About SubjectLines

SubjectLines is a free tool site for the one line of your email that decides whether the rest gets read. It runs every candidate through the same ten visible checks — length band, word count, concrete numbers, curiosity frames, second person, power words, emoji discipline, capitalization, exclamation stacking and a 175-word spam trigger list — then shows where Gmail, Outlook, Yahoo and Apple Mail would cut it off.

The tools are deterministic on purpose. The tester scores what you paste; the generator fills proven template shapes with industry nouns (same inputs, same ten lines — bookmarkable); the spam checker ranks 175 words by severity; the length analyzer renders each client's cut; and the inbox preview shows the subject-plus-preheader compound as subscribers actually see it. Nothing you type is uploaded — all math runs in your browser.

Where the numbers come from

Client truncation figures (~35 characters on the Gmail mobile app, ~45 in Outlook desktop, ~60 Yahoo, ~65 Apple Mail, ~70 Gmail web) are engineering approximations drawn from public client behavior — devices, font settings and zoom shift them, which is why every tool labels them approximate. The spam list is the classic trigger-word canon, split honestly into 107 words with no legitimate subject-line use and 68 ordinary commerce words that turn risky when stacked. Benchmark open-rate bands come from published industry data, with the standing caveat that Apple Mail Privacy Protection has inflated raw opens since 2021.

What this site is not

Not an inbox-placement guarantee — sender reputation, authentication (SPF/DKIM/DMARC) and reader engagement outrank vocabulary. Not a substitute for your own data: when the checklist and your list's behavior disagree, your list wins. And not an AI text generator — the generator is a transparent template engine, which is why we can show its entire scoring.

SubjectLines is part of a small independent network of tool sites, including ring size tools, painting cost calculators and trade calculators. Questions or corrections: contact page.