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About this tool
Work out export size, retina dimensions, minimum readable text size on mobile and a file-weight budget for an email header image.
An email header image has to be exported at twice its display size for high-density screens, then survive being scaled down again on a phone — and this planner does that arithmetic. Enter the body width (600px is the long-standing standard), an aspect ratio and a phone viewport, and it returns the export dimensions, the size the header actually renders at on the phone, the smallest text you can draw inside the image and still have it read at 14px, and whether your file-weight budget is realistic for the format you picked. It also runs the dark-mode and Outlook checks that catch most broken headers.
Open Newsletter Header Image Planner 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.
Shows the real render size on a phone and the design text size needed to survive it.
Bytes-per-pixel figures for JPEG, PNG-8, PNG-24, GIF and WebP with a suggested smaller export.
Outlook's Word rendering engine, first-frame-only GIFs and missing WebP support are all flagged.
For a 600px-wide email, export at 1200px wide for 2x screens and let it display at 600px. A 3:1 banner is 600 x 200 displayed, so 1200 x 400 exported. Heights above roughly 300px push your first line of copy out of the preview pane.
Because the image is scaled to fit the phone. A 600px-wide header inside a 375px viewport with 20px padding each side renders at 335px — about 56% of its design size — so 14px text in the design lands near 8px on screen. Design in-image text at 26px or larger, or keep the headline as live HTML text.
Avoid dark artwork on a transparent background, since several clients repaint the message background dark and the artwork disappears. Give logos an opaque background or a light outline, and keep headlines as live text so they can be restyled rather than baked into pixels.
No. Gmail clips a message when the HTML source passes roughly 102 KB, and image bytes are not part of that figure. Large images still hurt load time and data use, so a header under about 200 KB is a sensible target.