It was Tuesday night, but the Boerum Hill bar Fawkner was packed — not with sports fans or heavy drinkers, but small groups of millennials and Gen Xers sitting around, eagerly awaiting the next round of . . . trivia.

“Last week, there were so many teams, you couldn’t walk,” says Amy Smith, 46, a member of the True Blood Friends — one of the 300 or so teams that make up the NYC Trivia League, which holds competitions in bars all over town.

“We wanna be a place you can go [to] find trivia anywhere in the city,” NYC Trivia League founder Cullen Shaw, 35, tells The Post. Shaw has built himself something of a hobbyist group empire, and it’s become his full-time job. On Tuesdays, the league’s biggest night, about 32 hosts at bars in Brooklyn, Queens and Manhattan can be found asking patrons the same set of questions, most of which are concocted in an apartment in Astoria that’s home to three trivia league hosts.

“We have office hours,” says 32-year-old Pedar Benson Bate, Fawkner’s regular host. Bate, his wife, Meredith, and their roommate, Kyle, often brainstorm questions together in their living room.

The league is free to join for competitors and hosts 10-week seasons — the current one running through next month. Winning teams can expect prizes ranging from bar tabs to concert tickets. At the end of each season, there’s a championship called the King’s Cup, where the top three teams take home hundreds of dollars worth of swag — and cash. The venue for the next championship hasn’t been finalized, but it’ll be held on March 24. Although trivia fans are welcome to join in now and play noncompetitively — serious contenders for the King’s Cup should wait until after the championship, when the next season begins.

At Gebhard’s Beer Culture on the Upper West Side, a different set of regulars comes out for Monday night league trivia. “It’s all like-minded nerds,” Denise Page, 27, says of the crowd. She’s been coming to Gebhard’s trivia nights for a little more than a year with her team, the Blue Shells.

“If you play long enough, they’ll repeat entire rounds,” says Alice Vaughn, 29, another Gebhard’s regular. “But it’s been, like, nine months, so you don’t remember.”

At Gebhard’s, trivia-goers can rely on a few traditions: that the night will begin with 31-year-old host Joe Abrams making fun of the band Limp Bizkit and end with him playing Queen’s “We Are the Champions.” In between the two will be cries of “social!” which trivia players call out when they believe the answer is obvious.

As the evening wraps up at Fawkner’s, one of the bartenders, Jon Sokol, 26, joins the True Blood Friends’ huddle. “He’s our secret weapon,” says Fred Van Lente. “Whenever he can escape the bar, he comes by.”

At Gebhard’s, Vaughn’s team places second. While disappointed, they still lightly applaud when it’s announced that the Blue Shells took first. Both teams finish up their drinks and head out, saying goodbye to their host over the rousing sound of “We Are the Champions.”

For more details and to register, see NYCTriviaLeague.com. The next season starts on March 25.

Champ or dunce? Test your trivia know-how with these questions from past NYC Trivia League rounds.

1) Alexander Hamilton, who was born on the British Leeward Island of Nevis and immigrated to the US, founded what newspaper, which is now the fourth largest in the United States in terms of circulation?

2) What is the busiest subway station in New York City in terms of annual ridership?

3) The Very Hungry Caterpillar has two servings of what fruit on Tuesday? A) Pears B) Apples C) Strawberries D) Oranges

4) True or false: Central Park is the largest park in New York City.

5) What actress with an alliterative name has yet to win an Academy Award but has now been nominated six times? She won a Golden Globe for playing the character Margaret Keane in a 2014 Tim Burton movie.

(Answers below.)

1) The New York Post 2) Times Square 3) A 4) False – Pelham Bay Park
5) Amy Adams