A/B Subject Line Tester
Enter two subject lines and see which one scores higher across 9 proven engagement factors. Pre-screen your A/B tests before you hit send.
No signup required. Runs entirely in your browser. Your text is never sent to any server.
Enter Your Subject Lines
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But always test with your real audience -- scores predict, data confirms.
Category-by-Category Breakdown
See which subject line wins in each scoring category.
Why the Winner Scores Higher
Always validate with a real A/B test
This tool scores based on established best practices and pattern analysis, but your audience is unique. Send both versions to a small sample (at least 1,000 subscribers each), wait 2-4 hours, then send the winner to the rest of your list. Most email platforms offer built-in A/B testing for subject lines.
How the A/B Subject Line Tester Works
Enter Two Subject Lines
Type or paste both subject line candidates into the A and B fields. These are the two versions you are considering for your email campaign.
Instant Side-by-Side Scoring
Both subject lines are scored on 9 factors: character length, power words, spam triggers, numbers, question format, urgency, caps penalty, action verb start, and emoji usage. Scores range from 0 to 100.
See the Winner and Why
Get a clear winner badge, a category-by-category comparison, and specific reasons why one subject line outscores the other. Then go run your real A/B test with confidence.
What the Tester Evaluates
A/B Subject Line Testing FAQ
How do I A/B test email subject lines?
Most email marketing platforms have built-in A/B testing. You create two versions of your email with different subject lines, send each version to a small percentage of your list (typically 10-20% each), wait a set period (usually 1-4 hours), then automatically send the winning version to the remaining subscribers. Our tool here helps you pre-screen subject lines before you run a live test, so you go in with two strong contenders rather than wasting a test on a weak option.
How many subscribers do I need for a valid A/B test?
For statistically significant results, you generally need at least 1,000 subscribers per variant -- so a minimum list size of about 5,000 (1,000 for A, 1,000 for B, and 3,000 for the winning send). With smaller lists, your results will be directional rather than definitive. Lists under 1,000 total can still A/B test, but treat the results as suggestive insights rather than proven conclusions. The larger your sample, the more confident you can be in the winner.
How long should I run an email A/B test?
Most email opens happen within the first 2-4 hours after sending, so a 2-4 hour test window is standard for subject line tests. Some platforms default to 4 hours, which is a good balance between speed and accuracy. If your audience tends to check email at specific times (e.g., B2B audiences check in the morning), you may want to extend to 6-12 hours. Avoid running tests longer than 24 hours, as external factors start influencing results.
What should I A/B test besides subject lines?
Subject lines are the highest-impact element to test because they directly determine open rates. After that, test your preheader text (the preview snippet), send time (morning vs. afternoon vs. evening), sender name (company name vs. person name), call-to-action button text, email length, and personalization approaches. Test one variable at a time to isolate what actually moved the needle. Multivariate testing exists but requires much larger sample sizes.
What is a statistically significant result in email testing?
Statistical significance means the difference in performance between your two variants is unlikely to be due to random chance. In email marketing, most platforms use a 95% confidence level as the threshold -- meaning there is only a 5% probability the observed difference happened by luck. Factors that affect significance include sample size, the magnitude of the difference, and your baseline metrics. A 0.5% difference in open rates with 500 subscribers is probably noise. A 5% difference with 5,000 subscribers is almost certainly real.