What is taking up space on your C: drive?

Scan the whole drive with a tool that draws a treemap, start at the biggest rectangle, and drill down until you reach a folder you recognise. Run it as administrator so the folders Windows hides from you are counted.

Why the numbers never add up

You open C: in File Explorer, select everything, and the sizes come to far less than the used space. The gap is real, and it is made of things Explorer does not count or cannot open:

• Files you have no permission to read. System Volume Information (restore points) and parts of C:\Windows are skipped unless you run as administrator.
• Hidden system files. hiberfil.sys, pagefile.sys and swapfile.sys sit in the root of the drive and are hidden by default. Together they are often tens of gigabytes.
• Hard links. C:\Windows\WinSxS shows files that also appear elsewhere. Summing them double counts, and that is why the folder looks bigger than it is.
• Virtual disks. A single ext4.vhdx from WSL or docker_data.vhdx from Docker can be 50 GB or more, and it looks like one innocent file.

Step by step

1. Scan the drive as administrator. In Bytopsy, pick the drive. Windows asks for permission, or you can restart once as administrator. This counts the folders that normal scans skip.
2. Read the treemap. Every file is a rectangle sized by its bytes and coloured by kind (video, images, archives, code, programs, system). One large block of a single colour is your answer more often than you would expect.
3. Sort the folder list by size. Expand the largest folder, then its largest child. Three or four levels is usually enough to reach something you can name.
4. Check Largest files and File types. One shows single files, the other shows which kind of file adds up to the most, such as 200 GB of .mp4 spread over many folders.
5. Filter by age. Search for older:1y combined with a size such as >500M to find what you have not opened for a year.

Where the space usually hides

• C:\Users\<you>\AppData\Local: browser, app, game and developer caches.
• C:\Users\<you>\Downloads and your video folders.
• C:\ProgramData: shared app data, including installers and launcher libraries.
• C:\Windows\WinSxS, C:\Windows\Installer and C:\Windows\SoftwareDistribution: Windows itself, to be reduced only with Windows' own tools.
• Windows.old: the previous Windows installation, removed through Temporary files in Settings, never by hand.
• WSL, Docker and virtual machine disks, usually under AppData or a folder you chose.

Treemap or folder list?

Use both. The treemap shows shape: one huge block is obvious at a glance. The list shows names and exact figures. In Bytopsy a click in the treemap selects the file in the list, so you move between the two without losing your place. The same scanner is available in the terminal as bytopsy-cli, with an ncdu-style interface, for SSH sessions and scripts.

What to do next

Once you know what is big, free the space in a safe order. If the culprit is a Linux disk, see how to shrink WSL and Docker disks. Download Bytopsy to try it. It is free, open source and portable, and nothing is deleted unless you choose to.