As reported in a letter to shareholders
According to a letter Netflix sent shareholders yesterday (October 16), 64 million households viewed the third season of Stranger Things in its first four weeks on the platform, making it “the most-watched season to date”.
Though Netflix didn’t disclose in the letter how it arrived at that figure, The Verge confirmed that it “count[s] a viewer [for a season of a show] if they substantially complete at least one episode (70 percent)”.
In July, Netflix announced that 40.7 million households had watched Stranger Things season 3 within its first four days available on the service, and that 18.2 million had already completed the whole season.
Meanwhile, an independent study Nielsen conducted in July – which did not take into account viewers on mobile, tablets and laptops – showed that 26.4 million unique viewers in the United States had watched Stranger Things season 3 in its first three days on the platform.
Netflix renewed Stranger Things for a fourth season in September. Fans believe that filming for season 4 has already started, while theories have begun to swirl around the new season: Some believe Millie Bobby Brown’s character Eleven might be the new villain, or that she will spark a disaster of nuclear proportions.