Find What Is Using Disk With docker system df

Docker's disk space disappears in four places: images, containers, volumes and build cache. docker system df tells you how much each one takes, and the verbose form tells you which items. This page uses it to find out what is big, and ends with the rule I follow before I delete anything.

The summary

docker system df

TYPE            TOTAL     ACTIVE    SIZE      RECLAIMABLE
Images          41        16        35.64GB   20.79GB (58%)
Containers      16        4         14.27MB   8.417MB (58%)
Local Volumes   32        10        3.623GB   382.7MB (10%)
Build Cache     371       3         25.75GB   13.57GB

Four rows. TOTAL is how many items exist, ACTIVE how many are in use by a container, and RECLAIMABLE is what could be freed. The numbers above come from my own development computer, so yours will be different. Two things to read first: a large build cache is usually safe to clear, and a reclaimable volume may be a database you meant to keep. Volumes are the row to be careful with.

Which item is the big one

docker system df -v lists every image, container, volume and cache entry with its size. On a busy computer that is a long list with your own project names in it, so I will not show mine. Make a clear example instead: a volume with 20 MB in it, and an image that carries a 30 MB file.

FROM alpine:3.24
COPY filler.bin /filler.bin

Set-Location disk-demo
[IO.File]::WriteAllBytes("$PWD\filler.bin", (New-Object byte[] 30MB))
docker build -q -t disk-demo-image:1 . | Out-Null
docker volume create disk-demo-vol | Out-Null
docker run --rm -v disk-demo-vol:/data alpine:3.24 sh -c "head -c 20971520 /dev/zero > /data/filler.bin"
docker system df -v | Select-String "disk-demo" | ForEach-Object { $_.ToString().Trim() }

disk-demo-image   1   fc315d572e3c   2 seconds ago   44.4MB   12.93MB   31.5MB   0
disk-demo-vol                            0               20.97MB

I filtered the verbose output to my demo names, so only they appear. The image line shows 44.4 MB in total. Of that, 12.93 MB is the Alpine base, which other images share, and 31.5 MB is unique to this image: the 30 MB file I added. The volume line shows 20.97 MB, the file I wrote into it. Without a filter, use docker system df -v | more, or look at one section at a time, and sort by the size column.

For a single type, the normal commands also show sizes. A container shows its writable layer with docker ps -s, and an image has its size in docker image ls.

docker image ls disk-demo-image --format "{{.Repository}}:{{.Tag}}"

disk-demo-image:1

What to do about it

• Stopped containers and old images: remove them by name, one at a time. Stopped containers hold on to their image, so remove the containers first.
• Build cache: docker builder prune clears it. I did not run it for this page, because it would clear the cache for all my other projects. The next builds are slower, and that is the only cost.
• Volumes: look at the name and ask what put it there before you remove it. docker volume rm deletes the data with no undo.

docker image rm disk-demo-image:1
docker volume rm disk-demo-vol

Related course: Docker Intermediate.