Balancing subject line length and coming up with something catchy can be frustrating. Much like writing a tweet, one of the challenges is formulating an enticing phrase with a limited number of characters.
But how long can a subject line be?
From a technical perspective, RFC 2822 states that the maximum number of characters in a subject line is 998 characters. However, a lot of email clients will impose a 255/256 character limit.
Clearly, both limits are too much and one should not attempt to reach those boundaries. Despite those limits, most email clients will only “display” a certain number of characters before hiding the rest of the message. This limit is usually shorter than the maximum characters allowed. We’ve also found that this display limit varies wildly across email platforms.
Below is a list of email browsers and the number of characters they display by default.
| Email Client | Character Length |
| Outlook 2010 preview | 54 characters |
| Outlook 2010 compact | 73 characters |
| Thunderbird | 66 characters |
| Gmail | 70 characters |
| Outlook.com | 60 characters |
| Yahoo Mail | 46 characters |
| Android (480 x 320px) portrait | 27 characters |
| Android (480 x 320px) landscape | 46 characters |
| Android (800 x 480px) portrait | 30 characters |
| Android (800 x 480px) landscape | 62 characters |
| iPhone portrait | 41 characters |
| iPhone landscape | 64 characters |
| Windows Phone portrait | 42 characters |
| Windows Phone landscape | 61 characters |
(Note these are character limits are based off the default settings of an email client. Non-mobile based platforms are based off screen resoultions of 1024 x 768 pixels)
How subject line length can influence engagement
A report by MailerMailer found a correlation between subject line length and open and click through rates. Given the limited number of characters displayed across various devices, it was no surprise that shorter subject lines worked best.
Subject line length and their corresponding engagement rates:
| Characters | Open Rate | Click Rate |
| 4-15 | 15.2% | 3.1% |
| 16-27 | 11.6% | 3.8% |
| 28-39 | 12.2% | 4.0% |
| 40-50 | 11.9% | 2.8% |
| 51+ | 10.4% | 1.8% |
According to MailerMailer’s report, the ideal subject line length is between 28-39 characters – providing the best overall engagement rate.
Also, when drafting your next subject line, keep in consideration what character restrictions you might run into, especially if you are communicating to a mobile heavy audience, where the average subject line display space is shorter than desktop variants.
