Get the size of a Docker image before a pull?

Question

How does one get the size of a Docker image before they pull it to their machine?

Answer 1

When you search for a docker image on Docker hub, there will be 2 tabs- Repo Info and Tags. Open Tags tab and you will see the sizes of all the types of images you can pull for that image.

Answer 2

1. For image on Docker Hub:

curl -s -H "Authorization: JWT " "https://hub.docker.com/v2/repositories/library/<image-name>/tags/?page_size=100" | jq -r '.results[] | select(.name == "<tag-name>") | .images[0].size' | numfmt --to=iec-i 

2. For images on other registry like Microsoft Container Registry

   • Push the image to Docker Hub and you can get the compressed size of the image on Docker Hub website.

   • Use docker save to save image to a .tar file and then compress it a .tar.gz file.

docker save my-image:latest > my-image.tar  # Compress the .tar file gzip my-image.tar  # Check the size of the compressed image ls -lh my-image.tar.gz 

3. To manually view the manifest data

Use docker manifest inspect to observe the manifest data, which shows you the compressed size of the image.

• You need to first enable it by editing ~/.docker/config.json file and set experimental to enable. Example: { "experimental": "enabled" }. More info at official docs.

• Issue docker manifest inspect -v <registry-domain>/<image-name> and see add the size for the layers but only for your specific architecture (e.g. amd64).

docker manifest inspect -v <registry-domain>/<image-name> | grep size | awk -F ':' '{sum+=$NF} END {print sum}' | numfmt --to=iec-i 

Noted:

1. It's the compressed size of the layers, not their on-disk size on your server.
2. If the image is a multi-arch image (e.g. alpine linux contains arm, amd64 and several architectures), then you'll get the total of those while in actual usage docker only uses the relevant arch.