Free Email Subject Line Tester
Type a subject line to score it for length, spam phrases, capitals and punctuation, and see how it shows in a phone and a desktop inbox. Testing two? Compare them side by side.
Checked in your browser as you type. Nothing is sent or stored.
Results
Example result
–/ 100
Line B
–/ 100
Out of 100, higher is better. It’s a checklist, not a prediction of your open rate.
Your line has a deadline. Show the time left inside the email too, with a countdown that’s right at every open, built in our free countdown timer generator.
Subject sorted? Filters read the body too. Paste it into the spam checker.
What the score counts
Every line starts at 100 and loses points for what filters and readers hold against it: a line cut short on a phone, trigger phrases, capitals, stacked punctuation, a fake “Re:”, too many emoji, a missing preheader.
Each check shows what it cost, and one thing marked Fix keeps a line from reading “Ready to send”. It doesn’t predict your open rate: your list, your sender name and your timing decide that.
Put the point first
A phone gives the subject one line, and the rest gets an ellipsis. Lead with what the email is for: the offer, the deadline, the thing that changed.
“Your order has shipped” beats “Great news about your recent order with us”. The second one gets cut before it says anything.
Write the preheader
The preheader is the second line in the inbox, and the one most often left blank. Left empty, it shows whatever text comes first in the email.
Treat it as the rest of the subject. If the subject has the offer, the preheader can carry the code, the end date or the detail that makes it worth opening.
Frequently asked questions
How long should an email subject line be?
There’s no magic number. What matters is how much the inbox shows. A phone shows one line of subject, and the tester measures where yours gets cut, so put the offer and the deadline in that first part. Desktop inboxes show more, but they share the line with the preheader.
Do spam words in a subject line send an email to spam?
Rarely on their own. Mailbox providers weigh your sending reputation and authentication more than any single word. Trigger phrases add up, though, and a line full of hype hurts opens even when it lands in the inbox. Check the body with the spam checker and your SPF, DKIM and DMARC with the deliverability test.
Are emoji okay in subject lines?
Yes, in moderation. One emoji can stand in for a word and catch the eye. Each one looks different from platform to platform and takes about the room of two letters, so more than one or two crowds the part a phone shows.
What is preheader text?
The preview line inboxes show under or next to the subject. If you don’t set one, the inbox grabs the first text in the email, which is often “View in browser” or an image’s alt text. Set it in your ESP’s preheader or preview text field and use it to add what the subject leaves out.
How do merge tags show in the preview?
First-name tags (Klaviyo, Mailchimp, HubSpot, Salesforce and Liquid styles) preview as “Jane”. Any other field shows as its own name, so {{ event.extra.ProductName }} reads “Product Name”, and Liquid logic like {% if %} prints nothing. Real values run longer or shorter, so try the length with a typical one too.
Can I compare two subject lines?
Yes. Choose “Compare with another line” to score a second line beside the first and see both in the phone and desktop inboxes. The preheader carries over, since most tests change only the subject. The score shows which line has fewer problems; only a real send tells you which one gets more opens.
How accurate is the phone preview?
It’s close, not exact. The preview is drawn from iPhone Mail’s list layout at the default text size, and the cut-off is measured with your device’s system font. Different phones, apps and text size settings will cut a few characters earlier or later.
Does this tool save my subject lines?
No. Every check runs in your browser. Nothing you type is sent to a server or stored.
Still have questions? Contact us →
Got the open? Make it count.
Alterable draws countdown timers, live weather and personalized images the moment each email opens, so the deadline in your subject line is still right inside the email.
No trial, no time limit, no credit card.