Windows can compress folders and itself, and on some drives it should
Two different compression features, when each is worth using, and the case where it makes things slower.
Windows has had transparent file compression for decades and it is genuinely useful in specific places. It also has a newer feature, Compact OS, which compresses Windows itself and is why system binaries report a size on disk smaller than their length.
The two features
| NTFS compression | Compact OS | |
|---|---|---|
| What it compresses | Folders you choose | Windows system files |
| Turned on with | Folder Properties, Advanced | compact /compactos:always |
| Typical saving | Depends entirely on content | 2 to 4 GB |
| Cost | CPU on every read and write | Minimal on modern hardware |
What compresses and what does not
- Text, logs, source code, databases. Genuinely large savings, often half or better.
- Documents and spreadsheets. Already zipped internally, almost nothing.
- Photos, video, audio. Already compressed. Nothing, and you pay CPU for it.
- Games and program binaries. Modest, and on files read constantly the CPU cost can matter.
Using it well
# Compress one folder and everything under it
compact /c /s /i "C:\Archive"
# Check what Compact OS is doing
compact /compactos:queryThe sensible target is a folder of text or logs you keep and rarely read: archives, old project folders, exported data. Compressing a folder of video is pure loss.
The mistake to avoid
The checkbox in a drive's Properties labelled Compress this drive to save disk space applies to everything, including files that will not compress and files Windows reads constantly. On a modern machine it rarely breaks anything and it is a poor trade compared with compressing one folder deliberately.
Compact OS
On a machine with a small drive, Compact OS is worth knowing about and is usually already on where the manufacturer enabled it. It is why size on disk is smaller than size for system binaries, and why a disk tool that reads allocated size reports Windows as smaller than expected.
Common questions
Does NTFS compression slow down Windows?
On modern hardware, rarely enough to notice for text and archives. It does cost CPU on every read and write, which matters for files accessed constantly and for media that will not compress anyway.
What is Compact OS?
A Windows feature that stores system files compressed, saving a few gigabytes on small drives. It is why system binaries show a size on disk smaller than their length.
Should I tick Compress this drive to save disk space?
Usually not. It applies to everything, including media that will not compress and files Windows reads constantly. Compressing one archive folder deliberately is the better version.
What compresses well on Windows?
Text, logs, source code and uncompressed databases, often by half or more. Photos, video, audio and Office documents are already compressed and gain essentially nothing.