What is an email subject line tester?

A subject line tester is a tool that analyzes your headline to predict its open rate performance by checking character count, emotional impact, and spam triggers.

How does the AI improvement feature work?

Our tool uses advanced Language Models (LLMs) to understand your email context and generate three alternative subject lines optimized for engagement.

Should I use emojis in my subject line?

Yes, but sparingly. Using 1 emoji can boost open rates, but using more than 2 often looks like spam and can trigger filters.

Does this tool verify if my email will go to spam?

It checks for 'spam trigger words' in your subject line. For technical domain reputation checks, use our Email Health Checker.

Do you save my subject line data?

No. The subject line you input is processed instantly to generate the score and suggestions, and is not stored.

What is A/B testing for subject lines?

A/B testing involves sending two different subject lines to a small audience segment to see which one performs better before sending to the full list.

Email Subject Line Tester

Analyze your email subject lines to improve open rates. Get instant feedback, AI-powered suggestions, and mobile previews.

Master the First Impression

Your subject line decides whether your email gets opened or ignored. Our free tester uses data-backed rules and AI to help you write headlines that demand attention.

Why Test Your Subject Line?

Boost Open Rates

35% of recipients open emails based on the subject line alone. A higher score here means more eyes on your content.

Avoid the Spam Folder

We check for “spam trigger words” (like Free, Guarantee, Urgent) that can hurt your deliverability.

Mobile Optimization

Over 50% of emails are read on mobile. Our tool shows you exactly where your text will get cut off on smaller screens.

3 Ways to Write Better Subject Lines

Keep it Short

Aim for 40-60 characters. Long subject lines get truncated, hiding your most important message.

Use Power Words

Words like “Exclusive,” “Update,” and “Introducing” trigger curiosity. Our tool highlights positive keywords for you.

Leverage AI

Stuck? Use the AI Improvements feature in this tool to instantly generate 3 creative alternatives.

Action plan

Your Subject Line is Perfect. Now, Will It Land in the Inbox?

Ensure your prospect will see and open your emails. From technical setup with DMARC and SPF to using email warmup, deliverability is a complex topic. No worry, we can advise you on where to start.

Audit your current email setup

Get a complete, personalized action plan for your entire emailing system, from technical setup and list building to a full deliverability scorecard.

Take the 5-min Assessment

Setup email warmup

Improve your open rate by 60% thanks to Warmup Inbox.

Start a free trial

Your subject line is your first impression

In a crowded inbox, your subject line is your one chance to capture attention. This free tool analyzes your subject line against proven marketing data, giving you an instant score and AI-powered advice to turn a good subject line into a great one.

47%

That’s nearly half of all recipients who decide whether to open your email based on the subject line alone. It’s the single biggest lever you have to improve your open rate.

How our subject line tester works

Enter your subject line

Type or paste the subject line you want to analyze into the tool above.
Provide a little context

Select the purpose of your email (e.g., B2B outreach, B2C promotion). Our AI adjusts its advice based on your goal.
Get your score and AI suggestions

Instantly receive a score out of 100, a detailed analysis, and AI-powered suggestions to improve it on the spot.

See what makes a great subject line

Finding the right length

We check character and word counts to ensure your message is concise and avoids being cut off on mobile devices, where over half of all emails are read.
Ideal: 20-60 characters & 3-9 words.

Choosing the right words

The words you choose create an emotional response. Our tool checks for:

Helpful Words: Terms that create value and engagement.
Spam Triggers: Words that can hurt your deliverability.
Emotional Tone: Positive, negative, or neutral sentiment.

Making it easy to scan

In a busy inbox, your subject line needs to be easy to read. We check for capitalization style, and strategic use of numbers and emojis to make it stand out.

Adapting to your audience

A subject line for a cold B2B email should be different from a B2C promotional blast. Our tool adjusts its recommendations based on the context you provide.

Go beyond the score with these pro tips

Spark curiosity

Don’t give everything away. Instead of “Our new guide to email marketing,” try “The one thing you’re missing in your emails.”

Use urgency (when it’s real)

Phrases like “last chance” or “24 hours left” are powerful, but should only be used when they are authentic to avoid audience fatigue.

Personalize with purpose

Go beyond just {FirstName}. A subject line like “A question about Warmup Inbox” for a B2B lead is far more effective than a generic one.

Let your data decide with A/B tests

The only way to truly know what works for your audience is to test. Pit two subject lines against each other and let the data choose the winner.

Ready to boost your open rates?

Checkout our features

Email Subject Line Tester