A memory system includes a memory device and a controller. The memory device includes a plurality of memory blocks, wherein the plurality of memory blocks includes one or more first memory blocks, each storing at least invalid data and one or more second memory blocks, each of which is blank. The controller is configured to determine a time or a period for performing garbage collection to secure an additional second memory block based at least on a transition speed representing a speed in which the second memory blocks is converted to the first memory blocks, the transition speed being determined based on a change between a first count of the first memory blocks and a second count of the second memory blocks.