Photon energy: a photon is one quantum of light, and the energy it carries is set entirely by its frequency through E = h·f. This free calculator solves for photon energy or frequency — in J, eV, keV or MeV — shows the matching wavelength, and gives every step of the working.
Light is not perfectly continuous: it arrives in tiny packets called photons, and each photon carries a fixed amount of energy that depends only on its frequency. To calculate it, multiply the frequency by the Planck constant: E = h · f, where h = 6.62607015 × 10⁻³⁴ J·s. A higher frequency means a more energetic photon, which is why ultraviolet light damages skin while radio waves of the same intensity do not.
There are three steps. First, decide which quantity you want — energy or frequency — and select it in the calculator’s Solve for menu. Second, enter the value you already know and choose its unit; the calculator converts frequency (Hz, THz, PHz) and energy (J, eV, keV, MeV) to SI base units for you, so you never juggle powers of ten by hand. Third, read the answer with the worked steps, which show the formula, your numbers substituted in, and the final value — plus the matching wavelength in nanometres.
The equation rearranges in one move. If you know the energy and want the frequency, divide by the Planck constant: f = E ÷ h. Frequency and wavelength are themselves linked by the wave-speed relation c = f·λ, so the photon energy can equally be written E = h·c / λ. That form makes the inverse relationship clear — halving the wavelength doubles the photon energy. For an everyday refresher on how frequency and wavelength connect, see our guide to the frequency formula.
Because photon energies are minute in joules, physicists usually quote them in electronvolts (1 eV = 1.602176634 × 10⁻¹⁹ J). Visible light sits at a few eV per photon, X-rays at thousands (keV), and gamma rays at millions (MeV). This calculator lets you switch units instantly so the number is always in a convenient range.
Yellow-green light has a frequency of about f = 5 × 10¹⁴ Hz. Its photon energy is E = h·f = 6.62607015 × 10⁻³⁴ × 5 × 10¹⁴ ≈ 3.31 × 10⁻¹⁹ J. Converting to electronvolts gives 3.31 × 10⁻¹⁹ ÷ 1.602 × 10⁻¹⁹ ≈ 2.07 eV, and the matching wavelength is λ = c / f = 2.998 × 10⁸ ÷ 5 × 10¹⁴ ≈ 600 nm — squarely in the visible band. A blue photon (≈ 7 × 10¹⁴ Hz) carries about 2.9 eV, noticeably more for the shorter wavelength.
Photon energy underpins solar cells, LEDs and lasers, medical and astronomical imaging, the photoelectric effect, and the whole idea that energy at the atomic scale comes in discrete quanta — the foundation of quantum physics.
The energy of a single photon is E = h·f, where h is the Planck constant (6.62607015 × 10⁻³⁴ J·s) and f is the frequency. Because frequency and wavelength are linked by f = c/λ, the energy can also be written as E = hc/λ. The equation rearranges to f = E/h, so you can find frequency from a known energy.
Divide the energy in joules by the elementary charge, 1.602176634 × 10⁻¹⁹ C, since 1 eV = 1.602176634 × 10⁻¹⁹ J. For example, a 3.31 × 10⁻¹⁹ J photon equals about 2.07 eV. This calculator lets you read the answer directly in J, eV, keV or MeV.
Visible light spans roughly 400–700 nm, which corresponds to photon energies of about 1.8 eV (red) to 3.1 eV (violet), or around 3 × 10⁻¹⁹ to 5 × 10⁻¹⁹ J. Higher-frequency light — ultraviolet, X-rays and gamma rays — carries proportionally more energy per photon.
Energy is inversely proportional to wavelength: E = hc/λ. Shorter wavelengths mean higher frequency and higher energy, while longer wavelengths carry less energy. This calculator reports the wavelength in nanometres alongside the energy so you can see the relationship at a glance.
Experiments such as the photoelectric effect show that light delivers energy in discrete packets, or quanta, each carrying E = h·f. This quantisation, introduced by Planck and explained by Einstein, is a cornerstone of quantum physics and explains why only light above a threshold frequency can eject electrons from a metal.