Photon energy is the energy carried by a single photon. The amount of energy is directly proportional to the photon's electromagnetic frequency and thus, equivalently, is inversely proportional to the wavelength. The higher the photon's frequency, the higher its energy. Equivalently, the longer the photon's wavelength, the lower its energy.

Photon energy can be expressed using any energy unit. Among the units commonly used to denote photon energy are the electronvolt (eV) and the joule (as well as its multiples, such as the microjoule). As one joule equals 6.24×10¹⁸ eV, the larger units may be more useful in denoting the energy of photons with higher frequency and higher energy, such as gamma rays, as opposed to lower energy photons as in the optical and radio frequency regions of the electromagnetic spectrum.

Photon energy is directly proportional to frequency. $${\displaystyle E=hf}$$ where

This equation is known as the Planck relation.

Additionally, using equation f = c/λ, $${\displaystyle E={\frac {hc}{\lambda }}}$$ where

The photon energy at 1 Hz is equal to 6.62607015×10⁻³⁴ J, which is equal to 4.135667697×10⁻¹⁵ eV.

Photon energy is often measured in electronvolts. One electronvolt (eV) is exactly 1.602176634×10⁻¹⁹ J‍ or, using the atto prefix, 0.1602176634 aJ, in the SI system. To find the photon energy in electronvolt using the wavelength in micrometres, the equation is approximately

since ${\displaystyle hc/e}$ = 1.239841984...×10⁻⁶ eV⋅m where h is the Planck constant, c is the speed of light, and e is the elementary charge.