What’s the safe width for most clients?

Set your email width without thinking about it and you might end up with content clipped in Outlook's reading pane or text that requires horizontal scrolling on an iPhone. The width question matters more than it seems because different clients display email in different containers, and what looks fine in a full browser window may look cramped or broken in a narrow preview pane.

The industry standard is 600px. That number has held up for years because it fits within Outlook's preview pane, renders cleanly on most desktop clients, and scales down acceptably on mobile when you set your outer container with width: 100%; max-width: 600px;. Most email service providers use 600px as the default in their template builders, which is why it became the baseline in the first place. Email design best practices consistently point to 600px as the starting point for new templates.

Some senders push to 640px or 700px, and for most modern clients that's fine. Gmail on desktop and Apple Mail handle wider layouts comfortably, and if your audience skews toward those clients, you have more room to work with. But the further you go past 600px, the more risk you introduce in older Outlook versions and narrow preview panes. A template that looks great at 700px in Apple Mail might have its right edge cut off in Outlook 2019. Email client rendering differences are the main reason 600px remains the conservative consensus.

The right width for your list depends on your audience's email clients. If you've got a B2B list where most subscribers use Outlook at work, stick to 600px. If you're sending to a consumer audience who mostly reads on iOS, 640px is unlikely to cause problems. Check your ESP's analytics if it tracks email client usage, or look at device data from past campaigns. Testing across clients with a tool like Litmus will show you exactly how your chosen width renders before you commit to it.