Skip to main content


Optimizely Knowledge Base

Implement the one-line snippet for Optimizely

This article will help you:
  • Add one line of code to your site to start running Optimizely experiments (A/B tests)
  • Choose the pages where you'd like to add the Optimizely code snippet
  • Find your project code snippet in Optimizely
  • Decide where to add the code in your HTML

If this is your first time using Optimizely, you'll need to add a small snippet of code (here's how it works) to the <head> tag of your site's code to start running experiments. This snippet contains your unique Project ID and allows Optimizely experiments to run on your site. Once you add it, the snippet will update automatically; you don't need to do a thing.

After you complete one-line implementation of the snippet with the three steps below, you'll be able to run experiments anywhere on your site.

1. Decide where to add the snippet

Consider the architecture of your site and experimentation program when deciding where to add the snippet. Each Optimizely project, or workspace, has its own snippet.

  • If your account plan includes cross-project events, you can track metrics across project spaces. If not, make sure that all parts of your site where you measure results are in the same project. One snippet should run across all those pages.

  • If you have separate development and production environments, or different domains and subdomains, you'll likely need separate projects (and snippets) for each.

Site performance has become increasingly important in recent years. Download Optimizely's Performance Guide to decide which implementation is right for you.

Also, check out our whitepaper on optimizing the performance of client-side experimentation.

2. Retrieve the snippet

Next, retrieve the snippet that you’ll add to your site. Here's a walkthrough, with step-by-step instructions below:


  1. Navigate to Settings > Implementation.

  2. Click the action-button.png icon for to the snippet you want to copy.
    If your account uses custom snippets, you might see more than one snippet listed. Choose the one that's relevant to this part of your experimentation program.

  3. Click Copy Snippet Code.

  4. Click the name of the snippet.

  5. Enable the snippet for Optimizely and choose a snippet configuration. Skip this step if you're using Optimizely the first time.

  6. Click Save.

Never have more than one snippet on your site at a time.


If you ever need to find your Project ID, it's the number immediately preceding the ".js" in the snippet URL.

3. Specify origins where Optimizely will run

Next, specify the domains where Optimizely will run and track events.

An “origin” is a combination of a specific hostname, protocol, and port on your site. Tell Optimizely where you'll track events events. By default, events that you track in Optimizely can only be used to target changes on the same origin, so enable cross-origin targeting to track events across origins.

4. Add your snippet to the <head> tag of your site

For optimal performance, add the Optimizely snippet to the <head> tag of the HTML for your site. We recommend implementing the snippet as high up in the head tag as possible, generally after your <html> tag, charset declarations, and possibly other meta tags. Place the Optimizely snippet before code for any analytics or heatmapping platforms as well.

Place these before the snippet:

  • Charset declarations and other meta tags, like <meta charset='utf-8'>

  • jQuery, if you're using your own version and not Optimizely's version (see jQuery Settings)

Place these after the snippet:

  • All other elements on your page

  • Any analytics or heatmapping code

  • Other content in a tag manager 

Here's what the Optimizely snippet might look like on a page:

<!DOCTYPE html>
<html lang="en">
        <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
        <!-- Add other meta information here -->
        <!-- Add stylesheets here -->
        <script src=""></script>
        <!-- Add scripts and other content here -->

Why in the top of the head tag? Optimizely works by making changes to your page as the page loads. If the snippet is added to the top of the head tag, then Optimizely will make changes to the page as it loads.

If the snippet is added anywhere lower, it will technically still work. But if the page has already loaded content that the visitor sees before the Optimizely snippet loads, the original version of the page may load before being transformed into the variation by Optimizely. This is known as "page flashing." In most cases, the code executes too quickly for this to be visible. But to avoid any potential problems we suggest adding the snippet as early in the execution path as possible.


Please don't modify the snippet. Copy and paste it onto your site exactly as you see it within Optimizely. Otherwise, Optimizely may not run correctly on your site.

You'll need the ability to add the snippet to the head tag of your production environment and development environments (if you have both). If you don't have access, it's possible to implement Optimizely through a tag manager.


By default, the snippet is loaded synchronously, which is recommended. However, if you need to load Optimizely asynchronously, read this article to learn to implement asynchronous loading.

Check that the snippet is implemented

Here's how to check that the snippet has been implemented on your page correctly.

Load Optimizely with a tag manager or into a CMS

We don't recommend that you load the Optimizely snippet through a tag manager. 

Loading Optimizely through a tag manager can cause issues such as page flashing, where the original version of your page is briefly displayed to visitors before the Optimizely experiment. Google Tag Manager, for example, doesn't support synchronous loading. Tag managers can also cause issues with your analytics integrations. To deliver the best experience to your visitors, we strongly recommend that you implement Optimizely outside of a tag manager.

If you must load Optimizely through a tag manager, consider these pitfalls and implementation suggestions and reach out to your Customer Success Manager to discuss implications.

Here are some tag managers that customers have used:

Google Tag Manager | Ensighten | Tealium iQ | Signal

If you're trying to add the Optimizely snippet to a site built using one of the following CMS or site-building platforms, please read the relevant article for more information:

Wordpress | Marketo landing pages | Yahoo store | Volusion store | Drupal | PHP website