measured wavelength value (eg. 31.8) into the " Wavelength of Absorption peak (in nanometer)" column of the calculator. You should get the calculated results of the band gap in the "Calculated Result" field.

Theory Behind Calculations: UV Vis Spectroscopy absorption peak means the Electrons are absorbing the Energy at some specific wavelength. Electrons are absorbing Energy means the Electrons are going to excited state from its ground state. Electrons are going to excited state from its ground state means the material is having band gap, thus which can be determine by absorption wavelength.

Energy Equation of Quantum Mechanics:

Energy (E) = Planks Constant (h) * Speed of Light (C) / Wavelength (λ)

Where, Energy (E) = Band gap, Planks constant (h) = 6.626×10^-34 Joules sec, Velocity of Light (C) = 2.99×10^8 meter/sec and Wavelength (λ) = Absorption peak value. Also 1eV = 1.6×10^-19 Joules (Conversion factor)

By this formula band gap can be calculated easily, from UV Vis spectroscopy absorption peak.


  1. This is the useful and usual formula used to calculate the energy gap from the absorption spectra in the visible and ultraviolet regions and infrared region as well. This calculator which is given here will help the researcher to find directly the energy gap Eg.

  2. Kumaar Swamy Reddy B26 April 2017 at 11:46

    What if my sample has two absorption peaks or no peak at all? How do I calculate my bandgap?

  3. Vinil Namdev Jambhale5 October 2017 at 22:31

    How to calculate urbach energy from band gap.

  4. I can't understand how to calculate FWHM using xrd. Can u explain briefly using xrd graph without using any software?

  5. Akash the FWHM is very easy by using origion software for your XRD spectra.when you draw your xrd data in origion then make one peak zoom go to analysis then peak and base line then multiple peak fitting then open dialog change the gauss to gaussian then click ok a side window will open then open fitting a full history of the peak will appear then you can find the FWHM value of your peak in paprameters.

  6. Why the absorbance decreases at shorter wavelength (200-217nm)?

  7. Hi,
    1-open your XRD pattern by X'Pert High Score,
    2-push right click bottom on the pattern and then select search peaks.
    3-again push right click bottom on the pattern and choose the label peaks,
    4- active FWHM option,
    5- now you can see the FWHM value for each peak in your pattern.
    note that; in this method the FWHM unit shown in degree. if you want to calculate crystallite size or lattice strain, you should convert it to Radian.

  8. Mr. Sumit Ramdas Patil20 December 2018 at 17:35

    Actually I want to know about the reflectance mode bandgap calculation ?
    Please specify ?

  9. How to calculate band gap for solid sample? Plz answer

  10. Great explanation of uv vis spectroscopy. It would be Helpful for many students

  11. What is the formula for calculating bandgap from PL Spectra