Subject and Headline Testing API

Get up and running in 5 minutes.

What's this about?

This is the backend API that's powering both the Web and Slack versions of our Email Subject Line Tester.

If you have a service where your users enter headlines or email subjects into and would like to help them write better ones, you're in the right place.

What does an embedded integration look like?

The primary use case for this API is to provide a real-time score to a user entering a subject or headline into your app.

We refer to this as the embedded integration as opposed to a custom integration that would directly interact with the API endpoint.

To help you implement this as quickly as possible, the following will:

  • Attach the API to one or more <input type='text'> elements on a page.

  • Provide an auto-updating score next to the form element.

  • Let you style and position the score to match your own branding.

Put together this looks like the following:


Enter a Subject or Headline
Subject line tester api example

How to setup an Embedded Integration

1. Include the Subject Line Tester JS file in your site.

Add the following to your site alongside any existing js includes.

2. Add the slt-field class to any inputs you want tested.

It's likely that you want to Subject Line Tester to be embedded as part of an existing form within your application. Add the slt-field as an additional class on the fields you'd like the score to display next to.


<input type='text' name='subject' class='form-field form-field-inline slt-field'>

3. Style the Embedded Score

Add the following CSS to your application stylesheet and tweak as necessary.

/* Embedded Subject Line Score */
.sci-scores{position: relative;}
.sci-scores a,a:visited{text-decoration: none !important;}
.sci-scores img {position: absolute; margin-left: 10px;}
.sci-scores .number_rating {
  position: absolute;
  margin-left: 46px;
  margin-top: 6px;

If you're using Bootstrap, we recommend this as a starting point instead.

/* Boootstrap Embedded Subject Line Score */
.sci-scores{float: right; margin-top: -30px;}
.sci-scores a,a:visited{text-decoration: none;}
.sci-scores img {position: absolute; margin-left: 10px; }
.sci-scores .number_rating {position: absolute; margin-left: 46px; margin-top: 6px; font-size: 16px;}

4. Call subjectLineTester() from your on page Javascript.

subjectLineTester() will score the current values of slt-field input fields and immediately display a score. As users type in different subject lines / headlines the values in those fields will automatically update the score (throttled to one update / 500ms).

// Call this to start scoring

Codepen icon See a Working Example on Codepen

What's the charge or license for this?

Usage of the Embedded Subject Line Tester is free and unlimited for the general use case (letting users in your application or service enter headlines or subjects and having them scored).

The only requirement is that the link surrounding the letter grade and score remain intact. Clicking the link opens a new tab where the user can get more detailed information on how to improve their subject line.

If this behavior won't work for you, drop us an email at and we'll be happy to work something out.

Similarly, if you need to do things like mass checking of headlines/subjects, please email us. We'll typically grant access, higher rates, etc in exchange for attribution.

Our goal is to offer the Subject Tester API for free to the broadest number of people possible.

Need to do something else? Question not answered?

We sincerely want to help and encourage people to use the API. Email and let us know what we can do.