Email Subject Line Tester
Test and optimize your email subject lines for open rate, length, sentiment and spam triggers.
About Email Subject Tester
Analyze your email subject line for spam trigger words, length, power words and best practices to optimize your open rate.
About this calculator
Type your email subject and see its length, word count and warnings for spam-trigger words, all caps and excess punctuation.
Better subjects get more opens, and most inboxes show only the first 40 to 60 characters.
How to use it
- Type your subject line.
- Read the length and spam warnings.
- Edit until the score looks good.
What makes a subject work
Aim for 30 to 50 characters (about 6 to 10 words) so it isn't cut off on mobile. Make it specific and honest. Personalisation and a clear benefit help. Avoid ALL CAPS, many exclamation marks, and trigger words such as 'free!!!', 'act now' or '100% guaranteed'. Spam filters weigh many signals including sender reputation and authentication, so a clean subject alone doesn't guarantee inbox placement.
Worked example
Before and after
| FREE!!! ACT NOW!!! | Your invoice for October is ready |
| Newsletter | 3 tax-saving moves before 31 March |
Tips and common mistakes
- Front-load the important words.
- Use numbers when they are real.
- Test two versions (A/B).
- Match the subject to the email content.
- Authenticate your domain with SPF, DKIM and DMARC.
Frequently asked questions
How long should a subject line be?
30 to 50 characters.
Do emojis help?
Sometimes; test with your audience.
What triggers spam filters?
Shouting, excess punctuation and deceptive wording.
Does a good subject guarantee delivery?
No, sender reputation matters more.
What is A/B testing?
Sending two versions to compare opens.
Is it free?
Yes.