In a method of counting the number of memory cells in a nonvolatile memory device, a measurement range and a plurality of measurement intervals of a measurement window for a cell counting operation are set to a first range and a plurality of first intervals, respectively. The plurality of measurement intervals are included in the measurement range. A first sensing operation is performed on first memory cells included in a first region of a memory cell array based on the measurement window. A first shifting operation for shifting the measurement window is performed while a width of the measurement range and a width of each of the plurality of measurement intervals are maintained. A second sensing operation is performed on the first memory cells based on the measurement window shifted by the first shifting operation. A final count value for the first memory cells is obtained based on a result of the first sensing operation and a result of the second sensing operation.