Email Storage Calculator
Estimate your mailbox storage requirements based on user count, email volume, retention policy, and attachment sizes - and see how it maps to Microsoft 365 limits.
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Microsoft 365 Storage Limits by Plan
| Storage Feature | Basic (£4.60/user/mo) | Standard (£9.40/user/mo) | Premium (£16.60/user/mo) |
|---|---|---|---|
| Primary Mailbox | 50 GB | 50 GB | 50 GB |
| Archive Mailbox | 50 GB (auto-expanding) | 50 GB (auto-expanding) | 50 GB (auto-expanding) |
| Total per User (with archive) | 100 GB+ | 100 GB+ | 100 GB+ |
| Shared Mailbox | 50 GB (free) | 50 GB (free) | 50 GB (free) |
| OneDrive Storage | 1 TB per user | 1 TB per user | 1 TB per user |
| Max Message Size | 150 MB | 150 MB | 150 MB |
| Recoverable Items | 30 GB | 30 GB | 30 GB |
| Retention Policies | Basic | Basic | Advanced (Compliance) |
Storage limits are per Microsoft’s current allocation. Archive mailbox auto-expands beyond 50 GB for plans with Exchange Online Plan 2 or equivalent. View full Cloud Email details
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Frequently asked questions
Most Microsoft 365 plans include 50GB to 100GB of mailbox storage per user as standard, which comfortably covers typical usage of a few hundred emails a month with occasional attachments. Heavy attachment users, such as those regularly sending design files or large reports, may need closer monitoring to avoid hitting limits.
Once a mailbox nears its limit, users typically start receiving warnings, and eventually outgoing mail can be blocked until space is freed up, usually by archiving or deleting old items. Retention policies and auto-archiving to an online archive mailbox are the standard fix for teams that consistently generate high email volume.
Longer retention periods, often required for compliance in regulated industries, directly increase storage needs since old emails are kept rather than purged. A 7-year retention policy for financial or legal correspondence can multiply storage requirements several times over compared to a business with a 1-year deletion policy.
Base the estimate on current average mailbox size per user, projected headcount growth, and your retention policy, then add a buffer for attachment-heavy roles. This calculator uses your user count, email volume, and retention settings to project storage needs, helping you plan licence tiers before you hit capacity limits.
