It's 21 or fewer characters, according to a new report from Yes Lifecycle Marketing.
Although constituting less than 5% of all emails sent, emails with Subject lines under 21 characters generate the highest open rates, the report says.
In fact, Subject lines under 21 characters generate a 31 percent higher-than-average open rate.
The report, Subject Line Benchmarks: How Length and Personalization Impact Email Performance Across Message Type and Industry, bases its conclusions on an analysis of more than 7 billion emails.
The report also says:
"While shorter subject lines can sometimes stand out in the inbox, especially on mobile, length is only one of the many email components marketers should take into account when developing their campaigns," says Michael Fisher, president of Yes Lifecycle Marketing.
"The key is still to give subscribers enough information up front."
The full report can be found on the company's website.
Want to remember this rule-of-thumb for Subject line-length?
Email Guidance: No one over 21 admitted.