Email Subject Line Character Limit: Every Client, Desktop & Mobile

The short answer

Keep email subject lines under 50 characters if most of your audience reads on a phone. If you know your list skews desktop, you can stretch to 60. The technical maximum per RFC 2822 is 998 characters, but that number is meaningless in practice — every email client truncates the visible subject long before that.

The rest of this post is the exact display limits, client by client, with the numbers that actually decide whether your subject line gets read or gets cut off with an ellipsis.

Character limits by email client

Every email client truncates at a different point, and the mobile cutoff is always shorter than desktop. Here's what each one shows before the "..." appears.

These are approximate because font rendering, screen width and subject-line content (wide characters like "W" vs narrow ones like "i") all shift the cutoff by a few characters. But they're close enough to plan around.

Yahoo Mail is the harshest — just 30 characters on mobile. If even a small slice of your list uses Yahoo, anything past character 30 is invisible to them on their phone.

What the RFC limit actually means

RFC 2822 sets a hard ceiling of 998 characters per header line (including the "Subject: " prefix). That's the limit your mail server enforces. But no one will ever see 998 characters of your subject — they'll see 30 to 70, depending on the client. The RFC limit exists so email infrastructure can process the header. It has nothing to do with what a human reads.

Pre-header text: the second line people actually see

Below (or beside) the subject line, most email clients display a snippet of the email body as preview text. This is called the pre-header, and it's 35 to 140 characters of free real estate that many senders waste.

If you don't set a pre-header deliberately, the client pulls whatever text comes first in the email body — often "View in browser" or an image alt tag. That's a missed opportunity.

A strong pre-header extends the subject line. The subject says what, the pre-header says why open it. Together they give you roughly 90–180 characters of visible text in the inbox, which is far more than the subject alone.

Why shorter subject lines win on mobile

Over 60% of emails are opened on a mobile device. On mobile, the subject line shares screen space with the sender name, the pre-header snippet, and sometimes a timestamp. That leaves room for about 30–40 characters of your subject.

A 70-character subject line that reads perfectly on desktop gets cut to something like:

Your Exclusive Summer Sale Starts Tomo...

The reader sees a fragment. The call to action is gone. Shorter subjects avoid this entirely:

Summer Sale: 30% Off Today Only

That's 33 characters. It survives every client, every screen size, every font rendering quirk.

The 50-character guideline

Email marketing platforms (Mailchimp, Campaign Monitor, HubSpot) converge on the same recommendation: aim for 50 characters or fewer. At 50 characters you clear Gmail mobile, Outlook mobile, and Apple Mail mobile with room to spare. Yahoo mobile at 30 characters is the only client that still truncates — but Yahoo's market share is small enough that most senders accept the trade-off.

If you want to check your character count before hitting send, paste the subject line into a counter tool. It takes three seconds and saves you from truncation surprises.

Making subject lines stand out without extra characters

When every subject line in an inbox looks the same — plain text, normal weight — anything that breaks the pattern gets noticed. One approach: use Unicode bold or italic characters in the subject line.

A subject like "𝗦𝘂𝗺𝗺𝗲𝗿 𝗦𝗮𝗹𝗲: 30% Off" reads as bold in the inbox, even though email subjects have no native formatting. The bold characters are actual Unicode code points, not formatting — so they survive the plain-text subject field.

You can generate these with a bold text generator or a general text generator. There's a deeper walkthrough in bold email subject lines with Unicode.

A few caveats:

• Not every email client renders them identically. Gmail and Apple Mail handle Unicode bold well. Some older Outlook versions may show fallback glyphs.
• Screen readers may read them letter-by-letter. Use bold for a word or two of emphasis, not the entire subject.
• They still count as characters. Unicode bold letters are the same width (or wider) than normal letters, so they don't save you space — they just add visual weight.

Practical rules for subject line length

1. Write the subject at 50 characters or fewer. This is your primary constraint. If the subject is longer, cut words — don't just hope it fits.
2. Front-load the important words. Whatever gets truncated should be the least essential part of the subject. Put the offer, the name, or the action verb first.
3. Write a deliberate pre-header. Don't let the email client pull "View in browser" as your preview text. Write 60–100 characters that extend the subject line's pitch.
4. Test across clients. Tools like Litmus and Email on Acid show you what the subject looks like in every client. If you don't have those, at minimum send a test to your own Gmail, Outlook and iPhone.
5. Use Unicode formatting sparingly. One bold word can increase open rates. An entire subject in bold looks spammy and may trigger filters.

Quick reference

For more on character limits across platforms beyond email — Instagram, X, LinkedIn, TikTok and others — see the social media character limits guide. And if you're checking counts before posting anywhere, how to check character count before posting walks through the process.