How to Reduce Image Size for Email Attachments

The attachment limits

Email providers limit how large a message may be. Gmail allows 25 MB for the attachments of one outgoing email, counted together for all files, and offers to insert a link to Google Drive when you go over. Outlook.com limits attachments to 20 MB, and Microsoft 365 business mail (Exchange Online) allows messages of up to 35 MB by default, though administrators can change the limit. Other providers and company mail servers have their own limits, and some are much lower.

Check the current numbers with your provider, because limits change, and the limit of the recipient can differ from yours. A message that you can send may still be rejected by the other side if their server is stricter.

Why a file seems to grow

Email carries attachments as text, using an encoding called base64, which turns every three bytes of data into four characters. This makes the message about a third larger than the files in it, plus a small amount for headers. A set of attachments totalling 19 MB can therefore become roughly 25 MB in the message and be refused even though it looks like it is under the limit.

For this reason, leave a safety margin. If the limit is 25 MB, aim for attachments of at most about 18 MB in total. When you are not sure about the recipient's limit, aim for much less, such as 5 to 10 MB.

How big are phone photos?

A modern phone photo is often between 2 and 6 MB as a JPEG, and larger for high-resolution sensors or in raw formats. Four photos can fill a 20 MB limit. Screenshots saved as PNG can be 1 MB or more each. Sending an entire folder of pictures by email therefore tends to fail.

The pictures do not need that much detail for most email uses. A photo that the recipient will look at on a screen needs about 1600 to 2000 pixels on its longest side. A full-resolution original is only necessary if the recipient wants to print it large or edit it.

Shrinking the pictures

The most effective way is to reduce the pixel dimensions and then the quality. Add your photos to the compressor, set the maximum size to 1920 or 1600 pixels, and keep the quality at about 75. A 4 MB phone photo often becomes a few hundred kilobytes, and the picture looks the same on a normal screen. Download the files one by one or as a ZIP and attach them.

If the recipient needs the full quality, do not compress; use a link instead. Upload the original to a cloud storage service and share the link, or send the files in several messages. Compressing and a link can also be combined: send small versions in the email and offer the originals on request.

Zip files and why they do not help

Putting photos in a ZIP archive rarely reduces their size. JPEG, PNG and WebP are already compressed, so a ZIP can only gather the files in a single attachment, not make them smaller. A ZIP is useful for organizing several files, and the download all button of this tool creates one for that reason.

Some mail systems also block certain archive types for security reasons, so a ZIP is not always accepted. For a few photos, attach them directly after compressing.

Embedding versus attaching

When you insert a picture into the body of an email, it may be attached behind the scenes, so the same size limit applies. Some mail programs automatically resize pictures that you insert and offer a choice of small, medium and large. If your program does not, compressing the picture first gives you control over the result.

Remember that the recipient sees the picture at a small size in a mail window. A large file only costs their mailbox space and download time.

A short checklist

Decide how the recipient will use the picture. Resize to 1600 to 1920 pixels for viewing on a screen. Set the quality to 70 to 80 and compare the result with the original. Check the total size with a margin below the limit. And keep your originals, so that you can send a better version if someone asks.

Because the compressor runs in your browser, the photos are not uploaded anywhere, which is a good thing when the pictures are private, such as family photos, documents or identity cards.

If you often send photos from a phone, you can also change the share options in your mail app, which may offer to scale pictures down automatically. Check what size it chooses, because the default can be smaller than you want for printing.

Tips for sending several pictures

If you have many pictures, send them in two or three messages instead of one very large one, or use a cloud link. Name the files clearly, since a phone's default names such as IMG_4021 tell the recipient nothing. The compressor keeps the original name and adds -compressed to files it has changed, which makes it easy to recognize which file is which.

Finally, tell the recipient if the pictures were reduced and offer the originals. That avoids surprises if they want to print them, and it keeps the email small for everyone.