Email Signature Size Guide: Dimensions, File Size & Specs

Email clients treat signatures differently than message body content. Most delete anything outside your signature block, including external CSS, JavaScript, and certain HTML attributes. Gmail removes style tags and limits width to around 600 pixels. Outlook strips scripts and resets font sizes if they're declared globally. Apple Mail removes float and positioning properties, which breaks multi-column layouts. Understanding what each client deletes helps you build signatures that survive the journey to your recipient's inbox.

Your file size directly affects stripping behavior. If your HTML file exceeds 102 KB, Gmail starts removing elements to compress the file. Outlook has a 30 KB soft limit before it flags the signature as bloated. These aren't hard failures—emails still arrive—but your design degrades. A logo embedded as base64 can push you over these limits fast. A 100x100 pixel logo at high quality can add 15-25 KB alone.

Certain HTML tags never survive. Style tags get deleted by Gmail, Outlook, and Apple Mail. Conditional comments that work in Outlook desktop fail in Outlook web. The align attribute, deprecated in HTML5, strips in modern mail clients. Background images in table cells render fine in Outlook but break in Gmail and Apple Mail. Your signature gets rewritten by mail servers using their own sanitization rules before reaching recipients.

Margins and padding work differently across clients. Outlook respects padding on table cells. Gmail collapses padding on nested elements. Apple Mail honors both but may add extra space depending on your container structure. A 10-pixel margin in Outlook becomes 0 in Gmail. The safest approach: use table cells with explicit width and height attributes instead of CSS margins.

Line height changes how text displays. Outlook doubles line height on some text if you set it globally. Apple Mail ignores line-height declarations entirely unless attached to specific text elements. Gmail respects line-height but treats it as advisory rather than fixed. This is why your signature might look stretched in one client and cramped in another. Testing across platforms is mandatory before hitting send to your list.

Gmail imposes stricter limits than most other mail clients. Your signature HTML must stay under 102 KB total. If you exceed this, Gmail truncates the file and displays incomplete signatures. This includes all embedded images as base64. A signature with three logos and a headshot photograph can easily surpass this limit if images aren't compressed first.

Gmail sets a maximum display width of 600 pixels. Any elements wider than 600 pixels get clipped or repositioned. Most Gmail users view on desktop at 800-1024 pixel viewport widths, leaving roughly 600 pixels for your signature after accounting for margins and scrollbars. Build your signature with a maximum table width of 580-590 pixels to leave breathing room on all devices. Elements that exceed this width render as truncated boxes that users must scroll to see fully.

Font sizes in Gmail require explicit pixel values. Relative sizes (EM, percentages) work but produce inconsistent results across browsers and devices. Use 11px to 14px for body text, 16px to 18px for headings. Gmail displays these consistently. Anything below 10px becomes difficult to read on mobile. Anything above 20px for standard text looks oversized and takes up excessive vertical space.

Image dimensions matter because Gmail renders images as inline by default. A logo that's 200x200 pixels displays at full size. If your logo is 400x400 pixels, it exceeds the 600-pixel container and creates layout problems. Most professionals use logos between 100x150 and 150x200 pixels. Headshots work best at 100x120 pixels. These sizes remain visible without dominating the signature.

Gmail strips background images entirely. Any image set as a background in CSS or table cell style attributes vanishes. Text laid over background images becomes unreadable. Use inline image elements instead of background properties. This also prevents your signature from breaking in Outlook and Apple Mail, which handle background images unpredictably. Store images as separate <img> tags with explicit width and height attributes.

Outlook desktop and Outlook web behave differently despite sharing the same name. Outlook desktop (Windows) respects table-based layouts and inline styles reliably. Outlook web (OWA) strips some inline styles and rewrites others. You must test in both versions because a signature perfect in desktop breaks in web.

Outlook desktop has a 30 KB soft limit. Signatures under 30 KB render without issues. Between 30-50 KB, images may fail to display or load slowly. Above 50 KB, Outlook begins compressing or removing embedded content. Outlook web has a 40 KB practical limit because it runs through additional filtering. File size matters more in Outlook than Gmail because Outlook applies stricter compression on embedded base64 images.

Pixel width in Outlook desktop should not exceed 600 pixels for the outer table. Outlook respects width attributes on tables as long as you use pixels, not percentages. Set your main table to width="600" or width="580" to account for padding. Percentages work but create unpredictable scaling depending on screen resolution and how users resize their compose window. Explicit pixel values give you control.

Outlook desktop honors margin and padding on table cells, unlike Gmail. Use cellpadding and cellspacing attributes instead of CSS margins. These work across all Outlook versions. Set cellpadding="10" to create space inside cells. Use border-spacing in CSS as a fallback, but always pair it with cellpadding because Outlook ignores border-spacing unless you also declare it in the style attribute.

Outlook web strips certain CSS properties. Position, float, and display values get removed. Inline-block displays as inline. Float causes elements to stack unpredictably. Use table structures instead of divs with CSS positioning. A three-column signature layout works perfectly as a three-cell table row. The same layout using divs and float fails in Outlook web. Test in both Outlook desktop (Windows) and OWA (web.outlook.com) before finalizing your signature. One passes while the other fails frequently.

Apple Mail prioritizes simplicity and ignores most decorative CSS. It strips position, float, and z-index properties. Padding works on table cells but not on divs. Apple Mail renders tables reliably but abandons CSS Grid or Flexbox. Build for Apple Mail using only basic HTML and inline styles on table elements. If your signature works in Apple Mail, it works on iPhone Mail and iPad Mail automatically because they share the same rendering engine.

Apple Mail's maximum display width is 600 pixels on desktop and 320 pixels on mobile. A signature wider than 600 pixels still displays but forces horizontal scrolling on desktop—a terrible user experience. On iPhones, anything over 320 pixels requires scrolling. Design with a max-width of 560 pixels for desktop compatibility and responsive behavior on mobile. Apple Mail doesn't support media queries or responsive design, so you can't create different layouts for mobile and desktop. One fixed width must work everywhere.

Proton Mail sanitizes HTML aggressively. It removes all script tags, event handlers (onclick, onload), and external resources. Embedded base64 images work fine. Inline styles on table elements are preserved. Proton Mail respects table layouts and font specifications. Your signature displays in Proton Mail identically to Gmail if it's built using basic table structures. Proton Mail doesn't strip HTML as severely as Gmail, making it more forgiving of complex layouts—but those layouts still shouldn't use forbidden properties like position or float.

Mobile clients (iPhone Mail, Gmail app, Outlook app on iOS) display signatures at 320-375 pixel widths. These clients don't reflow wide signatures—they either truncate or force horizontal scrolling. Test your signature on a real iPhone or using Chrome's device emulation at 375px width. If text is cramped or images look oversized, your signature fails the mobile test. Most people check email on phones. A signature optimized for desktop but broken on mobile sends a bad signal about your attention to detail.

Android mail clients (Gmail app, Samsung Mail, Outlook app) typically display at 400-450 pixels. Android respects table layouts better than iOS in some cases. The Gmail app on Android and iOS both use the same rendering engine, so if your signature displays correctly in Gmail.com, it displays correctly in the Gmail app. However, Outlook app on Android and iOS have differences. Test on actual devices when possible. Emulation tools like BrowserStack let you test on real phones remotely.

Every image in your signature gets converted to base64 and embedded directly in the HTML. This conversion increases file size by roughly 33% compared to the image's original file size. A 50 KB PNG becomes approximately 67 KB when base64-encoded. A 100 KB JPEG becomes 133 KB encoded. This is why image optimization before encoding matters critically. A single unoptimized image can push you over Gmail's 102 KB limit.

JPEG format works best for photographs and headshots. Use 70-80% quality compression. A headshot photo at 100x120 pixels with 75% JPEG compression typically weighs 8-12 KB, which becomes 10-16 KB when base64-encoded. PNG format works best for logos with transparency or solid colors. A 150x80 pixel logo with solid colors might be 3-5 KB as PNG, becoming 4-7 KB encoded. Avoid BMP and GIF formats entirely—they produce unnecessarily large files. WebP format isn't supported by Outlook or Apple Mail, so don't use it despite its smaller file size.

Image resolution should match display size exactly. Don't upload a 400x400 pixel image if you display it at 100x100 pixels. The extra pixels add file size without improving visual quality. Create images at the exact dimensions you'll use in your signature. A logo designed for 150 pixels wide should be exported at 150 pixels wide, not 300 pixels wide at 50% scale. This single optimization reduces file size by 75%.

Compress images aggressively. Use TinyPNG, ImageOptim, or similar tools to remove metadata and reduce color palettes without visible quality loss. Most images lose 20-40% of file size through lossless compression without any visible degradation. A 12 KB PNG becomes 8 KB. A 15 KB JPEG becomes 11 KB. These seemingly small savings add up when you have multiple images. Metadata (EXIF data, color profiles, thumbnails embedded in JPEG files) adds 1-3 KB per image and serves no purpose in signatures—remove it.

Image dimensions should follow these rules for consistency: logos 100-150 pixels wide, headshots 80-120 pixels wide, social icons 20-24 pixels wide. Larger dimensions look unprofessional and consume valuable vertical space. Smaller dimensions become difficult to see. Use square or nearly-square aspect ratios for headshots (1:1) and logos (3:2 or 4:3). This creates balanced visual weight. Avoid ultra-wide images or tall, narrow images—they disrupt layout and waste space. Always declare width and height attributes on every <img> tag so mail clients can allocate space before images load, preventing layout shift.