S3 billing - why does the bill increase the amount of data as the month drags on when I'm not adding to it?

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I have a bucket that has backup data from a long time ago with about 530 GB of data. I haven't touched it in months.

Instead of my bill at the beginning of the month saying something like 530.00-GB-Mo

It says something like 25 GB-Mo Then, as the month goes on, the amount of data keeps increasing even though I'm not touching this bucket. Why? If I have 530 GB in the bucket on day 1 of the month, wouldn't AWS count that on day 1?

asked a month ago40 views
1 Answer
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Accepted Answer

The behavior you're observing with your S3 billing is actually normal and expected. AWS calculates storage usage for billing purposes on a per-GB, per-day basis throughout the month. This is why you see the amount of data increasing as the month progresses, even though you're not adding new data to your bucket.

When you see "GB-Mo" (Gigabyte-Months) in your bill, it's a cumulative measure of your storage usage over time. At the beginning of the month, you might see a lower number like 25 GB-Mo because that represents the storage used for just the first day or so of the month.

As each day passes, AWS adds the daily storage usage to the running total. By the end of the month, for a static 530 GB bucket, you would expect to see approximately 530 GB-Mo (give or take a small amount depending on the exact number of days in the month).

This billing method allows for accurate charging even when storage amounts fluctuate during the month. If AWS counted the full 530 GB on day one, it wouldn't account for scenarios where users might add or remove data throughout the month.

So, while you have 530 GB in your bucket from day one, AWS is essentially charging you for 530 GB * (number of days in the month) / (days in a month), accumulating daily. This results in the gradual increase you're observing in your bill, eventually reaching the full amount by the end of the billing period.

This approach ensures that you're billed accurately for the storage you use, even if you were to add or remove data during the month. It's not a sign of unexpected usage or errors, but rather the standard way AWS calculates S3 storage costs over time.
Sources
Billing and usage reporting for Amazon S3 - Amazon Simple Storage Service
AWS Billing reports for Amazon S3 - Amazon Simple Storage Service
Amazon Simple Storage Service (S3) — Cloud Storage — AWS

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