You ain’t dreaming. New York’s toniest buildings really are bigger on the inside than the outside.

For a small pool of New York buyers, a floor in the 90s is the new Park Avenue address — and developers are fudging numbers to accommodate them.

In 2013, Saudi retail magnate Fawaz Al Hokair signed an $87.7 million contract for a splendiferous privilege: owning the top floor of the Western Hemisphere’s tallest residential tower, 432 Park Ave. Al Hokair could boast that his 8,255-square-foot penthouse is on the 96th floor — six floors higher than billionaire Michael Dell’s then-record-breaking spread on the 90th floor of One57 (previously the city’s tallest residential tower).

Splendiferous, that is, if you ­ignore the fact that 432 Park is an 88-floor tower, eight floors less than advertised.

That’s not a fluke, it’s a power move. Nearly every new luxury condo in the city’s latest wave of super-tall construction mislabels floors to stroke buyers’ vanity.

“If you have the 95th floor in your address, that’s going to be impressive to pretty much everyone,” said Leonard Steinberg of Compass Real Estate. “Being on the 95th floor is unbelievable. In how many cities can you even live on the 95th floor?”

Like a short man in Cuban heels, One57 boasts a 90th-floor penthouse that is, technically, on the 75th floor. For more than a decade, billionaire developer Stephen Ross occupied the 80th-floor penthouse of his Time Warner Center, but has since moved up to the 92nd floor of his latest tower, 35 Hudson Yards. In reality, the Time Warner Center has 53 floors. His Hudson Yards building has 71.

“When [brokers] go see buildings under construction, we say, ‘Go to the top floor’ — which is often marketed as the 90th, but there will be a sign in the elevator that reads 63,” said broker Tristan Harper of Douglas Elliman.

This sleight of hand is achieved by developers in different ways, Harper and other experts explained. For one, most new residential skyscrapers have lobbies with tremendous ceiling heights. They might be counted as 10 or more stories. Many also have several floors of building ­mechanicals and amenity spaces that are counted.

Some — like One57 or 35 Hudson Yards — have hotels on the first couple dozen floors. Instead of counting from the first apartment, developers will divide building height by eight feet (the measure of a typical New York ceiling height) to get a total floor count that is higher than the actual number of residential floors. Or they count from the ground to determine on which floor an apartment would theoretically be.

That’s why residences at One57 start on the 22nd floor, while 35 Hudson Yards begins on the 53rd. In the industry, it’s known as marketing floors versus real floor, or “construction counting.”

“If we lived by the letter, buildings in New York would have a 13th floor — and I haven’t seen a 13th floor in a long time,” Steinberg said, adding that he considers the practice of embellishing floor numbers to be a mostly harmless example of “truthful hyperbole . . . Every developer wants to maximize value.”

Harry Macklowe is often credited with inventing vanity numbering with his Metropolitan Tower, which opened in 1985 on the south end of Central Park, now considered “Billionaires Row.” Macklowe advertised the building as having 78 floors, when it really has 66.

But it was Donald Trump who introduced 90th-floor fever. When Trump World Tower opened in 2001, he proclaimed it the “tallest residential building in the world” at 90 floors. (If you count them up, there are 72.)

“I chose 90 because I thought it was a good number,’’ Trump told The New York Times in 2003.

While the city allows developers to label floors as they please, it requires that the real number be disclosed on official offering plans.

But experts agree that, in a market where higher floors equal higher prestige and higher dollars, the rubber ruler is here to stay.

“If you repeat something often enough, it becomes the new normal,” said Steinberg. “There was a moment when a Botoxed face looked really weird. Now a face without Botox looks weird. It’s like that with real estate.”