The Substack Newsletter Header size (1200×600px)
On Substack, a header image sits at the top of a post and is the visual that gets pulled into the email version, the web archive, and the social preview when a piece is shared. A 1200x600 pixel image in a 2:1 ratio is a dependable size: wide enough to look like a proper banner across the reading column and social cards, and light enough to load quickly in an email client where large images can be slow or clipped.
The main thing to design around is how many contexts the same header appears in. In the email it spans the content width, on the web it headlines the article, and as a link preview it may be cropped toward a wider or squarer card by whichever platform renders it. Keeping any title text or key subject within the central area means the header still communicates whether it is shown full 2:1 or trimmed by a social unfurl, and it keeps the image from feeling empty in the reading view.
Sizing to the native 2:1 shape also avoids the stretching that happens when a portrait photo or square graphic is forced into the header slot. Because newsletters live and die on that first impression in a crowded inbox, a clean, on-brand header helps a reader recognize your publication at a glance. This tool resizes 100% in your browser with no upload, it is free, and it preserves the 2:1 aspect ratio. Substack can change how it crops headers and previews, so verify the current behavior before you send a big issue.