How is GB usage calculated for the next limit in Free Tier AWS EC2?

I have one instance with 15 gb of memory allocated, why does it say 13 gb instead of 15?

In Amazon EC2, the free tier allowance includes 30 GB of EBS General Purpose (SSD) or Magnetic storage. The calculation of this 30 GB limit is based on the total storage provisioned in a month and not how much data is currently stored.

If you have a 40 GB EBS volume created and attached to an instance for the entire month, you will be billed for 30 GB (Free) + 10 GB-months of usage (Paid), regardless of how much data you actually have on that volume. Deleting data from the volume will not reduce your GB-Months usage because the usage is based on the provisioned storage size, not the amount of data stored.

Remember, if you're trying to manage your EBS usage to stay within the free tier, you need to monitor the size of the volumes you're creating and how long you're keeping them, rather than the amount of data you're storing on them.

EBS Volumes in regards to Free Tier are calculated by the allocated storage. It doesn't matter if the disk space is fully utilized or not. If you want to reduce your storage allocation, you'll either need to reduce the disk size or replace the disk with a smaller one.