Paste your subject line
Add the exact subject line you plan to send. Keep the wording and punctuation intact so the test sees what the inbox will see.
subject line tester
Improve Any Subject Line in Seconds with Our AI Subject Line Optimizer
Paste your subject line to test clarity, curiosity, length, and filter risk, then get a stronger version you can send with confidence.
How to use the AI subject line optimizer
Add the subject line you plan to send. The tester checks clarity, curiosity, length, and filter risk, then gives you a stronger alternative.
- Step 01
- Step 02
Submit it for optimization
Run the analysis. The tester reviews clarity, curiosity, length, mobile truncation, and language that may raise filter risk.
- Step 03
Review the score and suggestion
See the score, the reason behind it, and a stronger alternative written to keep the same intent.
- Step 04
Apply the update and send
Use the suggested line or refine your original with the feedback, then move the winning version into your campaign.
How the subject line tester works
A cold email subject line has one job: earn the open without promising something the email does not deliver. The tester scores each line on whether it is specific, whether it creates a real reason to look, and whether it survives a phone inbox.
The line is scored for clarity, curiosity, length, and filter risk, then paired with a stronger alternative that keeps the original intent.
How this compares to paid tools
Subject line testing is usually bundled into a sending platform, which means you have to import your list before you can score a line.
| Feature |  | Paid sequencer | Paid copy tool |
|---|---|---|---|
| Price | Free | $79 to $199 per seat | $29 to $99 per month |
| Subject line score and rewrite | One at a time | One at a time | |
| Free with no card | Trial only | ||
| Clear reason behind the score | |||
| Mobile truncation preview | |||
| Filter risk flags | |||
| Suggested alternative line | |||
| No list import required |
Who this is for
SDRs testing a new angle
Before you split test across 500 contacts, rule out the lines that were never going to work.
Founders sending in small volumes
With 20 sends a week you do not have the volume for a statistically real A/B test. Score the line instead.
Marketers writing nurture subject lines
The same clarity and truncation rules apply to lifecycle email, not just cold outbound.
Sales managers reviewing sequences
Check every step's subject line in one pass rather than opening each one in the sequencer.
Agencies presenting options to clients
Bring three scored lines to the review instead of three lines and a preference.
Popular use cases
01 · Use case
Balancing clarity and curiosity
See whether the line gives away too much, says too little, or creates a useful reason to open without reading like bait.
02 · Use case
Fixing a sequence with low open rates
When opens are the problem, the body copy is not. Test the existing lines to find which step is dragging the sequence down.
03 · Use case
Checking how a line reads on a phone
Most cold email is first seen on a phone. The truncation preview shows what your prospect actually sees in the list view.
Frequently asked questions
Something not covered? Talk to our team.
Up to five. Each one is scored on its own and the strongest is called out, so you can see why one line beats another rather than judging it in isolation.
