Activision Blizzard Inc.’s esports league, the Overwatch League, is using a longtime TV measurement yardstick to make it easier for advertisers to consider its programming alongside traditional sports on broadcast and cable.

The league pits teams against each other in the Activision Blizzard videogame “Overwatch” during a recurring regular season that is modeled on traditional pro sports, with competitions viewable online and on TV around the world. The overall prize pool for the 2019 season is $5 million.

The Overwatch League in February began releasing Nielsen viewership reports on the average number of people who watch any minute of a league event, among other Nielsen-backed numbers.

This average-minute audience, or AMA, number has long been used by traditional sports leagues and broadcasters to show the reach of live sports across a full broadcast.

Nielsen metrics and the average audience numbers in particular help simplify the pitch to advertisers because they are familiar data points, said Frank Puma, investment business lead at ad agency Mindshare USA.

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“As advertisers look for more and more video inventory, especially within the 18-34 [demographic], this is something that will help Activision Blizzard be part of the conversation,” Mr. Puma said. “What they’re doing is saying: Here is our audience and we are willing to sell in the way you need to buy it. They are making it more digestible for ad buyers.”

Advertisers in the Overwatch League have included Coca-Cola, Bud Light and Toyota.

Providing average-minute ratings has helped Overwatch League sign deals with advertisers that aren’t endemic to esports, said Kasra Jafroodi, strategy and analytics lead at Activision Blizzard Esports.

Across the Overwatch League’s 2019 regular season, which began in February and will close with its grand finals in September, the league has averaged a global average-minute audience of 313,000 viewers, up 18% year over year, according to the Nielsen data. That includes viewing on digital platforms such as Twitch and Asian streaming players including Zhanqi and Huya, as well as linear broadcasts in the U.S. and Canada on ABC, ESPN and TSN. Less than a third of Overwatch League’s viewership was in the U.S.

The season has comprised 526 hours of content so far, according to Mr. Jafroodi.

Viewership for Overwatch League peaked during key moments of the regular season, which is broken up into four different stages before the grand finals in the fall. For example, the final week of stage three averaged 466,000 global viewers per minute, up 86% year over year, according to the Nielsen data.

Overwatch League trails far behind leagues like the NFL and NBA, which can pull in tens of millions of viewers in the U.S. alone during major games, but the esports league boasts a growing audience.

“It was critical for us to join the conversation about the most popular and most watched sports programming in the world,” said Pete Vlastelica, chief executive of Activision Blizzard Esports and commissioner of the Overwatch League.

“ “It was critical for us to join the conversation about the most popular and most watched sports programming in the world.” ” — Pete Vlastelica, chief executive of Activision Blizzard Esports and commissioner of the Overwatch League

“There were these old-school digital vanity metrics like unique viewers and peak concurrent viewers that the esports industry has been using for a long time to brag about reach—but those numbers don’t mean anything if you can’t make an apples-to-apples comparison with other programming that we are trying to compete with,” Mr. Vlastelica added.

A key part of Overwatch League’s pitch to advertisers is its reach among 18- to 34-year-olds, a demographic that is becoming increasingly harder to reach on traditional TV. Overwatch League averaged 55,000 U.S. viewers per minute in that group during the regular season, up 11% year over year.

Overwatch League was the first esports promoter to sign a deal with Nielsen to track its average audiences, but other esports giants including Tencent Holdings Ltd.-owned Riot Games have followed suit.

The two-year-old league will be profitable this year, Mr. Vlastelica said.

Write to Sahil Patel at sahil.patel@wsj.com