The formula for black hole entropy associated with Hawking is S = A/4. In geometrized units, S is the black hole’s entropy and A is the area of its event horizon.
Jacob Bekenstein first proposed that black hole entropy should be proportional to horizon area rather than volume. In 1974, Stephen Hawking’s calculations showing that black holes emit thermal radiation confirmed the idea and fixed the proportionality constant at one-quarter.
The fully expressed formula is S = k_B A/(4ℓ_P²), where k_B is the Boltzmann constant and ℓ_P is the Planck length. The shorter S = A/4 form assumes natural or geometrized units, which is why both versions appear in physics references.
A common mix-up is confusing this entropy formula with Hawking temperature, T = ħc³/(8πGMk_B), which describes how the temperature depends on a black hole’s mass. The entropy formula instead emphasizes the remarkable fact that information capacity scales with horizon area.