Free tool
What your subject line looks like in an inbox
This checks a subject line against the points where mobile and desktop inboxes cut it off, counts filter-bait words from a fixed list, and flags capitals and repeated punctuation. It also checks whether your preview text repeats the subject instead of extending it, which wastes the second line.
Last reviewed 27 August 2026
Result
Subject length
49 characters
9 words, 55 characters of preview text
Filter-bait words found
0
Nothing on the list matched
What each inbox shows
| Where | Cut at | What is visible |
|---|---|---|
| Mobile | 35 chars | Onboarding takes 14 days. Here is ... |
| Desktop | 60 chars | Onboarding takes 14 days. Here is where it breaks |
| Preview text | Varies | Three fixes we made, and the one that actually mattered |
Flags
- Cut on mobile at 35 characters. A reader on a phone sees "Onboarding takes 14 days. Here is" and nothing after it, so the meaning has to complete inside that.
- 9 words in a cold subject line. Cold outbound survives on looking like a message rather than a campaign, and short lines look like messages.
The main issue is truncation. Nothing here reads as promotional, so the fix is ordering: move the specific part to the front so it survives the cut.
How this works
What the numbers mean.
- 01Subject lines are cut, not shortened. Mobile inboxes show roughly the first 35 characters and desktop clients roughly the first 60, so the first few words carry the whole message. Both figures are editable because every client differs.
- 02Filter-bait words are counted from a fixed list of terms that appear in filter heuristics. Modern filters weight sender reputation and recipient engagement far more heavily than any word list, so treat a hit as a signal that the line reads like marketing, which is the more useful warning anyway.
- 03Capitals and repeated punctuation are flagged because they are the two formatting habits that read as promotional at a glance, whatever any filter thinks of them.
- 04Preview text is checked against the subject. Repeating the subject in the preview wastes the only other line you are given.
Assumptions and limits
- No open rate is predicted. Published open rate benchmarks are drawn from a single vendor's customer base, and since mail clients began prefetching images on the recipient's behalf, an open is a weaker measurement than it used to be.
- Truncation points vary by client, device, font size, orientation, and how long the sender name is. The 35 and 60 character defaults are conventions, which is exactly why both are inputs.
- The word list is a heuristic. A subject line can contain none of these terms and still land in spam because of domain reputation, and it can contain one and arrive fine.
- Nothing here checks authentication. If SPF, DKIM, and DMARC are not set up correctly, no subject line will save the send.
Questions about this tool
How long should a subject line be?
Short enough that the meaning completes inside roughly the first 35 characters, because that is about what a phone shows. Longer lines are not penalised, they are simply cut, so the real question is whether the part that survives still says something. Front-load the specific noun and the number.
Do spam trigger words still matter?
Less than they did, and less than most checkers imply. Filtering is now dominated by sender authentication, domain reputation, and how recipients have treated your previous mail. The words still matter for a different reason: they signal to a human that this is a campaign, and that judgement happens faster than any filter runs.
Why does this not predict an open rate?
Because every published benchmark is measured on one vendor's customer base, in one set of industries, at one point in time. Applying it to your list is borrowing someone else's history. Image prefetching by mail clients also inflates reported opens, so the underlying measurement is less solid than the confident percentages suggest.
What should the preview text do?
Extend the subject, never repeat it. Together they are two lines of a single sentence: the subject makes the claim and the preview adds the detail that makes it worth opening. If the preview is unset, most clients fall back to the first line of the email, which is usually a greeting and a wasted line.
This tool is free and there is nothing to sign up for. If you would rather have the work done than calculate it, that is what Zway does.