Email Subject Line Tester

Instant heuristics on any subject line: mobile truncation, filter-bait vocabulary, caps and punctuation problems — everything runs in your browser, nothing is stored.

Free · no signup · last reviewed August 29, 2026

Heuristics, not prophecy — the best subject line states plainly what the email contains for a list that trusts the sender. Nothing you type here leaves your browser.

Methodology & honest limits

How it works: plain heuristics applied in your browser — character length against the ~35-character mobile truncation point, a vocabulary list that correlates with filtered mail, caps/punctuation patterns, and a few positive signals (questions, numbers). Nothing you type is sent or stored anywhere.

Honest limits: no tool can predict opens — open rates depend mostly on who’s sending and whether past emails were worth opening. These checks catch the mechanical mistakes; the strategic rule stands on its own: state plainly what the email contains.

Keep going

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This is the work. Want it done?

Everything here is free to use yourself — that’s the point. If you’d rather a senior marketer just handle it, in your accounts and your name: 20-minute call, no pitch deck.

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