A volatile memory device includes a refresh controller configured to control a hidden refresh operation performed on a first portion of memory cells while a valid operation is performed on a second portion of the memory cells. The volatile memory device is configured to perform a regular refresh operation in response to receiving a refresh command. The refresh controller is configured to generate refresh information using a performance indicator of the hidden refresh operation during a first part of a reference time. The volatile memory device is configured to perform a desired number of the regular refresh operation during a remaining part of the reference time based on the refresh information. The desired number of the regular refresh operation is an integer based on a difference between a target number of refresh operations during the reference time and a count value of the hidden refresh operation during the reference time.