Estimate total directory size from file counts and average file sizes.
The `du` command reports space actually allocated on disk, which is always ≥ the sum of file byte sizes because every file consumes whole filesystem blocks (block-rounding / 'slack space'), and every directory entry and inode carries its own metadata. This calculator estimates total = allocated_data + directory_metadata + inode_metadata, so you can sanity-check `du -sh` output against a rough file-count × average-size model before running it on a live filesystem.
Total estimated size
total ≈ file_count × ceil(avg_size / block_size) × block_size + subdirs × block_size + file_count × 256
ls -l shows exact byte sizes; du reports blocks actually allocated on disk. A 100-byte file on a filesystem with 4096-byte blocks still consumes one full 4096-byte block, so many small files inflate du's total well above the sum of their logical sizes.
Block rounding (slack space) is worst for files much smaller than the block size — a 1KB file on 4K blocks wastes 3KB. Millions of small files also multiply inode and directory-entry metadata overhead, which is why archiving small files into a tar or using a filesystem with tail-packing (like Btrfs or ReiserFS) reduces total usage.
No — this gives a planning estimate before you have access to the filesystem (e.g. sizing a migration or a new volume). For an authoritative number on an existing directory, always run du -sh or du --apparent-size for the raw byte total.