Subject Line Checker — Email Preview & Preheader Tester

Preview how your subject line and preheader get cut in Gmail, Apple Mail, Outlook and Yahoo — before you hit send.

Subject Line Checker — see your email subject line and preheader exactly as Gmail, Apple Mail, Outlook and Yahoo will cut them, before you hit send. Most email marketers test subject lines the slow way: send yourself a test, check your phone, check desktop, open a different app, realise it's too long, fix it, start over. This extension replaces all of that. Type your subject line and preview text (preheader) straight into the extension — or select any text on a page and it auto-fills — and you instantly get a realistic inbox preview for every major client and device, plus a clear verdict for each: ✓ Fits — you're good to send ⚠ Risky — it may get cut on some devices ✕ Truncated — it will definitely be cut WHY IT'S MORE ACCURATE THAN A CHARACTER COUNTER Most subject line tools just count characters. Inboxes don't work that way — they cut by pixel width. An "l" is about a quarter the width of a "W", so "43 characters" tells you nothing on its own: illillillillillillillillillillillillillilli — 43 characters, fits easily WWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWW — 43 characters, cut off Subject Line Checker measures the actual rendered width of your text in each client's font, so narrow text gets more room and wide text gets less — and emoji are measured at their true width too. That's why the preview matches what your subscribers really see. WHAT IT COVERS Gmail — mobile and desktop Apple Mail — mobile (iPhone) and desktop Outlook — mobile and desktop Yahoo Mail — mobile WHO IT'S FOR Email marketers, newsletter writers, lifecycle and CRM folks, founders, and anyone who sends campaigns and cares how they land in the inbox. It works alongside whatever you draft in — just select your subject line on the page and check it. HOW TO USE IT 1. Click the icon in your toolbar 2. Enter your sender name, subject line and preview text 3. See results instantly across every supported client and device 4. Fix anything Risky or Truncated before you send Tip: select a subject line anywhere on a page before opening the extension and it fills in automatically — handy when you're drafting inside your ESP. Free. No account. No sign-up. And nothing leaves your browser: the extension makes no network requests and collects no data. Prefer not to install anything? There's a free web version: https://david.sendgoodemails.com/subject-line-checker/ Built by an email marketing professional who got tired of sending himself test emails to a Huawei just to check whether the subject line fit.