You can check your current usage directly from your Account Summary page. It shows the total count for each item type — notes, notebooks, devices, attachments, spaces, tags, and MB of storage used — along with the limit for your plan.
Note: Notes in the Trash also count towards the overall note limit. Attachments contained inside notes in the Trash will also count towards the total number of attachments.
If you want to check how much storage Evernote is taking up on your desktop device instead, visit this article.
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