The World Cup does not runneth over — with decent tips.

As New York City transforms into a giant international watch party, local bartenders and servers are discovering that many visitors didn’t get the memo about America’s tipping culture.

An estimated 1.2 million soccer fans are flooding the region for FIFA World Cup Matches at MetLife Stadium in East Rutherford, N.J., packing bars and restaurants from morning kickoffs to late-night celebrations.

The cash registers are ringing, the beer is flowing and the tables are full. But the tips are coming up short. And some cities are doing something about it.

Restaurants in places such as Kansas City, Atlanta and Philadelphia have begun adding automatic 20% gratuities to checks, during the tournament, according to industry groups and restaurant operators.

Supporters say the move is intended to protect American servers who rely heavily on gratuities and avoid confusion for visitors unfamiliar with American tipping norms.

At McCarthy’s Pub NYC on West 46th Street, waitress Louise Daggett said serving the international crowds has been equal parts exhausting and entertaining.

“Everyone has been really sweet . . . they’re loud and happy, very nice,” Daggett told The Post. But many are baffled by the American ritual of signing receipts and calculating tips.

“It’s very different. And a lot of people haven’t been leaving tips,” she said.

That can sting when a large group orders countless pints over several hours.

“There’s been a lot of tables that have come in and had a tab of almost $700,” Daggett said.

She says most visitors aren’t being rude — they really don’t know how things work here.

“They do ask,” she said. “They say, ‘Oh, how do I do this?” I explain to them that tipping is big here in the city, and it’s a nice habit to do.”

And despite the longer shifts, earlier openings and extra staff brought in to handle the crowds, Daggett isn’t losing sleep over it.

“All in all, I haven’t had a bad experience because they’re so happy and excited. And they try.”

A hostess at 5 Napkin Burger on 9th Avenue said they’d been overwhelmed with the amount of tourists filling booths. She laughed at the single television the eatery had showing fans the games.

A few blocks away at Hurley’s Saloon, co-owner Anne Calimano says the tournament has transformed what is usually one of the city’s slower seasons into a nonstop party.

“It’s busy, morning, noon and night,” said Calimano. “And then with the Knicks, go Knicks, it’s just been amazing, amazing. You couldn’t move in here Saturday night.”

The crowds have been great for business. The tipping? Not so much.

“I mean, obviously they don’t tip like normal,” Calimano said.

Still, she says most of the confusion comes down to culture, not stinginess.

In many countries, there is minimal to no tipping because European restaurants pay staff like any other profession and workers don’t rely on tips, so customers assume they’ve already paid for service.

“The bartender might say, ‘was everything alright with the service?’ And they say ‘yeah,'” Calimano said. “Because in Europe . . . [the tip] was already included in the price, and they think that.”

Her staff has learned to roll with it.

“They know they’re not going to get the 20% or whatever,” she said. “And they’re fine because they’re busy and they’re making money.”

Garvey’s Irish Pub on West 41st Street is also at full throttle.

“Just all day, through the day, even our mornings are busier than our night time now,” said bartender Cathal Reynolds. “Scotland has just take over this city.”

As far as tipping, Reynolds understands why some visitors aren’t leaving American-sized gratuities.

“They’ve already spent a lot of money to be here in the first place,” he said. “The tipping, I’m not too fussed about it.”

His advice? Embrace the local customs.

“I think if you do come here, you should just engage in the culture because I think you’ll have a better time.”