Get your video attachment under the Outlook limit
Drop a video and FitSize shrinks it to what Outlook will send, in your browser. Nothing is uploaded to a server.
Instant client-side compression · Up to 2 GB per file · Zero server upload
- Attachments limit
- 25 MB
- FitSize aims for
- 18.1 MB (room for email encoding)
- Last checked
- 30 September 2026
- Read on
- support.microsoft.com
There is more than one Outlook. The personal Outlook.com account, a work or school Microsoft 365 mailbox and the phone app each have their own limit, and the work limit is set by whoever runs your organisation's email. That is why the same file sends for a colleague and bounces for you.
Why Outlook has this limit
Why does Outlook block your attachment? Outlook and Exchange servers enforce a 20 MB or 25 MB message ceiling including message body and base64 transmission overhead. FitSize calculates precise bit-budgets using the W3C WebCodecs API to ensure attachments clear mail server gateways with zero-byte server upload privacy.
Microsoft documents that messages sent outside its own datacentres get an extra 33% encoding increase, and that Outlook on the web restricts the size you can send to 25% below the configured setting to allow for it. Its default maximum for a mailbox is 35 MB when sending, and an administrator can set anything from 1 MB to 150 MB.
Source: learn.microsoft.com
What FitSize does with your file
FitSize starts from the smallest personal limit and lets you pick the work or phone limit instead. Because Microsoft itself says encoding adds weight, the target it aims for is smaller than the limit on the box, and the page shows you that real usable size. If you can share a OneDrive link instead of attaching a file, that route avoids the attachment limit entirely.
Best fit for a 30-second clip at 18.1 MB: 720p at Balanced quality (that resolution holds up to 59 s). Best fit for a 2-minute clip at 18.1 MB: 480p at Balanced quality (that resolution holds up to 2 min 05 s).
Your file never leaves your device. It is read and compressed by your browser, and nothing is uploaded to a server.
Three things to watch on Outlook
- Your work limit is chosen by your admin. The documented default for a Microsoft 365 mailbox is 35 MB for sending and 36 MB for receiving, and administrators can specify anything from 1 MB to 150 MB. The size you can actually send also depends on what your email client supports. Source: learn.microsoft.com
- The phone app is stricter. Outlook for iOS and Android has a message size limit of 33 MB, lower than the default, and the mobile apps honour the lowest of the attachment, send and request size settings in the mailbox policy. Source: learn.microsoft.com
- A link avoids the limit. For Outlook.com the attachment size limit for files is 25 MB, while files shared from OneDrive can be up to 2 GB. Sharing a link rather than attaching is the documented way past the limit. Source: support.microsoft.com
Outlook limits by account type
| Account | Limit | FitSize aims for | Checked |
|---|---|---|---|
| Attachments | 25 MB | 18.1 MB | 30 September 2026 |
| Work or school mailbox, default send limit (whole message) | 35 MB | 25.4 MB | 30 September 2026 |
| Outlook for iOS and Android, whole message | 33 MB | 23.9 MB | 30 September 2026 |
FitSize vs traditional cloud converters
| Architecture Feature | FitSize (In-Browser) | Traditional Cloud Converters |
|---|---|---|
| Latency | Instant / 0s queue delay | 15–45s upload queue & network delay |
| Privacy | 0-Byte Server Upload (100% on-device) | Files uploaded & stored on third-party servers |
| Bandwidth | 0 MB upload bandwidth consumed | Full upload and download bandwidth consumed |
| Exact File Size Guarantee | Deterministic bit-budget target | Trial-and-error guesswork re-compression |
Frequently asked questions about Outlook compression
What is the file attachment limit for Outlook and Exchange?
Outlook.com sets a 25 MB total message limit, while corporate Microsoft Exchange accounts commonly restrict attachments to 20 MB or 10 MB depending on administrator security policies.
Why does Outlook reject attachments that appear smaller than 20 MB?
Internet email uses MIME base64 encoding, which inflates binary file size by approximately 33%. A 16 MB file often expands to over 21 MB during delivery, triggering server rejection.
Can I compress proprietary corporate documents safely here?
Absolutely. FitSize processes every file directly inside your browser sandbox via WebCodecs and OPFS. No file data is sent across the internet, ensuring enterprise-grade confidentiality.