Every free way to see what is filling a Windows drive, compared

There are more free ways to answer this on Windows than people realise, and the differences between them are real.

The options

Start with the two already installed

Settings, System, Storage finds temporary files, previous installations and large apps faster than anything else, because it knows what they are. It cannot show folders, which is where it stops.

Get-ChildItem C:\ -Force -Directory -EA SilentlyContinue | ForEach-Object {
  $s = (Get-ChildItem $_.FullName -Recurse -Force -File -EA SilentlyContinue | Measure-Object Length -Sum).Sum
  [PSCustomObject]@{ GB = [math]::Round($s/1GB,2); Folder = $_.Name }
} | Sort-Object GB -Descending | Select-Object -First 15

Slow, because it walks everything, and honest, which the built in tools are not: they only report categories they know how to remove.

When a graphical tool is worth it

• You want the answer in seconds. WizTree reads the file table rather than walking, which is a genuine difference rather than a marketing one.
• You want to drill in repeatedly without rerunning a command.
• You want to see proportion rather than read numbers.
• You do not know what the folder is. None of the pure measuring tools solve this.

The question none of them answer

Every tool above will tell you a folder is 6 GB. None will tell you it belongs to software you uninstalled two years ago, that it is a cache that will be back next week, or that deleting it breaks Windows' ability to uninstall anything. On Windows that last one is not hypothetical: it is the Installer folder.

That is why the guides here are organised by what a folder is rather than by how large it is, starting with the seven places Windows hides space.