This article describes how to estimate storage usage for Office 365 domain. Storage usage determines infrastructure costs and is important when you request a custom quote.
1. Log in Office 365 administrator panel and go to Reports -> Usage
2. To estimate total space occupied by user accounts and shared mailboxes you need to sum up Exchange and OneDrive space usage. In 'Select a report' dropdown you can find Exchange -> Mailbox usage and OneDrive -> Usage reports and then view Storage tab in each report.
3. SharePoint sites usage (including Group and Team sites) can be looked up the similar way in SharePoint -> Site usage report.