Email subject line tester with preheader preview

There is no hard email subject line character limit that applies to the inbox list. The RFC 5322 standard allows long headers, and email tools accept subjects of hundreds of characters. What limits you is the space an inbox gives each message row, which depends on the client, device, font and window width.

A practical rule: put the meaning in the first 35–40 characters. That is roughly what fits on a phone in portrait mode, where most emails are opened. Subjects of about 30–50 characters usually survive on mobile and desktop; longer ones can work if the start stands on its own.

The calculator uses approximate visible lengths for a standard inbox list. They are typical values seen in default layouts, not documented limits:

Characters are counted as the reader sees them: an emoji with a skin tone modifier is one character, and wide letters take more space than narrow ones, so treat each number as a range of a few characters either way.

The preheader (preview text) is the short line shown next to or under the subject in the inbox. If you do not set one, most clients take the first text from the email body — often “View this email in your browser” or the alt text of a logo. That wastes the second most visible line of your email.

• Continue the subject, do not repeat it. The subject sparks interest; the preheader adds the detail, the benefit or the deadline.
• Front-load it too. On a phone only the first 40–90 characters are shown, depending on the app.
• Fill the space. If the preheader is short, some clients append body text after it. Many email templates add invisible spacer characters after the preheader to prevent this.
• Keep it consistent with the email. A preheader that promises something the email does not deliver hurts trust and future opens.

Personalization tokens

Merge tags insert data for each contact: Mailchimp uses *|FNAME|*, many platforms use Liquid or Handlebars-style tags in double curly braces, others use %FIRSTNAME% or single braces. The checker recognizes these syntaxes, flags unbalanced braces and unclosed tags, and fills name tokens with a sample value so that you count the length recipients actually see. Always set a fallback value: a subject that starts with “, your report is ready” looks broken.

Spam filters today rely mostly on sender reputation, authentication (SPF, DKIM, DMARC), engagement and the content of the whole message. A single word like “free” in a subject will not send a legitimate email to spam. The hints in this tool are heuristics: they point out patterns that look promotional to people and can add to other negative signals.

• All caps and repeated exclamation marks read as shouting and are old spam patterns.
• Trigger phrases such as “act now”, “risk-free” or “winner” are typical of scams; use them only when they are literally true and necessary.
• Fake RE: and FWD: pretend there is a conversation. Besides hurting trust, misleading subject lines are prohibited by anti-spam laws such as the US CAN-SPAM Act.
• Emoji can make a subject stand out, but render differently across platforms. One relevant emoji is usually enough.

Test, do not guess

Previews help avoid obvious truncation and mistakes; opens and clicks decide which subject works. Split-test two subjects on part of your list and check that the difference is significant with the A/B test calculator. Track clicks from each version with the UTM builder, and keep the email copy readable with the readability checker.

1. RFC 5322, Internet Message Format. IETF, 2008 — header line length limits for the Subject field.
2. Federal Trade Commission. CAN-SPAM Act: A Compliance Guide for Business — the requirement that subject lines not be deceptive.
3. Unicode Standard Annex #29, Unicode Text Segmentation — grapheme clusters used to count visible characters.
4. Visible lengths per client on this page are approximations of default inbox layouts, not published specifications.