Email Subject Line Checker

What the subject line checker looks at

The subject line and the preview text are all most people see of an email before they decide to open it, skip it or delete it. This checker shows both the way an inbox row would: on a phone and on desktop, cut where the room runs out. Below the preview it counts what matters for length and style:

• Characters and words, counted as you see them, so an emoji or a flag is one character.
• Emoji, symbols shown as pictures. Text symbols like © or ™ do not count.
• Words in capitals, like SALE or FREE, and acronyms like SEO, which are fine.
• Punctuation and symbols, and runs like !!!, ?! or $$$. Marks inside words, as in don't or 3.5, are not counted.
• Personalization tags such as {{ first_name|default:'there' }}, *|FNAME|* and %%FirstName%%, filled with a sample name in the preview.

The default line, "Your March reading list: 7 books we loved", has 41 characters, 8 words and 1 punctuation mark.

How long should a subject line be?

Mailchimp recommends no more than 9 words and 60 characters (Mailchimp: Best Practices for Email Subject Lines). That is a good target, but a character count cannot tell you where a line gets cut. What fits depends on the room in the row: a line in capitals or with wide letters like W and M fills a row sooner than one in lowercase. Screens, fonts, text sizes and apps differ too, so no single number fits every inbox; Klaviyo notes that preview text limits vary by device and email client.

That is why the preview measures your text in pixels, in the system font of your device, against row widths we set for a typical phone and a desktop list. Treat it as an approximation. On mobile the room is smallest, so lead with the words that make the point, and let the end be the part that may get cut.

Preview text: the second line

Most email tools have a field for preview text, sometimes called preheader. Klaviyo's help center explains that if you do not add it, inboxes pull it from the first line of text in the email, which can include alt text or navigation content (Klaviyo: How to insert preview text into an email). "View in browser" is a waste of that space.

Write it as the second half of the subject: the detail that makes the reader open. If the subject says what the email is, the preview text says why it matters now. Do not repeat the subject; the checker warns when the preview starts with the same words.

Spam words: what the official sources say

Lists of "spam trigger words" are easy to find online, but we found no official one. Gmail's sender guidelines contain no list of words. Instead they ask for accurate messages (Gmail Help: Email sender guidelines):

• Headers and content should be accurate and not misleading or deceptive, and the subject should accurately represent the message.
• Do not start a subject line with Re: or Fwd: unless the message really is a reply or a forward. The checker flags this one as a problem.
• Do not use emoji or other non-standard characters to imitate graphic elements with the intent to deceive or influence recipients.

In the US, the CAN-SPAM Act makes accuracy a legal duty. The FTC's compliance guide says not to use deceptive subject lines: the subject line must accurately reflect the content of the message, and each separate email that violates the law can draw a penalty (FTC: CAN-SPAM Act, A Compliance Guide for Business). No tool can check that for you; only you know what the email says.

On style, Mailchimp warns that too many punctuation marks can make an email look like spam, especially with a lot of special characters, and suggests no more than 3 per subject line. The checker counts them and flags runs like !!! and $$$. The example "🔥🔥 LAST CHANCE!!! 50% OFF EVERYTHING $$$" has 7 marks and 4 words in capitals.

Emoji, capitals and punctuation

Mailchimp's research suggests no more than 1 emoji at a time, and testing them, because different operating systems render different versions of emoji. Emoji should support the words, not replace them. Capitals are a style question: a line in capitals reads like shouting and takes more room, so keep them for acronyms and, at most, one word of emphasis. These are hints, not filter rules, and the checker labels them that way.

Personalization tags and fallbacks

Email tools fill tags with each contact's data. Mailchimp writes merge tags like *|FNAME|* (Mailchimp: All the Merge Tags Cheat Sheet). Klaviyo uses double braces, and its first-name tag works in subject lines, with optional default text for contacts whose first name is not set, as in {{ first_name|title|default:'there' }} (Klaviyo: How to add a first name to an email, SMS, or push message).

The risk is the contact without a name. The example "{{ first_name|default:'Friend' }}, your plan renews on Friday" reads "Alex, your plan renews on Friday" for Alex and "Friend, your plan renews on Friday" for someone with no first name, thanks to its default. Without a default, the same line would start with a comma. Clear the "Fill tags with" field to see what those contacts get.

Weak and better subject lines

How to use the email subject line checker

1. 1Type or paste your subject line and, if you have one, the preview text. Add your sender name for a realistic inbox row.
2. 2Look at the phone and desktop preview to see where each line gets cut. Move the important words to the front if they disappear.
3. 3Read the counts and the checks. Fix problems first, then decide which style hints apply to your email.
4. 4If you use tags, clear the "Fill tags with" field to see a contact without a name. Copy the report when you are done, then send yourself a test.

Related tools: the character counter counts any text against a limit, and the SERP snippet preview does the same width check for search result titles.

Built by Tobias Kurth, designer. Updated October 13, 2026.

Frequently asked questions