An email signature banner should be 600 px wide, or 1200 px for a double-resolution file (@2x): that is the width ATOM Signatures recommends for a campaign banner. This free resizer, which runs in your browser, sets your image to that width, shows it at 320 px wide, like a narrow screen, and downloads it.
Resize a signature banner
What size should a signature banner be in Outlook?
Aim for 600 px wide. That is the width recommended in the ATOM console, and it matches the 600 × 200 px Exclaimer recommends in its seasonal signatures guide.
| Criterion | Guideline | Source |
|---|---|---|
| Recommended width | 600 px, or 1200 px for a double-resolution file (@2x) | ATOM console |
| Warning | ATOM warns above 700 px wide | ATOM console |
| Dimensions | 600 × 200 px (a 3 to 1 ratio) | Exclaimer |
| Outlook for Windows | Can clip images wider than 624 px in the reading pane | Exclaimer |
| Phones | Outlook for iOS and Android scale the banner to the device width | Exclaimer |
| Weight | Under 100 kB (Exclaimer); up to 3 MB in ATOM. The first is a recommendation, the second a limit | Exclaimer, ATOM console |
| Formats | ATOM accepts PNG, JPEG, GIF and WebP; SVG is not supported in email signatures | ATOM console, Microsoft Learn |
max-width | Partially supported in classic Outlook for Windows, supported in Outlook for Mac, Outlook.com, iOS and Android | caniemail |
Tip: always set a width attribute in pixels on the image in the signature's code, rather than relying on max-width. For a double-resolution file, double the numbers: 1200 × 400 px is the 600 × 200 px banner with twice the pixels, displayed at 600 px.
How do I use the resizer?
- Choose the banner image (PNG, JPEG, GIF or WebP) from your computer.
- Choose the output width: 600 px, or 1200 px if you want a double-resolution file.
- Choose the file format: PNG (the default) or JPEG, lighter for a photo.
- Look at the preview at 320 px wide: the banner text must stay readable.
- Check the weight shown, then click Download.
The height follows the image's ratio: a 1800 × 600 px image gives 600 × 200 px at a width of 600 px. If the original image is narrower than the output, the tool tells you: enlarging it makes it blurry, and it is better to start again from a larger file. For a campaign in ATOM, the file you produce is uploaded in the Visual step; the guide to the holiday banner in the signature covers the rest.
Why check the banner at 320 px wide?
According to Exclaimer, Outlook for iOS and Android scale the banner to the device width, so it must stay readable down to 320 px wide. The tool's preview simulates a narrow screen: if the text becomes unreadable, make it larger or shorter rather than shrinking the image. It is not a measure of your recipients' screens, only a design guideline.
For other image problems in a signature, see Outlook signature logo not displaying, and for mobile signature with a logo in Outlook for Android and iPhone or Outlook signature on Mac and mobile. To check the code of a complete signature, there is the email signature checker for Outlook.
Which format and weight for the banner?
Exclaimer advises PNG, falling back to JPG, and a file under 100 kB. ATOM accepts PNG, JPEG, GIF and WebP images up to 3 MB: that limit is the console's, not a recommended weight. A photo weighs far less as a JPEG than as a PNG; flat colours with text stay sharp as a PNG. According to Microsoft, SVG is not supported in email signatures: export the banner as PNG or JPG before loading it here.
Do not rely on the image alone for essential information: when images are blocked, only the alternative text is left. ATOM shows one banner per email. To place ATOM against another tool on campaigns, see the ATOM vs Exclaimer comparison.
Frequently asked questions
What size should an email signature banner be for Outlook?
ATOM recommends 600 px wide, or 1200 px for a double-resolution file, and warns above 700 px. Exclaimer recommends 600 × 200 px and notes that Outlook for Windows can clip images wider than 624 px in the reading pane.
Why is the preview 320 px wide?
According to Exclaimer, Outlook for iOS and Android scale the banner to the device width. The 320 px preview simulates a narrow screen to check that the text stays readable; it is a design guideline, not a measurement.
Is my image sent to a server?
No. The image is read, resized and downloaded in your browser. The tool sends nothing to ATOM Signatures or anyone else.
Which format should I choose, PNG or JPEG?
Exclaimer advises PNG, falling back to JPG. JPEG is lighter for a photo; PNG suits flat colours with text. Microsoft states that SVG is not supported in email signatures.
How heavy should the banner file be?
Exclaimer advises under 100 kB. ATOM accepts up to 3 MB: that is a limit, not a recommended weight. The tool shows the weight of the file it produces so you can compare.
Roll the banner out to every employee. Banner campaigns are included in ATOM Signatures at no extra cost: CA$1 per user per month, plus taxes, with a 14-day free trial.
Sources
- Exclaimer — Seasonal email signatures and marketing campaigns (600 × 200 px, 100 kB, 624 px, 320 px, PNG and JPG): exclaimer.com
- Microsoft Learn — Office.Body, SVG files in email signatures: learn.microsoft.com
- caniemail — CSS max-width: caniemail.com
Sources consulted on November 2, 2026. Recommended width, warning above 700 px, formats and the 3 MB limit: the ATOM Signatures console.