Therefore, a file’s “allocated” size will usually be larger than its actual size. And so on.Īs a result, any file that has a size which is not an exact multiple of the file system’s cluster size (and the vast majority aren’t) will “waste” a portion of its last cluster. Any file containing between 40 bytes will use two clusters. For example, if you are using NTFS with a 4K cluster size, any file containing between bytes of data will consume a single cluster.
Since all files are stored within one or more clusters, their “size on disk” (allocated size) is always a multiple of the file system’s cluster size. Most people today running the Microsoft Windows operating system are using NTFS, which has a default cluster size of 4K (4096 bytes). The size of a cluster can vary depending upon several factors, including what file system is used ( NTFS, FAT32, etc.) and partition size. In this blog entry, I will discuss what these metrics represent and how they differ.ĭisk space is allocated to files in units called clusters. Some software tools, including our FolderSizes disk space analyzer, are capable of reporting two size metrics for each file system object it encounters – “size” and “allocated size” (the latter is sometimes also called “size on disk”).