Black holes lose mass through radiation and eventually evaporate. This process is very slow. For example, a black hole with the mass of our sun would take approximately 10⁵⁷ times the current age of the universe to evaporate. As black holes evaporate, their horizons shrink and their entropy decreases. In keeping with the 2^nd law of thermodynamics, the overall entropy of the universe increases in the process since the radiation carries away a larger amount of entropy than a black hole loses.

Black holes also grow as material falls into their horizons. This process competes with the process of evaporation. As material falls in, the mass and entropy of a black hole increase while the outside entropy decreases. As a result, the horizon must grow. Since the temperatures of large black holes are so low, black holes generally grow overall under current conditions rather than shrinking.

The temperature of a black hole is inversely proportional to its mass. The temperatures of astrophysical black holes is generally very small. For example, a 3 solar mass black hole has a mass of around 6 × 10³⁰ kg, which gives it a temperature of approximately 2.1 × 10^-8 K. The supermassive black hole at the center of our galaxy has a mass of about 5 × 10³⁶ kg, which means that it's temperature is about 2.4 × 10^-14 K. However, a black hole with the mass of Mt Everest would have a horizon the size of an atomic nucleus and a temperature hotter than the temperature at the center of a star.