How to find disk usage of folders in Windows [duplicate]

WinDirStat is a port of KDirStat for Linux. It's free, lightweight, small (650kb installer), fast, portable (as a standalone .exe file), and works on multiple versions of Windows. Besides showing folders and percentages (for the entire disk or any subset of folders), it also displays an (optional) graphical usage map. Works well with NTFS Junction folders, avoiding counting folders multiple times.

SpaceSniffer is another possibility. It can scan Alternate Data Streams (ADS) and correctly ignores junctions. However, it is not hard-link aware. If a file has multiple links, they will show up in the scan more than once. I've personally tested all this information to be accurate on Windows XP as of version 1.1.2.0.

TreeSize is pretty sweet.

Its advantage over the others is that on NTFS drives, it works on the MFT (Master File Table) and reaches extremely high scan speeds.

They have three versions of the product: Free, Personal and Professional.

The free version doesn't have fancy visualizations and reports but should be sufficient in most cases.

JDiskReport (also available for Mac OS X and Linux).