Free Email Subject Line Tester and Mobile Preview
A subject-line tester is a pre-send checklist, not a guaranteed open-rate predictor. It helps you catch truncation, vague wording, excessive punctuation and avoidable spam-risk patterns before a real audience sees the message.
How this free tool works
Check email subject-line length, readability, urgency and spam-risk patterns before you send. Free, private and no signup. The result is designed to be transparent: you can see the inputs, the formula and the assumptions instead of receiving a black-box score.
The score is a transparent heuristic based on length, clarity, punctuation, personalization and common risk patterns. It is not a deliverability guarantee.
Best for
- Newsletter and lifecycle marketers
- Founders writing outbound or launch emails
- Teams that want a fast mobile preview before an A/B test
Frequently asked questions
Can a subject-line score predict open rate?+
No. The score is a writing and QA heuristic. Audience, sender reputation, timing, offer and inbox placement still matter.
How long should an email subject line be?+
Keep the important words early and preview the subject on a narrow mobile width. There is no single length that works for every client.
Does the tester send or store my subject line?+
No. The first version runs entirely in the browser and does not require an account.
References and methodology
We use primary documentation where available and treat calculators as planning aids, not guarantees. Check the linked source when a platform changes its rules.
- Mailchimp: Email subject line best practices
Practical guidance from a major email platform.
- Google Workspace: Gmail sender guidelines
Official sender and deliverability guidance; scoring cannot replace it.
Turn this answer into a repeatable growth workflow.
Use the free result as your starting point, then move the next campaign, creative or growth decision into GrowthGPT when the work becomes repetitive.