Provided is a memory device configured to perform a calibration operation without having a ZQ pin. The memory device includes a calibration circuit configured to generate a pull-up calibration code and a pull-down calibration code which termination of a data input/output pad for impedance matching in the data input/output pad is controlled. The calibration circuit performs a first calibration operation for trimming first and second reference resistors based on an external resistor to be connected to a pad, and a second calibration operation for generating the pull-up calibration code and the pull-down calibration code based on the trimmed second reference resistor.