Email Subject Line Tester
This subject line tester scores a cold or sales email subject line out of 100. It checks the length, spam-style words, capitals, exclamation marks, questions, numbers, emoji and fake Re: prefixes, and explains each result using published data from Hunter, Gong and Lavender. Type several lines to compare them.
Type a subject line. See its score and how to improve it.
100% free • No signup • Runs in your browser
How to use the Email Subject Line Tester
- 1
Type your subject line
Enter one subject line, or several on separate lines to compare them.
- 2
Read the score
Each line gets a score out of 100 and a check-by-check breakdown, with a preview of how it looks in a phone inbox.
- 3
Fix and compare
Change the words the checks flag and watch the score update. Several lines are ranked best first.
The rules behind the score
The main checks come from published studies and rules, each linked to its source. The rest (spam words, capitals, exclamation marks, prices) flag what makes a subject line read like an ad.
Keep it short
Five or six words did best for replies across 31M emails.
Hunter, 2025Fewer words for executives
With executives, open rates fell as subject lines got longer; one to four words did best.
Gong, 2026Skip numbers and buzzwords
Buzzwords and numbers cut open rates by up to 17.9% across 28M+ cold emails.
Gong and Outbound Squad, 2025Make it a statement
Question subject lines cut opens by 56%. Lavender also advises against emoji in cold email subject lines.
LavenderAvoid "Quick question"
It underperformed other subject lines: a 2.5% reply rate against 2.9%.
Hunter, 2025Never fake a reply
A made-up "Re:" or "Fwd:" misleads the reader, and US law requires the subject line to reflect the message.
FTC, CAN-SPAM guide
Need the rest of the email? The cold email templates come with subject lines to start from, and the Email Spam Checker checks the whole message before you send.
Frequently Asked Questions
What makes a good cold email subject line?
A few plain words about their world, written like an email from a colleague: their company, their project, or the problem you noticed. Hunter found five or six words did best for replies, and Gong found one to four got the most opens with executives. Leave out spam words, capitals and exclamation marks.
How long should an email subject line be?
For cold email, short: roughly two to seven words. Hunter's data on 31M emails put the sweet spot for replies at five or six words. Phones also cut long subject lines off, so put the words that matter first.
Should cold email subject lines be questions?
Usually not. Lavender found question subject lines cut opens by 56%, and Gong advises against them. Turn "Struggling with hiring?" into "hiring at Acme" and save the question for the email.
Should I put the prospect's name in the subject line?
The data is split. Hunter found subject lines with two personalized details got more opens, while Lavender found first names in the subject cost replies. Their company name or project is a safer detail than their first name.
Do spam words in a subject line send it to spam?
Less than people think: inbox providers weigh your sender reputation and authentication far more. But words like "free", "act now" or "guaranteed" make the email read like an ad, which costs you opens and replies either way. Test the whole email with the Spam Checker.
Does this work for newsletter subject lines?
It's built for cold and sales emails. Newsletters play by different rules, where numbers and emoji can work, so treat those warnings as cold-email advice. The checks for spam words, capitals and fake replies apply to every email.