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An email preheader checker counts the characters in your subject line and preview text, then compares your preheader against the approximate points where Gmail, Outlook and Apple Mail on iPhone typically start cutting it off in the inbox list.
The preheader, sometimes called preview text or a Johnson box, is the snippet of text that appears right after or below your subject line in someone's inbox, before they ever open the email. Left unset, most email clients grab the first line of visible text in your email body, which is often an unhelpful "view in browser" link or an image alt tag nobody meant to be read. Setting a deliberate preheader, usually through hidden preview text at the top of your email template, turns that wasted space into a second sales line working alongside your subject.
Every email client renders the inbox list with its own column width, font size and sender name length, so the number of preheader characters actually visible is only ever an estimate rather than a fixed rule. Gmail on desktop and Apple Mail on an iPhone tend to show noticeably less preview text than Outlook on desktop, and all three shift depending on screen size, zoom level and how long the sender name is. That is why this checker shows approximate cutoffs for a few common inboxes rather than promising an exact number that will hold everywhere.
A useful working range is roughly 40 to 100 characters. Much shorter than that and you are leaving inbox space unused that could reinforce your subject line. Much longer and the second half is likely to be cut off before it adds anything, especially on mobile screens where the visible window is smallest. Within that range, the highest-value preheaders add something the subject line did not have room for, like a specific number, a deadline or the actual offer, rather than simply repeating the subject in different words.
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