HEIC · Image formats
Email attachment size limits, and how to get under them
Gmail allows 25 MB per message on a personal account. The Outlook desktop app applies a 20 MB limit to internet email accounts, and a 10 MB default to Exchange accounts, which is why a work address frequently refuses a file that a personal one accepted. Two details catch people out. The limit covers the whole assembled message rather than the file on disk, and attachments are encoded in a way that inflates them by roughly a third — so a 25 MB cap accepts a file closer to 18 MB. And the recipient has a limit too; the smaller of the two wins, and you cannot see theirs. Converting photographs to JPG at a set size budget is usually enough, and a link beats an attachment once you are near the line.
The published limits
Gmail states a 25 MB limit for personal accounts. Work and school accounts run on limits their administrator sets, which may be higher or lower, and nothing in the interface tells you which.
Microsoft publishes different numbers for different products, and this is the source of most confusion. Outlook.com allows 25 MB per message. The Outlook desktop application applies its own limit before the server ever sees the message: 20 MB for internet email accounts such as Outlook.com or Gmail, and 10 MB by default for Exchange accounts.
That last figure is the one that surprises people. A photograph that sent perfectly from a phone can be refused by the same person sitting at a work laptop, because the desktop client is enforcing a 10 MB Exchange default rather than the 25 MB the mail service would have allowed. The file did not change; the client did.
Why the real limit is lower than the number
An email is plain text. An attachment is binary, so it has to be re-expressed using only printable characters before it can travel, and the encoding used for this — base64 — represents every three bytes as four. That is a fixed overhead of about 33%.
Where a provider applies its limit to the assembled message, as is usual, the number you are given is the size after that inflation. A 25 MB limit therefore accepts a file of roughly 18 MB, and a 20 MB limit roughly 15 MB. The message body, any signature images and the headers come out of the same budget.
This is why a 24 MB file is rejected by a 25 MB limit with an error that looks wrong. Nothing is broken and nobody is miscounting — you are comparing the size on disk against a limit that applies to something larger.
The recipient has a limit too, and it is invisible
Both ends enforce their own maximum, and the effective limit for any message is the smaller of the two. Your provider accepting a 24 MB message does not oblige theirs to deliver it.
The failure is also asymmetric in a way that wastes time. When your own provider refuses the message you find out immediately. When the recipient server refuses it, the message leaves your outbox, appears in your sent folder, and the bounce arrives minutes or hours later — and bounce messages from corporate mail systems are often terse enough that the size is not obviously the cause.
If you are sending to an address you do not control, particularly a corporate one, treating 10 MB as the practical ceiling avoids the whole category of problem.
Getting a photo under the line
For photographs, converting to JPG with a size budget is almost always sufficient and is the least disruptive option. A modern phone photograph is 3 to 12 MB, and a handful of them cross any of these limits quickly; the same set converted to fit 1 MB each is indistinguishable in an email client and comfortably under every limit above.
Aim at a number rather than a quality percentage. Quality sliders are not comparable between tools and tell you nothing about the result, whereas a budget is exactly the thing the limit is expressed in.
HEIC photographs from an iPhone deserve a separate mention. They are already small, so they rarely breach a size limit — but many desktop mail clients and most recipients cannot open them at all, so the message arrives with an attachment nobody can read. Converting to JPG makes it larger and far more useful.
Past roughly 20 MB in total, stop attaching and send a link. Both Gmail and Outlook will offer this themselves, through Drive and OneDrive respectively, and a link is not subject to any of the limits above.
Frequently asked questions
Why was my 24 MB file rejected by a 25 MB limit?
Because attachments are base64-encoded for transport, which inflates them by about a third, and the limit applies to the encoded message. A 24 MB file becomes roughly 32 MB in the message. As a rule of thumb, multiply your file size by 1.33 before comparing it against any published limit.
Gmail says 25 MB but my work Outlook refuses 12 MB. Why?
The Outlook desktop application enforces its own limit before the message reaches any server, and for Exchange accounts that default is 10 MB. It is a client-side restriction on a different product from the web service, so the two numbers genuinely differ for the same person.
How many photos can I attach to one email?
It depends entirely on the photos, not the count. Phone photographs run 3 to 12 MB each, so three or four can exceed a 25 MB limit once encoding overhead is counted. Converting them to fit 1 MB each lets you attach fifteen or more with room to spare.
Does compressing a photo for email ruin it?
Not at a sensible budget. A photograph fitted to 1 MB is essentially indistinguishable from the original when viewed in an email client or on a phone. It would matter for printing at large sizes or for archival copies, which is a reason to send the compressed version and keep your original rather than a reason to send the original.