Email subject line tester
Preview your subject line and preheader the way an inbox shows them, on a phone and a desktop, with every check explained in plain words. Compare A/B versions side by side. No signup.
Cut-offs vary by app, screen, font size and settings. These previews show the general shape, not an exact device.
| Subject line | Score | Chars | Words | First ~35 chars on a phone | Flags |
|---|
4 steps, no signup
- 01
Type your subject line
Add the sender name and preheader too: inboxes show all three together, so they should be judged together.
- 02
Check the inbox previews
See roughly where a phone and a desktop inbox cut the line, and what the preheader fills in after it.
- 03
Read the flags
Length, ALL CAPS, punctuation, emoji, personalization tokens and spam-trigger words are each explained, not just scored.
- 04
Compare A/B versions
Switch to A/B compare and paste several subject lines, one per line, to see them side by side.
What this email subject line tester does
The email subject line tester shows your sender name, subject and preheader together, the way an inbox lists them, in a phone-width and a desktop-width row. Then it runs a set of plain checks and explains each one: length, how much survives on a phone, capital letters, punctuation, emoji, merge tags and words that often read as spammy.
Many subject line testers ask for an email address before showing results, or give a score with no explanation. This one shows every check and the points it moved, previews the preheader (which most testers skip), and lets you line up A/B versions in one table.
How the score works
| Check | Max | Full points when |
|---|---|---|
| Length | 25 | Roughly 20–50 characters and 3–9 words |
| Front-loading | 15 | The first ~35 characters end on a whole word and carry meaning |
| Capitals | 15 | No fully capitalised words longer than four letters |
| Punctuation and symbols | 15 | At most one ! or ?, no repeats, no $$$ or %%% |
| Emoji | 10 | None or one |
| Spam-style wording | 15 | No phrases from the short list below |
| Preheader | 5 | Set, and not a repeat of the subject |
Personalization tokens are detected and shown, but they neither add nor remove points: whether they help depends on your data.
Where inboxes cut subject lines
No mailbox provider publishes a fixed character limit for how much of a subject line is shown. The app simply fills the available width, so what you see changes with the device, orientation, font size and display density. On a typical phone in portrait, expect somewhere around the first 30 to 45 characters; desktop inboxes show more but put the preheader on the same line. Those ranges are rough rules of thumb, not measurements, which is why the previews use real widths and ellipses rather than a character count. The practical rule holds either way: put the words that matter first.
About spam-trigger words
Lists of "spam words" are everywhere, and most overstate their effect. Mailbox providers decide using many signals, chiefly whether you are authenticated, your sending reputation, and how often recipients mark you as spam. Google's sender guidelines for bulk senders centre on authentication (SPF, DKIM, DMARC), easy unsubscribes and keeping reported spam rates low, and ask that the subject line accurately represent the message (Google email sender guidelines). In the US, the FTC's CAN-SPAM guidance says the subject line must not be deceptive (FTC).
So the tool flags a short list of phrases that tend to read as pushy or misleading to people: "act now", "100% free", "risk-free", "guaranteed", "cash", "winner", "no cost", "urgent", a fake "RE:" or "FWD:", and similar. A flag is a reason to reread, not a prediction that the email will land in spam. This page quotes no open-rate statistics: the widely shared ones trace to vendor data that has not been published for checking.
Checks at a glance
| Flag | Why it matters |
|---|---|
| ALL CAPS words | Reads as shouting; short acronyms like SEO or USA are allowed |
| !!! or ?! | Repeated punctuation looks like an advert |
| Several emoji | Render differently across apps; Google advises against using them to imitate graphics |
| Merge tags | Formats like {{first_name}}, *|FNAME|*, %FIRSTNAME%, [Name]: check a fallback is set |
| Fake RE: / FWD: | Misleading unless the email really is a reply or forward |
| Empty or repeated preheader | The inbox fills the gap with the first text in your email |
Tips for subject lines that get opened
- Say what is inside. A specific promise beats a teaser that hides the point.
- Front-load. Assume only the first few words survive on a phone.
- Write the preheader as part two. Do not repeat the subject; extend it.
- Test one change at a time. Use A/B compare to draft, then your email platform to send a real split test.
- Measure clicks, not just opens. Some apps load images automatically, which inflates open counts.
Privacy
Everything runs in your browser. Nothing you type is sent anywhere. A/B drafts are saved only in your own browser storage.
Last checked and updated: . Everything on this page runs in your browser; nothing you enter is sent anywhere.