A calibration circuit includes a first, second and third pull-up units each connected to a first power supply node, and first and second pull-down units each connected to a second power supply node. A first code generator is configured to generate a first code by comparing a voltage of a pad at which the first pull-up unit is connected to an external resistor with a reference voltage, and a second code generator is configured to generate a second code by comparing a voltage of a first intermediate node with the reference voltage and output the second code to the first and second pull-down units. A third code generator is configured to generate a third code by comparing a voltage of a second intermediate node between the second pull-down unit and the third pull-up unit with the reference voltage.