Guide from the Subject Line Tester
How Long Should a Subject Line Be?
Short answer: aim for a few words that name something the reader cares about, and keep the whole line short enough to show in full in an inbox. There is no magic number. Here is what public research says, what is simply good writing practice, and how to check your own subject line.
Aim for a few words
Gong and 30 Minutes to President's Club (30MPC) looked at cold email subject lines that got opened. The ones that did well looked like internal notes: a few words, normal capital letters, and a real priority for the reader. Picture the subject a colleague would type when forwarding you something useful.
Boomerang studied ordinary everyday email and found that short, plain writing is easier to answer. That was one-to-one email in general, not cold sales outreach, so treat it as a supporting hint rather than a sales rule.
Based on: Gong / 30MPC, 2025Boomerang, 2016
Keep it inside the inbox line
Many mail apps cut the subject off once it runs past the width of the screen, and phones show even less. A good habit is to keep the subject under about 60 characters and put the most important words first, so nothing that matters gets cut off. This is a writing practice, not a study result.
Writing practice, not a study result.
Shorter is not the same as vague
Cutting words only helps if what is left still says what the email is about. “Quick question” is short, but it tells the reader nothing. “Late shipments at Northline” (an invented example) is just as short, and it names a topic the reader will recognize.
Curiosity can help, but only when it points at one specific thing the reader could answer. A vague teaser is not the same thing. This idea comes from psychology research on curiosity, not from an email study.
Based on: Psychological Bulletin, 1994
What makes a subject feel long
Length is not only a word count. A few habits make any subject feel heavier than it is. Gong's cold email research found that short language about the reader's priority does better than a product pitch, a buzzword, or a hype number. Boomerang found that subjects in all capital letters read like yelling.
Based on: Gong, 2025Boomerang, 2017
A quick checklist
- A few words, written the way a colleague would write them.
- The most important words first.
- Under about 60 characters, so it shows in full.
- Names a topic or priority the reader will recognize.
- Normal capital letters, with no hype and no pitch.
Writing practice, not a study result.
What this guide does not promise
No subject line length guarantees opens or replies. The research above describes patterns, and a good subject still needs a good email behind it. Use the checklist to catch problems, then write the subject you would want to receive.
Writing practice, not a study result.
Test your subject line
Paste your subject line into the Subject Line Tester. It checks length, the inbox line, tone, and clarity, explains each point in plain words, and links the research behind it. It is free, and nothing you type is stored.
Try the Subject Line TesterSources
- Gong / 30MPC. 4 Data-Backed Subject Lines to Get Your Cold Emails Opened (2025). Read the source
- Boomerang. 7 Tips for Getting More Responses to Your Emails (With Data!) (2016). Read the source
- Psychological Bulletin. The psychology of curiosity: A review and reinterpretation (1994). Read the source
- Gong. Does cold email even work any more? Here's what the data says (2025). Read the source
- Boomerang. The One Thing You Should Never Do In An Email Subject (Based on Data) (2017). Read the source
Advice marked as writing practice is common sense for clear email, not a study result. See where all of our guidance comes from. No statistic on this page is invented.