Email Header Image Sizes — Mailchimp, Klaviyo, Kit, Constant Contact & Substack

Where 600px Comes From — and Why No One Owns It

There is no standards body for email width. 600px is convention, and the convention has a cause: Outlook’s three-pane reading view on a 1024x768 monitor left roughly that much horizontal room, and nobody has had a reason to move since. What that means in practice is that the number is remarkably consistent across platforms even though none of them can point at an authority for it.

Mailchimp states it directly: Mailchimp templates are designed to be no greater than 600px wide to fit the display capabilities of most email clients. Klaviyo makes it the default and warns you off changing it — Klaviyo doesn’t recommend increasing the width of your template unless this is required for a more advanced layout. Constant Contact says its templates are about 600 pixels wide. Kit never says it out loud at all; the figure appears only as max-width: 600px inside the CSS of its default templates.

So four platforms agree on 600px through four different degrees of commitment, and none of them tells you how tall a header should be. Height is a design decision, not a spec. If you want a starting point, Substack is the only platform that has made one: 1100x220 is a 5:1 banner.

Mailchimp Header Image Size

The most-quoted Mailchimp number — 600px — is the width of the template shell, not the width of your image. Mailchimp publishes two different content widths depending on which builder you are in, and neither of them is 600.

• New builder: 660px content width. Upload between 660 and 1320px wide. The 1320 is the 2x retina export.
• Legacy builder: 564px content width. Upload between 600 and 1200px wide. Mailchimp’s own guidance: it’s best to use images with a range of 660-1320 pixels in the new builder and 600-1200 pixels in the legacy builder.
• 1 MB is a hard cap, not a suggestion. Mailchimp recommends a maximum file size of 1MB for images, and Content Studio enforces it — you can upload images up to 1MB each. Non-image files get 10MB. This is the tightest image ceiling of the five platforms here by an order of magnitude.
• No header height exists. Mailchimp publishes a full-width image width and stops. The 600x200 and 1200x400 figures circulating in guides appear in no Mailchimp document.
• JPG, PNG or GIF, in RGB. Mailchimp is explicit that CMYK doesn’t render accurately online, and that PDF, PSD and AI files cannot be used. 72dpi is generally sufficient for the web, but isn’t required.

Klaviyo Header Image Size

Klaviyo publishes the most complete image guidance of the five, and it is the only one that bounds the height — not as a header spec, but as a ceiling for full-width images generally.

• 600px default template width, adjustable. Klaviyo asks you not to raise it, and adds that if you adjust the width above 600px we also recommend conducting robust testing.
• Upload 600–1000px wide, under 2000px tall. Klaviyo’s wording: keep your images between 600 and 1000 px in width and less than 2000px in height for full-width images.
• 10 MB maximum, 1 MB recommended. The maximum image file size supported by Klaviyo is 10MB, but we recommend using images that are 1MB or smaller, as larger images can load slowly. Uploads through the Images API are capped at 5 MB.
• Retina is official policy: insert an image 2x the desired size and then reduce the image size in your template.
• WebP is accepted but converted. Klaviyo supports JPEG, PNG and GIF, and accepts WebP by converting it to PNG — except that conversion is not supported for animated .webp files. Animated GIFs must be under 5 MB.

Kit (formerly ConvertKit) Header Image Size

ConvertKit rebranded to Kit in 2024, and convertkit.com now forwards to kit.com. It is also the platform with the least to say about images, which is a design position rather than an oversight: Kit’s default templates are plain-text-forward and ship no header image block at all.

• There is no header slot to size. Kit’s default templates render a single content placeholder. Images go inline in the body, not into a dedicated hero.
• ~600px, findable only in the CSS. The default Modern and Classic templates carry .message-content { max-width: 600px }, and images inside are fluid at max-width: 100%. Kit never states a width as a spec.
• 10 MB, recommended rather than enforced: a maximum file size of 10 MB is recommended — that file size is the maximum allowed by many email clients.
• No published formats, widths, or retina guidance for email images. Kit lists uploads by category (“Images”, “PDFs”, “Documents”) and never enumerates extensions.
• Do not borrow Kit’s form dimensions. Kit does publish 1920x1080, 1080x1920 and 1000x1000 — but that article is scoped to forms and landing pages, not email. Those numbers are wrong for a header image.

Constant Contact Header Image Size

Constant Contact is the one platform where uploading a bigger file actively costs you resolution, because it resizes on the way in. This puts its retina advice and its upload pipeline on a collision course, and the collision lands exactly at 1200px.

• 1200px wide is the ceiling, enforced silently. Constant Contact compresses any image that’s over 1200 pixels wide, reducing the image’s height proportionally. Upload a 1600px master and you get a 1200px image back.
• 1200px is also the retina target. Constant Contact recommends an image at least 600 to 650 pixels wide for a full-span slot, and separately advises that the industry-standard recommendation is to make an image twice the size (2x) of the template maximum. Twice 600 is 1200 — the compression threshold. Land on it exactly; going over gains nothing.
• Tall images get cut twice. If an image’s height is greater than its width, then it’ll be compressed to 1800 pixels — and separately, older versions of Outlook may crop images taller than 1728 pixels.
• 20 MB upload cap, 1 MB target. The maximum supported file size is 20MB, but Constant Contact asks you to try keeping your images under 1MB each. Library storage runs to 25 GB depending on plan.
• JPG, PNG, GIF. TIFF and BMP are not supported. 72 PPI is recommended, not required.

Substack Email Banner Size

Substack is the only platform in this list that publishes a dimension for the image at the top of the email itself. It is worth being precise about which image that is, because Substack has several and they are easy to conflate.

• Email banner: 1100 x 220, transparent background. Substack’s wording is recommended 1100 x 220 px with a transparent background, but could be taller — an unusually permissive spec, and a 5:1 letterbox.
• That is not the post hero. The image at the top of a post is a different asset at a different size. See Substack image sizes for the post hero, section headers and the rest.
• Publication logo: at least 256 x 256, also transparent. It sits in the email header alongside the banner.
• Cover image: at least 600 x 600.
• Formats: AVIF, GIF, JPG, JPEG, PNG and WebP. Substack accepts the widest format range here — and publishes no image file size limit at all.

Responsive Email Image Best Practices

Email is not the web. The rendering engine behind Outlook on Windows is Microsoft Word, and it ignores most of what you would reach for in a browser. These are the rules that actually govern whether your header arrives looking like the thing you designed.

• Export at 2x, constrain with the HTML width attribute — not CSS. Outlook on Windows does not support CSS widths and heights; if you omit the width attribute it will display your image at its full natural size. A 1200px header meant for a 600px column needs width="600" on the <img> tag itself. This is also why max-width is not a reliable substitute in Outlook.
• Gmail clips the whole email past ~102KB of code. The threshold covers HTML and CSS weight only — the 102kb limit does not include images — but a clipped email hides everything below the fold behind a “View entire message” link, including your unsubscribe footer. Tracking links injected by your platform count toward it, so leave headroom.
• Assume the header may not load, and write alt text that stands alone. Outlook on Windows blocks images by default. Gmail displays them by default and has since 2013, routing them through Google’s image proxy; since March 2024 it auto-displays images from trusted senders. Gmail renders styled alt text; Outlook renders plain alt text only.
• Transparent PNG logos disappear in dark mode — and the platforms disagree about it. Clients recolor the background behind your image without touching the pixels inside it, so a dark logo on transparency vanishes against an inverted background. Klaviyo’s advice is blunt: use image files that include background colors, rather than using a transparent background, and add a light-colored shadow to a dark logo with a transparent background. Substack, which controls its own email chrome, recommends transparency instead. Follow whichever platform is actually sending.
• Prefer live text to text baked into the header. Klaviyo again: use text wherever possible, rather than using images that contain text. This allows full color inversion (plus, it’s better for accessibility). A header that is one flat image of words is unreadable when images are blocked and uninvertible in dark mode.
• Background images need VML in Outlook. The Word engine ignores CSS background-image. A header built as a background with text on top needs Outlook-only VML in a conditional comment, or it renders as bare text on white.
• Keep the file near 1 MB regardless of the ceiling. Mailchimp enforces 1 MB. Klaviyo and Constant Contact both allow far more and still recommend 1 MB. The permissive limit is the one you should ignore.

Frequently Asked Questions