There is confusion around how the storage is calculated in SharePoint Online. I believe, in SharePoint Online 1 TB is 1024 GB (based on powers of two), although the SI Prefix is for numbers based on powers of 10 (1TB = 1000GB, Wikipedia). In this post I would like to summarize the results of my investigations and I hope Microsoft or the community can confirm or disconfirm this.
First, let me explain why we care about it. The storage in SharePont is limited and we need to keep an eye on it. Especially in our case, where we need to track storage utilization across different parts of the organization/our tenant. The storage in SharePoint is calculated like so: