Quick answer: A subject line tester scores your email subject line on the factors that affect opens — length, word count, spam-trigger words, personalization, numbers and emoji — and shows how it will truncate on mobile. Aim for roughly 28–50 characters, 3–9 words, no spam words, and one clear hook.
Email subject line tester
Type or paste a subject line — it is scored live.
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Enter a subject line
Mobile inbox preview (~33 chars)
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Great subject line, but landing in spam? Bluey Email ships with deliverability tools and send-based pricing on every plan. See the full feature set.
How the subject line score works
The score blends the factors email teams optimise for: length (around 28–50 characters reads fully on most clients), word count (3–9 words), spam-trigger words (phrases like “free”, “act now” and “100%” hurt deliverability), personalization and numbers (both lift opens), and over-punctuation or ALL CAPS (which trip spam filters). It is a directional guide, not a guarantee — always A/B test against your own audience.
Frequently asked questions
What is the ideal email subject line length?Roughly 28–50 characters, or 3–9 words. Mobile inboxes cut off around 33 characters, so front-load the hook.
Which words trigger spam filters?Money and pressure words like “free”, “guarantee”, “act now”, “risk-free”, “cash” and “100%”, plus excessive punctuation and ALL CAPS. Use them sparingly.
Do emojis help subject lines?Sometimes — a single relevant emoji can lift opens, but too many look spammy. Test with your audience.
Is this tool free?Yes — free and private. Your subject line is analysed in your browser and never sent anywhere.