Free Online Email Subject Line Tester
Score email subject lines on spam triggers, length, and mobile preview
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Why use Email Subject Line Tester
- Email marketers can catch spam-trigger words before sending a campaign, preventing deliverability issues that waste an entire send.
- The mobile inbox preview reveals truncation problems that desktop-only testing misses, ensuring your hook appears on every screen size.
- Sales teams standardize outreach quality by running every sequence subject through the scorer to maintain consistently high open rates.
- Newsletter editors get instant, specific feedback on what to fix rather than guessing why open rates dropped after a send.
- Founders testing cold email variations can compare scores side by side to pick the strongest subject before committing to a campaign.
How it works
The scorer evaluates your subject line across seven checks. First, it measures character count against the 30-60 character ideal range and flags anything over 80 as too long. Second, it checks whether the text fits within the roughly 40-character mobile preview window. Third, it scans against a list of 35+ known spam-trigger words and phrases, penalizing more heavily when multiple triggers appear together. Fourth, it calculates the ratio of uppercase letters to total letters, flagging subjects where more than 40% of letters are capitalized. Fifth, it detects repeated or excessive punctuation marks like multiple exclamation points. Sixth, it searches for engagement boosters including personalisation tokens, numbers, emojis, urgency language, and curiosity triggers such as questions. Seventh, it checks word count against the 6-10 word sweet spot. Each check adds or subtracts from a base score of 100, clamped to 0-100, which maps to a letter grade from A+ to F.
About this tool
Test and score your email subject lines before you hit send with this free online email subject line tester. Paste any subject line and get an instant score from 0 to 100 along with a letter grade, detailed feedback on length, spam triggers, mobile preview truncation, capitalization, punctuation, and engagement signals. The scorer checks your text against a database of 35+ known spam-trigger words and phrases that email providers like Gmail, Outlook, and Yahoo flag during filtering. It measures character count against the 30-60 character sweet spot that research shows delivers the highest open rates, and warns you when your subject exceeds the roughly 40-character preview width on iPhone and Android lock screens. Capitalization analysis catches ALL CAPS shouting that both readers and spam filters penalize, while punctuation checks flag repeated exclamation marks and question marks that reduce credibility. On the positive side, the engine detects engagement boosters like personalisation tokens, numbers and statistics, emoji usage, urgency cues, and curiosity triggers such as questions or intriguing word choices. A live mobile inbox preview shows approximately how your subject will appear on a phone screen, so you can see what gets cut off before sending. Email marketers use this tool to A/B test subject variations side by side, picking the version with the highest score. Newsletter editors paste their drafts to catch spam triggers that would tank deliverability. Founders writing cold outreach emails check that their subject stays under the mobile truncation line and includes at least one engagement signal. Sales teams run every campaign subject through the checker to maintain consistently high open rates across sequences. All scoring runs entirely in your browser with no data sent to any server, so confidential campaign details stay private.
How to use Email Subject Line Tester
- Type or paste your subject line. Enter your email subject line into the text input at the top of the tool. Scoring begins instantly as you type.
- Review your score and grade. Check the overall score (0-100) and letter grade displayed prominently above the detailed breakdown.
- Read the per-check feedback. Review each check - length, mobile preview, spam triggers, capitalization, punctuation, and engagement boosters - to see what passed, what needs attention, and specific suggestions.
- Check the mobile preview. Look at the simulated mobile inbox preview to see approximately how your subject will appear on a phone screen, including where it gets truncated.
- Refine and re-test. Edit your subject line based on the feedback and watch the score update in real time. Iterate until you reach a score you are happy with.
Use cases
- An email marketer preparing a Black Friday campaign tests three subject line variations to find the one with the highest score and no spam triggers.
- A SaaS founder writing cold outreach emails checks that each subject fits within mobile preview width and contains at least one engagement signal.
- A newsletter editor pastes their weekly subject line draft to catch spam-trigger words that could hurt deliverability to Gmail subscribers.
- A nonprofit fundraiser tests donation appeal subjects to balance urgency cues with professionalism, avoiding overly aggressive language that triggers filters.
- A recruiter crafting candidate outreach emails scores each subject to maximize open rates while keeping the tone professional and personalized.
Frequently Asked Questions
Research consistently shows that subject lines between 30 and 60 characters get the highest open rates. Under 30 can lack context, while over 60 gets truncated in most desktop email clients. Mobile devices show even less - typically only the first 35-40 characters on a phone lock screen notification.
Common spam triggers include 'free', 'buy now', 'act now', 'winner', 'congratulations', 'guaranteed', 'no obligation', 'click here', '100%', and 'earn money'. Using one occasionally is fine, but stacking multiple spam-trigger words in a single subject significantly increases the chance of landing in junk or spam folders.
Start with a strong subject line: keep it under 60 characters, include a personalisation token like the recipient's name, add a number or statistic for specificity, use a question to spark curiosity, and avoid spam-trigger words. Also test sending at different times, segment your audience, and make sure your preview text (preheader) complements the subject.
Yes. Writing entire subject lines in uppercase is one of the strongest spam signals for email providers. It looks like shouting to readers and triggers automated filters. If you want to emphasize a word, capitalize just that single word rather than the entire subject.
Emojis can boost open rates by making your email stand out in a crowded inbox. However, overusing them or choosing irrelevant emojis can look unprofessional. One or two well-placed emojis that relate to your content work best. Keep in mind that emoji rendering varies across email clients, so stick to widely supported ones.
Subject lines with 6 to 10 words tend to perform best. Fewer than 3 words may not give enough context, while more than 12 words often get cut off and dilute the message. Focus on conveying one clear idea or benefit in as few words as possible.
Most smartphones display approximately 35-40 characters of a subject line on the lock screen and 30-35 characters in the inbox list. This means the first few words carry the most weight. Front-load your key message or hook so readers see it even on the smallest screen.
No. This tool runs entirely in your browser using client-side JavaScript. Your subject line text is never transmitted to any server, stored in a database, or logged anywhere. You can safely test confidential campaign subjects and outreach messages without any privacy concerns.
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