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We can search for available image files on the docker website like this:

• https://index.docker.io/search?q=ubuntu

How can I tell what the download size(s) will be prior to pulling?

docker.io pull [image]

Looking at the API for Docker, Docker Remote API v1.10, it doesn't appear there is any way to get the sizes of the images. Section "2.2 Images" shows the spec for how to query about images.

Example

    GET /images/json?all=0 HTTP/1.1



**Example response**:



    HTTP/1.1 200 OK

    Content-Type: application/json



    [

      {

         "RepoTags": [

           "ubuntu:12.04",

           "ubuntu:precise",

           "ubuntu:latest"

         ],

         "Id": "8dbd9e392a964056420e5d58ca5cc376ef18e2de93b5cc90e868a1bbc8318c1c",

         "Created": 1365714795,

         "Size": 131506275,

         "VirtualSize": 131506275

      },

      {

         "RepoTags": [

           "ubuntu:12.10",

           "ubuntu:quantal"

         ],

         "ParentId": "27cf784147099545",

         "Id": "b750fe79269d2ec9a3c593ef05b4332b1d1a02a62b4accb2c21d589ff2f5f2dc",

         "Created": 1364102658,

         "Size": 24653,

         "VirtualSize": 180116135

      }

    ]

But this query needs to go against an actual Docker instance. Here's an example showing how one could use the above RESTful query:

$ echo -e "GET /images/json HTTP/1.0rn" | nc -U /var/run/docker.sock

HTTP/1.0 200 OK

Content-Type: application/json

Content-Length: 858

Connection: close

Date: Fri, 20 Dec 2013 16:02:41 GMT



[{"Repository":"ubuntu","Tag":"12.04","Id":"8dbd9e392...",

"Created":1365714795,"Size":131502179,"VirtualSize":131502179},

{"Repository":"ubuntu","Tag":"latest","Id":"8dbd9e392...",

"Created":1365714795,"Size":131502179,"VirtualSize":131502179},

{"Repository":"ubuntu","Tag":"precise","Id":"8dbd9e392...",

"Created":1365714795,"Size":131502179,"VirtualSize":131502179},

{"Repository":"ubuntu","Tag":"12.10","Id":"b750fe792...",

"Created":1364102658,"Size":24653,"VirtualSize":180116135},

{"Repository":"ubuntu","Tag":"quantal","Id":"b750fe792...",

"Created":1364102658,"Size":24653,"VirtualSize":180116135}]

I saw no way to query the public repositories using this particular RESTful call. The only other RESTful method that looked like you could query docker.io's images was via search, GET /images/search, but the API doesn't show any size attributes being returned for this.

References

• DOCKER FROM A DISTANCE - THE REMOTE API

This is not a direct answer to your question but I hope it will be helpful nonetheless.

In the disk-usage script
in my Docker experiments
I use something like this:

docker run --entrypoint=/bin/sh $image -c 'du -sh / 2>/dev/null | cut -f1'

So you can run, e.g.:

docker run --entrypoint=/bin/sh ubuntu -c 'du -sh / 2>/dev/null | cut -f1'

Or you can download that script: disk-usage
and run e.g. ./disk-usage "ubuntu busybox gcc" to have the disk usage (as reported by du -sh) displayed for those 3 images:

Image   Disk usage

-----   ----------

ubuntu  209M

busybox 2.6M

gcc     1.5G

Please note that it doesn't show the actual download required for any given image, and it will display the result after downloading the image, but it gives some idea on how bloated is a given image as compared to others.

You can run it on one machine to decide whether you want to download that images on other machines, or to use it at all.

If you really look into the docker code for pull operation, I think your answer is there. If the image of the container is not cached, then during pulling of the image, docker first collects the information about the image from the registry like number of layers, size of each layers etc. etc.

I would refer to read this file.

https://github.com/moxiegirl/docker/blob/master/distribution/xfer/download.go

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 image on other registry like Microsoft Container Registry. I figure out 3 ways.

   
   
   
   
   
   • Use docker manifest inspect to observe the manifest data, which can give you idea on how to gain the compressed size of the image.
   
   
   
   
   
   
   

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

   
   
   

   To enable docker manifest inspect, edit ~/.docker/config.json file and set experimental to enable.(Reference: docker manifest inspect)

   
   
   
   
   

   • 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