Compress Images for Email Attachments — Stay Under Size Limits
· 5 min read
Gmail and Outlook cap attachments at 25 MB, and many corporate servers stop at 10. Resize to about 2000 pixels wide and compress at quality 70, which typically takes a 6 MB phone photo under 500 KB. Encoding inflates the message by roughly a third, so aim below the stated limit rather than at it.
Most mail servers reject attachments over 25 MB, and plenty of corporate ones stop at 10. Here is how to get a set of photos under the limit without emailing a link instead.
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The limits you are actually up against
Gmail and Outlook both cap attachments at around 25 MB. Many company mail servers are stricter — 10 MB is common and some are at 5. The limit applies to the encoded message, which is roughly a third larger than the raw files, so budget for less than the stated number.
The recipient's server matters as much as yours. You can send a 24 MB attachment from Gmail perfectly happily and have it bounce because the company at the other end caps inbound mail at 10 MB. When in doubt, aim for the stricter figure.
| Provider | Limit | Safe target |
|---|---|---|
| Gmail | 25 MB | under 20 MB |
| Outlook.com | 20 MB | under 16 MB |
| Microsoft 365 | 25 MB, admin-configurable | under 15 MB |
| Yahoo Mail | 25 MB | under 20 MB |
| iCloud Mail | 20 MB | under 16 MB |
| Typical corporate server | 10 MB | under 8 MB |
Why the encoded message is bigger than the file
Email cannot carry raw binary data, so attachments are re-encoded into plain text using base64. That encoding costs roughly 33% — three bytes of file become four characters of message.
This is the single most common reason a send fails when the file looked like it should fit. A 20 MB folder of photos becomes about 27 MB of message and bounces off a 25 MB cap. Sizing to about three quarters of the stated limit avoids the problem entirely.
Compress ImageShrink PNG, JPG and WebP without visible quality lossHow much to compress
For photos that will be looked at on a screen rather than printed, quality 70 and a maximum width of about 2000 pixels is plenty. That usually takes a 6 MB phone photo to well under 500 KB, which means a dozen of them fit comfortably in one email.
If the recipient will print them, keep the dimensions and lower the quality less — around 85. Printing needs pixels far more than it needs a low compression setting.
- On-screen viewing: 2000px wide, quality 70
- Print at A4: keep full dimensions, quality 85
- Reference or record-keeping shots: 1200px wide, quality 65
- Anything the recipient will edit: do not compress, send the original by link
Resize before you compress
A modern phone photo is around 4000 pixels wide. Nobody reading it in an email client sees more than about 1000. Reducing the dimensions removes far more weight than any quality setting, and it removes it without touching how the visible pixels look.
Resize first, then compress. Doing only the second is why people end up disappointed with how little they saved. Halving the width quarters the pixel count, and file size follows pixel count far more closely than it follows the quality slider.
Sending several at once
If you are attaching a set, the limit applies to the whole message rather than to each file. Ten photos at 2 MB each will not fit under a 10 MB cap however you feel about each one individually.
Compress them as a batch with the same settings so they look consistent, then check the total before attaching. A batch compressed with mismatched settings is obvious to the recipient — some pictures crisp, some soft.
When compressing is the wrong answer
If you are sending original photographs for someone to edit, do not compress them. Lossy compression discards data permanently and an editor will notice — pushing shadows or correcting colour on a compressed file brings out blocking that was invisible before.
Compression is for delivery, not for archiving. Keep your originals whatever you do.
What to do when it still will not fit
Send fewer per message. Three emails of four photos each is unglamorous but it works everywhere, needs no third-party service, and does not put your files on anyone else's server.
A shared link avoids the limits entirely, at the cost of uploading the images somewhere. That is a reasonable trade for holiday photographs and a poor one for anything confidential.
Common questions
What is the maximum image size for Gmail?
Gmail allows attachments up to 25 MB total across the message. Beyond that it offers to send a Google Drive link instead. Because base64 encoding inflates the message by about a third, keep the actual files under roughly 18 MB.
How do I send a large image by email?
Resize it to about 2000 pixels on the long edge and compress at quality 70. That takes a typical phone photo from several megabytes to under 500 KB with no visible difference on screen. For genuinely huge originals, send a link instead.
Should I use JPG or PNG for email attachments?
JPEG for photographs — it compresses far smaller than PNG for the same visual result. PNG only when you need transparency or the image is a screenshot with text, where JPEG smears the letters.
What image size is best for email newsletters?
Header banners under 100 KB at 600px wide, product photos under 200 KB, thumbnails under 50 KB. Keep the whole email under about 600 KB — some clients clip long messages and slow images hurt open-to-click rates.
Why did my email bounce when the attachment was under the limit?
Almost always base64 encoding. Attachments travel as text, which costs about 33% in size, so a 20 MB folder becomes a 27 MB message. The receiving server may also have a lower cap than the one you sent from.
Does compressing an image lose quality permanently?
Yes for lossy formats like JPEG — the discarded detail cannot be recovered, and re-saving repeatedly compounds it. Always compress a copy and keep the original.
Is it safe to compress work documents on an online tool?
It depends entirely on whether the tool uploads them. FileCrisp compresses in your browser, so the images never leave your device. Tools that process on a server have your file, whatever their privacy policy says about it.