Reducing a photo's size to send it by email
Resize first, compress second: the method for taking a 5 MB photo down to 300 KB with no visible artefacts.
The message goes out, then comes back with a curt 552 message size exceeds fixed limit. Or it sits in the outbox for ten minutes before giving up. Three photos taken on a phone are enough to break the limit on a company server set to 10 MB — by far the most common default setting. And in the vast majority of cases the culprit is not compression: it is the dimensions. You are sending an image 4032 pixels wide that your recipient will look at in an 800-pixel window.
How big a photo really is, and what a mailbox will accept
The orders of magnitude, to frame the problem:
- A 12-megapixel sensor produces 4032 × 3024 px, which comes out as a JPEG of 3 to 6 MB depending on how much detail the scene holds.
- A 48-megapixel sensor in full-resolution mode climbs to 8000 × 6000 px and 10 to 15 MB per shot.
- A desktop screenshot saved as PNG (2560 × 1440) sits around 1 to 3 MB, more if it contains a photo or a gradient.
- A photograph saved as PNG by mistake can reach 20 MB for content JPEG would handle in 800 KB.
On the receiving side, Gmail caps attachments at 25 MB, Outlook.com at around 20 MB, and plenty of company servers are configured far lower — 10 MB is a common default on Exchange. The detail everyone forgets: attachments travel base64-encoded, which adds roughly a third to the size actually transmitted. A stated limit of 25 MB therefore works out in practice at a little under 19 MB of actual files.
Another trap: it is not only your own limit that decides — the recipient's counts too. Your server accepts 25 MB, theirs refuses anything over 8 MB, and the rejection lands several minutes after you press send. In a work context, aiming for 5 MB for the whole message avoids virtually every refusal.
The real lever is pixels, not the quality slider
Cutting the resolution before you touch compression changes everything. Going from 4032 to 1600 pixels on the long edge divides the pixel count by 6.3: the file size drops by a factor of about five, with no visible loss at all, since nobody was ever seeing those pixels anyway.
A few reference points for choosing the target width:
| Use | Long edge | Expected size in JPEG |
|---|---|---|
| Illustration viewed in the body of the email | 1000 to 1200 px | 150 to 300 KB |
| Photo the recipient will open full screen | 1600 to 2000 px | 300 to 700 KB |
| The recipient needs to print it at A4 | 2400 px and up | 1 to 2 MB |
For a photo meant to be opened full screen, as long as you do not drop below 1600 px it stays comfortable on an ordinary computer display. Below 1000 px it becomes a thumbnail the moment the recipient zooms in.
The order of operations matters
Do it in this order, never the other way round.
- Crop. If a third of the image is empty tarmac or blank sky, that is a third of the file size gone without argument.
- Resize. Bring the long edge down to the figure in the table above with an image resizing tool, keeping the proportions locked.
- Compress. Only now apply JPEG compression.
Compressing before resizing means spending time optimising pixels you are about to throw away at the next step — and the second re-encode stacks its own artefacts on top of the first lot.
Which JPEG quality level
Quality 85 is the balance point: the difference from the original is invisible to the naked eye, and the file has already shed a great deal. At 75 the image is still perfectly respectable for everyday sending. Below 70 the flaws show up first where the image is most exposed: sky gradients breaking into bands, skin tones going blotchy, halos around crisp edges — the familiar mosquito noise.
Two habits worth keeping: always start again from the original file rather than from an already compressed copy, because JPEG loses information with every successive save. And check the result at full size, not in the file explorer thumbnail.
Choosing the right format before you send
JPEG for every photograph. It is the universal format, opened by any mail client, any phone, any version of Windows.
PNG for anything that is not photographic: screenshots, diagrams, text, logos, flat colour. A screenshot saved as JPEG shows grey fringing around the letters, and the file is often larger than the equivalent PNG. The reverse holds too: a photograph in PNG is a disaster for file size.
Mind the transparency: JPEG does not have it. Converting a transparent PNG to JPEG fills the empty area with black or white, which leaves an ugly rectangle behind a logo. If you need transparency, stay with PNG.
HEIC is the default format on recent iPhones. It is half the size of JPEG at the same quality, but a recipient on Windows or Android can end up with a file they cannot open. Before sending, run the HEIC to JPG conversion — or switch the phone's camera to “Most Compatible”.
WebP is 25 to 35% smaller than JPEG, but the built-in preview in some desktop mail clients will not display it. Excellent for a website, best avoided for an attachment going to someone whose setup you do not know.
Several photos to send at once
Squeezing a batch of JPEGs into a ZIP archive achieves almost nothing: JPEG is already compressed, and ZIP claws back 1 to 2%. The only benefit is bundling the files into a single download.
Two alternatives that are more practical, even if they are no smaller. Combine the photos into a single PDF: the recipient scrolls instead of opening twelve attachments, and the file stays printable as it is. Or put the originals on a file-sharing service and send nothing but the link, which remains the only sensible answer beyond twenty photos or so.
When it still will not go through
“The file is still enormous after resizing.” Check the extension: a 2000 px PNG can still be 5 MB. Convert it to JPEG if the content is photographic.
“I have been asked for a photo at 300 dpi and under 500 KB.” DPI has no effect on file size, nor on how the image looks on screen: it is simply a label in the header telling the printer what physical size is intended. Pixels are what count. A 35 × 45 mm passport photo at 300 dpi comes to 413 × 531 px — a file of a few tens of kilobytes. For a full-page A4 print at 200 dpi, reckon on 1654 × 2339 px.
“The photo is a scanned document and the text has become unreadable.” Do not go below 200 effective dpi, which is about 1650 px wide for an A4 page, and prefer PNG if the scan is crisp black and white. Aggressive JPEG compression goes for fine outlines first, and that means the characters.
“The message is rejected even though I am under the limit.” Remember the encoding overhead: add a third to the total of your attachments to estimate the real size of the message, signature and inline images included.
A word about metadata
A photo taken on a phone carries EXIF data: GPS coordinates of where it was shot, the exact date and time, the device model, sometimes the serial number. Sending a stranger a photo of your living room potentially hands them your address. Resizing and re-encoding generally strip those fields along the way — a welcome side effect of the process described here, but one worth checking rather than assuming if the subject is sensitive.
On Convertu, image resizing and compression run directly in the browser: the photo is never sent to a server. Both tools require an account and are covered by the single subscription at €7 a year. HEIC conversion and PDF assembly go through our servers, with one conversion free every day without signing up and that same subscription beyond. The maximum file size accepted is shown on each tool's own page.