Email signature size limits: Gmail, Outlook and image sizes explained

Signatures fail in two ways when they get too big: the email client refuses to save them, or they arrive slow and clipped. Here are the limits that matter in practice.

Gmail: 10,000 characters

Gmail rejects signatures longer than 10,000 characters. That limit applies to the HTML source, not the visible text. Inline styles, table markup and long tracking links all count. A signature with a headshot, a logo, five social icons, a banner and a disclaimer can reach it quickly.

Ways to stay under:

• Prefer a compact template. Nested tables are necessary, but deep nesting adds up.
• Remove styles that repeat defaults, such as margin:0 on every cell.
• Shorten the disclaimer. Two or three sentences are usually enough.
• Use one banner, not several.

Sendoff shows a live character counter and warns you before you reach the limit.

Outlook

Outlook does not publish a hard character limit for signatures, but very large signatures slow down composing, and classic Outlook converts signatures to Word’s format, which can bloat them further. Keep the same discipline you would for Gmail.

Gmail message clipping

Separately from the signature limit, Gmail clips messages larger than about 102 KB of HTML and shows a “View entire message” link. A heavy signature repeated in a long reply thread can contribute to clipping, hiding the latest content.

Image sizes

• Width: keep the whole signature at or under 600 pixels wide. Most signatures look best between 400 and 500.
• Headshots: display at 70–100 pixels; save the file at twice that for sharp results on retina screens.
• Logos: display at 100–160 pixels wide.
• Banners: display at 400–600 pixels wide and keep the file under about 100 KB.
• Social icons: 16–24 pixels.

Always set width and height attributes on images. Outlook relies on them.

Plain-text fallback

Some recipients read email as plain text. A good signature still makes sense when the HTML is stripped: your name, title and phone number are real text, not part of an image.