The Carnot efficiency relation is η = 1 − T_C/T_H, where T_H is the hot-reservoir temperature and T_C is the cold-reservoir temperature. Temperatures must be measured on an absolute scale such as kelvins.
The formula describes the maximum efficiency of a reversible heat engine operating between two reservoirs. It follows from the Carnot cycle, whose reversible heat transfers satisfy Q_C/Q_H = T_C/T_H. Since efficiency is η = 1 − Q_C/Q_H, substitution gives the stated expression. A larger temperature gap produces a higher possible efficiency, while equal reservoir temperatures give zero efficiency.
The relation is a limit, not a promise that every engine reaches it. Real engines are irreversible because of friction, finite temperature differences, turbulence, and heat loss. The expressions η = T_C/T_H and η = T_H/T_C − 1 are not the Carnot efficiency; they omit the subtraction structure or reverse the physical roles of the reservoirs.