How are Amazon EBS snapshot's sizes calculated?

Solution 1:

This may answer Q2 (from http://aws.amazon.com/ebs/):

Even though the snapshots are saved incrementally, when you delete a snapshot, only the data not needed for any other snapshot is removed. So regardless of which prior snapshots have been deleted, all active snapshots will contain all the information needed to restore the volume

In your example after deleting the first snapshot you would not pay anymore for the 1GB in the first overwritten by the second snapshot, and you won't be able to restore the state of the first snapshot.

But it's still quite opaque about how much a set of snapshots costs in terms of S3 usage.

Solution 2:

See this message and the reply two messages below it. Essentially, there is only one copy of each block and multiple snapshots can refer to the same block. Snapshots can be deleted in any order and any snapshot can be used to restore the volume to the state it was in at the moment the snapshot was made.