The average inbox receives over 120 emails a day. Your subject line is the single most important factor in whether yours gets opened or scrolled past. After analysing millions of sends across the InboxQuarry platform, we found ten patterns that consistently outperform the rest.
1. Lead with curiosity, not the answer
Subject lines that open a loop — without giving away the ending — pull readers in. "We tried sending at 3am. Here's what happened" beats "Best send times for email" every time. The brain hates an unfinished story.
2. Use numbers and specifics
Specificity signals substance. "Grow your list by 2,140 subscribers this quarter" is far more clickable than "Grow your email list." Round numbers feel made up; precise ones feel earned.
3. Personalise beyond the first name
Merge tags for first names are table stakes. The real lift comes from behavioural personalisation — referencing a recent purchase, a city, or a past open. Relevance is the strongest persuasion there is.
The best subject line isn't the cleverest one — it's the one that feels like it was written for a single person.
4. Create urgency honestly
Genuine deadlines work; fake ones erode trust. "Doors close Friday" is fine if they actually close Friday. Readers remember when "last chance" shows up three weeks in a row.
5. Keep it short for mobile
Over 60% of opens happen on a phone, where only 30–40 characters are visible. Front-load the hook. If your value proposition lives at the end of a 70-character line, most readers will never see it.
Putting it together
You don't need all ten in a single line — pick one or two and test relentlessly. The subscribers who open are the only ones who can click, and the ones who click are the only ones who convert. It all starts here.
- Write five subject lines for every campaign, then cut to two.
- A/B test the two on a small slice of your list.
- Send the winner to everyone else automatically.
Inside InboxQuarry, subject-line A/B testing is built in — pick your variants, set the test size, and we send the winner to the rest of your list for you.