Email Subject Line Tester
Score your subject lines, preview inbox rendering, and get AI-powered improvements.
3 free tests/day
Test your subject lines
Enter one subject line to score it, or multiple (one per line) to compare them.
Score your subject lines, preview inbox rendering, and get AI-powered improvements.
Enter one subject line to score it, or multiple (one per line) to compare them.
An email subject line tester analyzes your subject lines before you send them. It scores them on key factors that determine open rates: length, spam risk, power words, personalization, clarity, and mobile rendering. Instead of guessing which subject line will perform best, you get data-backed analysis and AI-generated improvements.
This tool supports two modes: score a single subject line for detailed analysis with 5 improved alternatives, or compare multiple subject lines side-by-side to find the strongest performer before sending.
Your subject line is the only thing standing between your email and the trash folder. Studies show that 47% of email recipients open email based on the subject line alone, while 69% report email as spam based solely on the subject line.
The difference between a 15% and 30% open rate often comes down to 5-10 words. Length, word choice, personalization, and mobile rendering all play measurable roles. Testing subject lines before sending is the highest-leverage activity in email marketing.
Each subject line is scored on 6 dimensions: Length (ideal 30-50 characters), Spam Risk (trigger words and formatting red flags), Power Words (action verbs and emotional triggers), Personalization (merge tags and specificity), Clarity (clear value proposition), and Mobile-Friendliness (visible on small screens).
Each dimension gets a score from 1-10. The overall score is a weighted average converted to a 0-100 scale, with length and spam risk weighted most heavily since they have the largest measurable impact on open rates.
Subject lines render differently across email clients. Gmail shows about 77 characters on desktop, Outlook shows 73, and mobile devices show only 35-40 characters. If your core message gets cut off on mobile, you lose a significant portion of opens.
The inbox preview feature shows exactly how your subject line and preheader text will appear in Gmail, Outlook, and on mobile devices. Lines that fit fully get a green checkmark. Truncated lines get an amber warning so you can adjust before sending.
Keep subject lines between 30 and 50 characters for optimal open rates. Front-load the most important words since mobile truncates after 35-40 characters. Use personalization tokens like the recipient's name or company when possible.
Avoid spam triggers: ALL CAPS, excessive punctuation (!!!), and words like 'free,' 'guarantee,' or 'act now.' Use power words that create urgency or curiosity without triggering spam filters. Test two variants at a time for clean A/B results.
A good subject line is 30-50 characters, clearly communicates value, avoids spam trigger words, includes personalization where possible, and delivers its core message within the first 35 characters for mobile visibility.
The ideal length is 30-50 characters (5-9 words). Subject lines under 20 characters lack context, while those over 60 characters get truncated on most devices. The most critical content should appear in the first 35 characters for mobile readers.
Common spam trigger words include: free, guarantee, act now, limited time, click here, buy now, earn money, no obligation, winner, congratulations, and urgent. ALL CAPS words, excessive punctuation (!!!), and misleading claims also increase spam risk.
Cold emails should be short (under 40 chars), personalized with the recipient's name or company, and reference a specific pain point. Newsletter subject lines can be longer, use curiosity hooks, and reference the content inside. Avoid salesy language in cold emails.
Average open rates vary by industry: B2B averages 15-25%, while B2C newsletters average 20-30%. Cold outreach typically sees 10-20% open rates. A subject line scoring above 75 on this tool should consistently outperform industry averages.
Preheader text appears after the subject line in most email clients and can increase open rates by 7-20%. It provides additional context and reinforcement. The best preheaders complement the subject line rather than repeating it, and they should be 40-130 characters.