Expressions are derived for error due to mismatch when a UHF or microwave power meter is calibrated by comparison with a standard power meter. Three different methods are considered: (a) alternate connection to a stable power source, (b) the use of a microwave junction which simultaneously supplies power to the uncalibrated power meter and the standard power meter in a known ratio, (c) alternate connection to a microwave junction. The relative merits of the methods are discussed. Expressions are derived for error due to mismatch when using a calibrated power meter in the following situations: (a) direct connection of power meter to power source, (b) reduction of power into the power meter by means of an attenuator, (c) reduction of power into the power meter by means of a directional coupler.