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About this tool
Get the export size, retina multiplier and img markup for an email header, plus how it renders in Outlook, Gmail and Apple Mail.
An email header image has two sizes: the width it occupies in the template and the larger size you actually export so it stays sharp on high-density screens. This guide takes the display size, applies the retina multiplier, and returns the export dimensions, the width and height attributes Outlook's Word rendering engine requires, the rendered size on 320, 375 and 414 px phone viewports, and the img markup ready to paste. It also flags a header taller than 60% of its width, which pushes your first line of copy out of the preview pane.
Open Email Header Image Size Guide on AltFTool — it loads instantly in your browser.
Upload or drag in the file you want to work with.
Adjust the settings and preview the result.
Download the finished file to your device.
Export at 2x, place at 1x with explicit attributes — the pattern that works across every major client.
See the exact rendered height at three common phone widths before the send, not after.
Outlook's Word engine, Gmail's clipping limit and dark-mode recolouring are all called out.
Match the template width, which is 600 px for most email templates and up to 640 px for wide ones, and keep the height under about 60% of the width so copy stays visible in the preview pane. Export at twice those numbers for retina: a 600 x 200 px header becomes a 1200 x 400 px file placed with width="600" height="200".
Outlook on Windows uses the Word rendering engine and scales images by the system DPI setting, so a 1x export gets stretched and softened on a high-DPI display. Export at 2x and constrain it with the width and height attributes on the img tag; Outlook ignores CSS max-width, so those attributes are what actually control the size.
Gmail truncates a message and shows a "View entire message" link once the HTML passes about 102 KB. Linked images do not count towards that, but base64-embedded images do, which is one reason to host header images rather than inline them. Keep the header file itself under about 200 KB so it loads quickly on mobile data.
Prefer live text over an all-image header. Many recipients block images by default, so an image-only header shows nothing but alt text, and image-heavy emails are more likely to be filtered. Use an image for the visual and real HTML text for the headline and call to action.