The invention provides a capacity acquiring method and system for solving the problem of causing a burden on a system due to overlarge IO (Input/Output) consumption in a capacity acquiring process. In order to solve the problem, the invention discloses a capacity acquiring method which comprises the following steps of: sending a request for calling disk metadata to a file system of a disk; receiving the returned disk metadata which contains the total amount of storage blocks and the idle amount of storage blocks in the disk; and computing the total amount of storage blocks and the idle amount of storage blocks in the disk to acquire the capacity information of the disk. Through the invention, the disk metadata is directly called, and the IO consumption can not be generated, so that the utilization ratio of the disk is always kept in a normal level and the burden on the system can not be caused.