Email signature size and dimensions

An email signature is the block of contact details, branding, and sometimes a logo or headshot that sits at the bottom of an email. Getting its size right matters because email clients render images very differently from a browser, and a signature that looks correct in one inbox can overflow, blur, or break entirely in another. Two numbers decide whether it holds up: the pixel dimensions of the image and the file size in kilobytes.

Recommended email signature dimensions

The single constraint that drives every other number is the email content column, which is conventionally 600px wide. Most templates and clients render the body at or near that width, so a signature wider than 600px forces horizontal scrolling on desktop and shrinks unpredictably on mobile. These dimensions stay inside that column and render consistently across clients:

Sharpness depends on one more step. Export images at twice the display size and constrain them down in the HTML. A 100px headshot should be a 200px image displayed at 100px, because many recipients read mail on high-density (Retina) screens that render a 1x image soft. Set the intended size with width and height attributes on the image so the layout holds even before the image loads.

File size and format

Image weight affects more than load time. Gmail clips any message larger than 102KB, hiding the rest behind a "View entire message" link, and a heavy signature on a long thread can push a message past that limit. Keep each image under 50KB and the whole signature under 100KB.

Format choice comes down to what the image contains. PNG is right for logos, icons, and anything with text or a transparent background, because it keeps edges crisp and supports transparency. JPG is right for photographs and headshots, where its compression produces a much smaller file than PNG at the same visual quality. SVG is tempting for logos, but most email clients strip it, so it is not a safe choice for sent mail.

Lighter images also help deliverability. A high image-to-text ratio is one of the signals spam filters weigh, and oversized images slow rendering on mobile connections. The same discipline that keeps a signature crisp also keeps it from working against the inbox, which is part of the broader case for treating image weight as a deliverability concern.

How signatures render across clients

There is no shared rendering engine across email clients, so a signature has to survive several different ones. The three that cover most recipients are Gmail, Outlook, and Apple Mail, and each has a quirk worth designing around.

• Gmail clips messages over 102KB and strips most embedded styles, so a light, image-hosted signature with inline styling renders most reliably.
• Outlook on Windows uses Microsoft Word's rendering engine, which ignores many CSS properties and can add unwanted spacing around images. Explicit width and height attributes (not CSS sizing) keep images at the intended dimensions here.
• Apple Mail renders close to a real browser and scales images for Retina displays, which is exactly why exporting at 2x matters: a 1x image that looks fine in Outlook can look soft here.

Across all three, the most conservative choice renders everywhere: fixed pixel dimensions on hosted images, inline styles, and a layout that does not depend on CSS the stricter clients discard.

Common mistakes

Most broken signatures come down to three avoidable errors, all of them about size in one form or another.

• Oversized image files. A multi-megabyte logo exported straight from a design tool slows loading, gets the message clipped in Gmail, and counts against spam filters. Compress every image before it goes into the signature.
• A signature wider than the content column. Anything past 600px forces horizontal scrolling on desktop and gets scaled down on mobile, often shrinking the text to an unreadable size.
• No mobile consideration. Fixed-width tables and tiny tap targets that work on a desktop monitor fall apart on a phone, where more than half of email is now read. Keep the layout simple and the touch targets reasonable.

FAQ

Bottom line

Two constraints cover almost every case. Keep the signature within the 600px content column so it never forces horizontal scrolling, and keep every image light and exported at 2x so it loads fast, stays sharp on Retina screens, and never trips Gmail's 102KB clip. Hosted PNGs for logos and icons, hosted JPGs for photos, and fixed pixel dimensions on each one will render consistently across Gmail, Outlook, and Apple Mail.