How is antenna percent efficiency calculated?

Efficiency is always a measure of useful output divided by total input. For an antenna, the useful output is represented by radiation resistance (power turned into radio waves), and the total input is represented by total resistance (radiation resistance plus ohmic losses). To find the percent efficiency, you divide the radiation resistance by the total resistance, and then multiply the result by 100. For example, if radiation resistance is 40 ohms and total resistance is 50 ohms, the ratio is 0.8, which equals 80% efficiency. Inverting the formula to put total resistance on top would result in an efficiency greater than 100%, which violates the laws of physics. Using Effective Radiated Power (ERP) is also incorrect, as ERP includes transmitter power and antenna gain rather than purely the antenna's electrical efficiency.