Marketing & GrowthFree
Subject Line Generator With Preview Text
Describe your email and get 20 subject lines, each paired with preview text, sized for phone inboxes, plus three ready-made pairs to test.
River writes 20 subject lines for the email you describe or paste, grouped by approach: a clear benefit, curiosity, a deadline where one is real, a question, a personal note and news. Each comes with its own preview text, the line most inboxes show beside or beneath the subject, so the two work as a pair instead of repeating each other. Character counts are shown for both lines, and the important words always come first, where a phone shows them.
Many subject line generators chase opens with tricks: fake urgency, shouting capitals, "Re:" on emails that are not replies. Those can lift a single send and still cost you, because readers who feel misled report the email as spam, and spam reports are what mailbox providers watch. Every line here has to be true to the email it sits on. You also get three test pairs that change one thing at a time, so a test actually tells you something.
It is for small business owners, newsletter writers and email marketers who send regularly and want better options than their first idea. Paste a few past subject lines that did well and River matches your voice and length. Writing a whole series rather than one send? The welcome email sequence writer drafts seven emails with subject lines included. Once some sends have gone out, the report across every campaign shows which patterns earn clicks on your list, and the list health audit explains what is wrong when emails stop reaching inboxes.
What a subject line has to get right
Start with honesty, because in the United States it is the law. The FTC's CAN-SPAM compliance guide says the subject line must accurately reflect the content of the message. It is also good business, whatever the country. Gmail's sender guidelines ask senders to keep the spam rate reported in Postmaster Tools below 0.1% and never reach 0.3%, and a subject line that tricks people into opening is a reliable way to collect spam reports.
Next, be careful what you learn from open rates. Apple's Mail Privacy Protection prevents senders from seeing whether you opened a message, and Mailchimp recommends excluding Apple privacy activity and other bot activity from reports. On a list with many Apple Mail readers, an open-rate test between two subject lines can be decided by mail apps rather than people. Clicks, with bot activity filtered out, are the steadier signal for picking a winner.
Finally, write for the phone in someone's hand. Phone inboxes show far fewer characters than desktop ones, so the words that matter go first, and the subject should still make sense if the end is cut. Preview text is the second line of the pitch. If you leave it empty, many inboxes fill it with the first words of the email, which is often a link to view it in a browser. Write it deliberately, and make it add something the subject did not say.
How it works
Describe the email
Say what it is about and who gets it, or paste the email and a few past subject lines.
River writes twenty
Subject lines grouped by approach, each with preview text and a character count beside it.
Pick and test
Choose a favourite, or run one of the three test pairs on your next send to your list.
Refine in chat
Ask for a shorter set, a different tone, or subject lines for the rest of a sequence.
What you get
- Twenty subject lines grouped by approach, from a plain benefit to a question or news
- Matching preview text for every subject line, written to add to it rather than repeat it
- Character counts for every line, with the key words placed first for phone inboxes
- Every line true to the email, with no fake urgency, false replies or misleading claims
- Three test pairs that each change one thing, so the result means something
- River's three top picks for this email and audience, with the reason for each
Common questions
How long should an email subject line be?
Short enough that the point survives on a phone. There is no fixed limit, because each email app shows a different number of characters, and phone inboxes show far fewer than desktop ones. Put the important words first so the subject still makes sense if the end is cut off. River shows the character count beside every option.
What is preview text, and do I need it?
Preview text is the short line most inboxes show beside or beneath the subject. If you do not set it, many inboxes show the first words of the email instead, which may be a navigation link. Treat it as the second half of the subject line, and use it to add a detail, a reason or a payoff the subject left out.
Should I use emojis in subject lines?
Only if they suit your brand and your readers, and never in place of words. An emoji can help a subject stand out in a busy inbox, but it can display differently across email apps and can look out of place in professional email. If you want to know for your list, test one subject with an emoji against the same subject without it.
Do spam trigger words really matter?
Less than people think. Filters weigh how recipients respond and the sender's reputation heavily, so complaints and engagement matter more than any single word. What does hurt is a subject that misleads people into opening, because readers who feel tricked report the email as spam. Gmail asks senders never to reach a reported spam rate of 0.3%.
How do I test subject lines properly?
Change one thing at a time, such as a question against a statement, and keep everything else the same. Send each version to a random part of your list large enough to show a real difference. Because privacy features inflate opens, judge the winner on clicks where you can. River sets up three pairs this way for every email you describe.
Can it match my brand voice?
Yes. Paste a few past subject lines that performed well, or the email itself, and River matches their tone, length and style. Describing the brand in a sentence, such as friendly and plain or formal and brief, works too. Without either, River writes in a clear, friendly tone and tells you that is what it chose.
Sources
Built by
Chandler Supple
Co-founder & CEO, River
Chandler has spent the last six years working in AI, building tools for millions of users and Fortune 100 companies. In 2025, he co-founded River to build a new kind of operating system for AI agents.
Subject Line Generator With Preview Text
Describe what you need, add any files you have, and your workspace opens with the work already underway.