Character counts are a rough guide at best. Inboxes truncate by pixel width, not character count, so "MMMMM" and "iiiii" are both five characters but take up wildly different space. This tool measures your subject line and email preheader in pixels, using each client's real interface font, against typical list widths for every client and device combination.
The limits differ more than most people expect. Apple Mail on iPhone is the most generous of the mobile clients at roughly 48 visible characters, while the Gmail app can cut off around 30. Apple Mail on iPad actually shows less than on iPhone because of its split-pane layout, and Outlook's desktop list is narrower than Gmail's. The safe target across every tested device is about 33 characters for the subject and 37 for the preheader.
The built-in email spam words checker flags all-caps, repeated punctuation, and common spam trigger words as you type. Everything runs client-side; your subject lines are never uploaded, logged, or stored.
The subject line is the short line of text recipients see in their inbox before opening an email, next to the sender name and above the preheader preview. It's the single biggest factor in whether an email gets opened, which is why testing how it displays, and where it gets cut off, matters more than most senders realize.
Strong preheader CTAs extend the subject rather than repeat it. Some proven patterns: "Your timeline is inside, two dates need your call", "Reply by Friday to lock your spot", "See the three changes before they go live", "Open for the full breakdown", "Your code expires tonight", "Here's what we'd do differently", and "The answer surprised us too". The common thread: each adds new information or urgency the subject didn't cover, in the first 37 characters.
Sentence case is the safer default for most email: it reads as personal and conversational, while title case can feel promotional and all-caps is a spam signal. Newsletters and B2C promos sometimes use title case deliberately. If you need to convert between formats, our Title Capitalization Tool handles AP, Chicago and sentence case instantly.
Around 33 characters survives untruncated on every major client and device; under 50 is a reasonable ceiling if you front-load the key message. The honest answer is that it varies by pixel width, not characters: the Gmail app cuts off around 30 characters while Gmail desktop shows roughly 70, which is exactly why this tool measures your actual text in pixels per client instead of quoting one universal number.
Technically, the email standard allows up to 998 characters in a subject header, so there's effectively no sending limit. The real limit is display truncation: mobile inboxes show roughly 30 to 48 characters depending on client and device, and desktop clients show around 50 to 70. Anything past the visible cutoff exists but is never seen.
The preheader is the grey preview text shown after or below the subject line in the inbox list. If you don't set one deliberately, email clients pull the first text found in your email body, which is how "View this email in your browser" ends up as the preview for so many campaigns. A written preheader is free space to extend your subject's message.
Classic triggers include "free!!", "act now", "urgent", "winner", "cash", "limited time", "click here", "guarantee", "risk-free", "buy now" and "congratulations", especially combined with all-caps or repeated punctuation. No single word gets an email blocked on its own; filters weigh patterns and sender reputation together. The checker built into this tool flags these as you type so you can judge the risk in context.
Front-load the key message in the first 30 characters, be specific rather than clever, write the preheader as a deliberate second line rather than leaving it to chance, and avoid all-caps and spam trigger words. Then test it: run it through this subject line checker on the mobile clients your audience actually uses, since that's where most truncation happens.
Indirectly. Studies disagree on an "optimal" length because they measure different audiences and email types, but truncation clearly hurts: a message cut off mid-thought can't do its job. The consistent finding is that what the visible portion says matters more than raw length, which makes checking where the cutoff lands more useful than chasing a magic character count.
Your text is rendered in the browser using each client's typical interface font, Roboto for Gmail, SF Pro for Apple Mail, Segoe UI for Outlook, and measured in pixels against typical inbox list widths for the selected device. Real truncation varies with screen size, settings and font rendering, so the preview is labeled as typical rather than exact, but it's far closer to reality than a flat character count.