Do short subject lines always perform better?

You've probably heard it a hundred times: keep your subject line short and your open rates will soar. It sounds tidy. It's also not really true.

Short subject lines can be punchy and attention-grabbing. But long subject lines can build intrigue, set expectations, and give readers exactly the context they need to click. Both can work. Both can flop. Length alone isn't the lever.

What actually drives opens is a combination of three things: relevance (does this feel written for me?), clarity (do I know what's inside?), and timing (does this land when I'm ready to read?). A six-word subject line that's vague beats nothing. A 14-word subject line that nails the reader's exact pain beats both.

That said, there's one practical reason to keep an eye on length: mobile preview cutoffs. Most phones show somewhere between 30 and 50 characters before the subject gets clipped. Gmail on mobile sits around 40 characters. Outlook mobile tends to be similar. If your most important words are buried at the end of a 70-character subject line, some readers will never see them. The fix isn't to go short by default. It's to front-load what matters most, so even a truncated preview still lands.

The honest answer is that what resonates depends entirely on your audience and your content. B2B readers might respond better to a specific, descriptive subject. A lifestyle newsletter audience might open more often for something short and mysterious. You won't know until you test.

Here's a simple testing framework worth running:

• Pick one campaign type you send regularly (weekly digest, product update, promotional blast).
• Write two versions: one short (under 40 characters), one longer (50 to 70 characters).
• Keep everything else identical: send time, audience segment, preview text, content.
• Run the test across at least 3 to 5 sends before drawing conclusions. One send is noise.
• Look at open rates, but also click rates. Opens tell you the subject did its job. Clicks tell you the subject set the right expectation.

So one more thing worth knowing: subject line content can affect spam filtering in ways that length never does. Certain words and patterns trip filters. Length? Not so much. That's a separate conversation entirely, but worth keeping in mind when you're writing your tests.

You can run a quick check on your subject lines with our free subject line tester if you want a sanity check before you send.