How to reduce image size for email attachments

Photographs from a modern phone or camera are routinely 5 to 15 MB each. Attach four of them and most email systems will refuse to send the message. This guide covers why the limits are lower than advertised, and the two different ways of getting under them.

The limits are smaller than the numbers suggest

Common attachment limits sit around 25 MB for Gmail and Outlook, and often 10 MB or less for corporate mail servers.

But there is a catch that catches people out constantly: attachments are encoded for transmission, and that encoding inflates them by roughly a third. A 20 MB file becomes about 27 MB on the wire. So a "25 MB limit" is really closer to an 18 MB limit on the actual files.

The receiving server matters too, and it may be stricter than yours. A message that sends fine can be rejected on arrival by the recipient's system. Aiming for 10 MB total is a safe target that avoids arguing with infrastructure you cannot see.

Resizing and compressing are not the same thing

These get used interchangeably and they do quite different things.

Resizing changes the pixel dimensions — how many pixels the image contains. A 6000 × 4000 photo resized to 1500 × 1000 has one sixteenth as many pixels. This is a large, predictable reduction.

Compressing keeps the dimensions and reduces how many bytes are used to describe each pixel, by discarding detail. The image stays 6000 × 4000 but the file gets smaller and, past a point, visibly degrades.

For email, resizing is almost always the right first move, and it is the one people skip. The reason is simple: nobody views an email attachment at 6000 pixels wide. A typical laptop screen is around 1900 pixels across. You are sending fifteen times more pixel data than the recipient can possibly see.

Pick a sensible target size

What dimensions you need depends entirely on what the recipient will do with the image.

• Just looking at it on screen: 1600 pixels on the long edge is plenty, and usually lands under 500 KB as a JPG.
• They may zoom in on detail: 2400 pixels on the long edge preserves useful detail at roughly 1 MB.
• They will print it: this is the one case where you need the original resolution. A good print needs around 300 pixels per inch, so a 6 × 4 inch print wants about 1800 × 1200 at minimum. Do not aggressively downsize anything destined for paper.
• It is going into a document or slide deck: 1200 pixels on the long edge is generous for anything that will occupy part of a page.

A workflow that reliably works

For a batch of photos that need to fit in one email:

• Resize first. Set the long edge to 1600 pixels. This alone typically cuts each file by 90 percent or more.
• Then compress, at JPG quality 75 to 80. Applied after resizing, this gets you the rest of the way without visible damage.
• Check the total. Ten photos at 400 KB each is 4 MB — comfortably inside any limit.
• Keep your originals. Resizing is destructive; the discarded pixels are gone. Work on copies, always.

The order matters. Compressing a full-resolution image and then resizing it wastes the compression work and can produce worse results than doing it the other way round.

When to send a link instead

Sometimes the right answer is not to attach the files at all.

If you are sending more than about ten photographs, or the recipient needs full-resolution originals, or the images are for printing, a shared link is better than fighting the attachment limit. Degrading a set of photos to squeeze them into an email, when the recipient specifically needed the quality, solves the wrong problem.

Most mail clients will offer this automatically when you exceed the limit. Be aware that this uploads the files to that provider's cloud storage — which is fine for holiday photos and is worth thinking about for anything confidential.

Combining images into one PDF

If you are sending several images that belong together — scanned receipts, pages of a signed document, photographs of damage for an insurance claim — putting them into a single PDF is often better than attaching them separately.

The recipient gets one file in a fixed order rather than a pile of loose attachments to work out. It also lets you resize the images on the way in, so the resulting PDF can be smaller than the sum of the originals.

Do resize before combining, though. A PDF built from ten full-resolution photos will be just as oversized as the ten photos were.

Common mistakes

A few things that regularly go wrong:

Compressing repeatedly. Each pass on an already-compressed JPG degrades it further. If a file is still too big, go back to the original and use stronger settings once, rather than compressing the compressed version again.

Resizing upward. Enlarging cannot recover detail that was never captured. If someone needs a bigger image than you have, you need a bigger original.

Sending PNG photographs. A photo saved as PNG can be several times larger than the JPG equivalent with no visible benefit. Check the format before you conclude the file is unavoidably large.

Forgetting the phone-format problem. iPhones shoot HEIC by default, which many Windows and Android recipients cannot open. Converting to JPG before sending avoids a reply asking what the file is.