Stranger Things 4 has eclipsed the milestone of 1 billion hours viewed, Netflix said Tuesday, solidifying its status as the second most popular title the streaming service has ever released. 

In the week ended Sunday, Stranger Things 4 has been watched for a cumulative 1.15 billion hours. That's behind only Squid Game in popularity. By comparison, Squid Game was viewed for a total of 1.65 billion hours in its first 28 days of release. Stranger Things will continue to amass viewing hours toward its ranking through July 28, potentially giving it time to surpass Squid Game if it remains popular. 

But Stranger Things 4's watch-time ranking requires a couple caveats.  

First, the watch-time popularity rankings favor shows and movies with unusually long runtimes, like Stranger Things 4. It's why Martin Scorcese's film The Irishman, at three-and-a-half hours long, suddenly became Netflix's No. 7 top film ever as soon as the company started going by watch-time rather than the number of accounts that streamed it.

Stranger Things 4's runtime, at 778 minutes, is nearly 60% longer than that of Squid Game. That gives Stranger Things the advantage of simply have more hours in a single season for each account to stream. (Stranger Things' latest season is also nearly twice as long as the first season of the show was.)

Second, its hours-watched total has an advantage because the season was released in two separate batches of episodes. 

Generally, Netflix gives shows and movies only 28 days from their release for all watch hours to count toward the all-time popularity rankings. But Netflix has different rules for shows that release their latest episodes in separate "volumes," like Stranger Things 4 did. Instead of tallying up viewing hours in its first 28 days of release, as Netflix did for Squid Game, Netflix is racking up Stranger Things 4's views over the course of 56 days total: All the viewing of the first seven episodes was counted in the 28 days following their May 27 release, and all the viewing of the season's final two episodes is being counted for 28 days following their release on Friday.

While that means each individual episode's viewing is counted no more than 28 days, other series with a two-volume season have seen their watch-time totals inflate dramatically as soon as Netflix changed the rule. 

For years, Netflix was notoriously tight-lipped about its viewership. Beau Willimon -- creator of House of Cards, which put Netflix's original programming on the map -- once said the company wouldn't even share viewership metrics with him. 

But within the last two years, Netflix has become much more open about the popularity of its shows and movies to help it recruit talent and stoke buzz. First, Netflix added a top-trending ranking to its service, so people can see the most popular titles streaming on Netflix in their country on any given day. Then it also started publicly sharing popularity stats for certain titles, publicizing the number of accounts that watched two minutes of a particular title in its first month of release.

Netflix eventually launched a website in mid-November posting weekly charts of its most popular shows and movies, as well as a global ranking of all-time most watched titles. The charts are updated every week and ranked by the total number of hours that subscribers spent watching them. 

The rankings represent an unprecedented trove of data about what's popular on Netflix, detailing the most popular titles in the last week not only globally but also for more than 90 individual countries. It is, by far, the most transparency that Netflix has ever adopted for its viewership. It will also help subscribers like you have a better grip on what's most popular on the world's biggest subscription streaming service.