Mayor Eric Adams has asked the federal government for $1 billion to handle the crush of migrants who’ve surged into the Big Apple this year — but the cost was calculated when the population was far less than it is now and before a judge decided to open the floodgates at the southern border, The Post has learned.

The Federal Emergency Management Agency confirmed Wednesday that its Emergency Food and Shelter Program’s National Board received the city’s funding request and was “reviewing their application.”

But FEMA “does not share information regarding pending applications and will only share award amounts upon application approval,” a spokesperson said in an email.

Adams first floated the $1 billion price tag for providing shelter and other services when he declared a state of emergency over the city’s migrant crisis in October. The city’s estimated migrant population then was around 17,400.

As of Sunday, that number had ballooned to more than 30,300, with over 20,900 living in taxpayer-funded shelters, according to City Hall.

The migrant population is poised to further explode when a pandemic-related immigration restriction known as “Title 42” gets lifted on Dec. 21, based on a decision by Washington, DC, federal Judge Emmet Sullivan, who said it was imposed illegally by former President Donald Trump.

That ruling was appealed by President Biden’s administration, with 19 Republican-led states also filing an emergency request Monday to stay the judge’s order.

The administration fears that border crossings could skyrocket to 14,000 a day and is considering a five-month ban on asylum-seekers who enter the country illegally or arrive at ports of entry without authorization, Axios reported Tuesday, citing sources.

The projected figure would nearly double the average 7,442 daily “encounters” recorded by US Customs and Border Protection during October, the latest month for which statistics are available.

In the hard-hit border city of El Paso, Texas — which has bused more than 10,700 unwanted migrants to New York City — officials last week predicted that daily arrivals there would jump from 1,700 to as many as 2,380.

A Homeland Security Intelligence memo obtained Wednesday by CNN also warned that ending the policy would “likely increase migrant flows immediately,” with hordes of people camped out along Mexico’s northern border expected to cross into the US.

City Council Minority Leader Joe Borelli (R-Staten Island) said that lifting Title 42 would “put the Biden administration in a liberal conundrum.”

“They will have no tools to enforce the border nor will they have political capital within their own party to change the policy,” he said.

“The feds will have even less ability to stop people at the border. They will make their way here.”

Staten Island Borough President Vito Fossella also said the end of Title 42 would be “not good for the city of New York.”

“You just have to put an iPhone on the border and see the crossings on a daily basis and many of these people end up being supported by New York City taxpayers — and with no end in sight,” he said.

“Unless somebody comes to the rescue, New York City taxpayers will be on the hook for the foreseeable future.”

City Hall said it was “closely monitoring the situation with our state and federal partners” but declined to say whether officials had estimated the potential impact on the city of lifting the order and how it could increase the burden on taxpayers.

Meanwhile, a Post analysis of city records showed that officials have publicly accounted for less than $100 million in spending on the migrant crisis thus far.

That includes $23.5 million for an emergency family shelter in Manhattan’s NYMA Hotel near the Empire State Building, $12 million for another family shelter in Queens and $8.2 million for translation services.

The information was contained in contracts filed with the city Comptroller’s Office and notices filed in the City Record.

But officials have refused to reveal the total price tag for Adams’ controversial tent cities at Orchard Beach in The Bronx and on Randall’s Island — with the former never having opened and the latter under-utilized for just a few weeks.

Partial costs were revealed by Emergency Management Commissioner Zach Iscol, who said it cost about $650,000 total to “demobilize from Orchard Beach” and rebuild the facility on Randall’s Island.

Officials have also refused to say which hotels responded to the city’s request for 5,000 hotel rooms at the height of the migrant crisis rants or how much those rooms cost — an expense that The Post has estimated as high as $300 million.