Picture an empty apartment — there are thousands in Manhattan’s new towers — and fill it with the city’s chattiest real estate developers. How do you quiet the room?

Ask about their sales.

Among the more than 16,200 condo units across 682 new buildings completed in New York City since 2013, one in four remain unsold, or roughly 4,100 apartments — most of them in luxury buildings, according to a new analysis by the listing website StreetEasy.

“I think we’re being really conservative,” said Grant Long, the website’s senior economist, noting that the study looked specifically at ground-up new construction that has begun to close contracts. Sales in buildings converted to condos, a relatively small segment, were not counted, because they are harder to reliably track. And there are thousands more units in under-construction buildings that have not begun closings but suffer from the same market dynamics.

Projects have not stalled as they did in the post-recession market of 2008, and new buildings are still on the rise, but there are signs that some developers are nearing a turning point. Already the prices at several new towers have been reduced, either directly or through concessions like waived common charges and transfer taxes, and some may soon be forced to cut deeper. Tactics from past cycles could also be making a comeback: bulk sales of unsold units to investors, condos converting to rentals en masse, and multimillion-dollar “rent-to-own” options for sprawling apartments — a four-bedroom, yours for just $22,500 a month.

The slowdown is uneven and some projects are faring better than others, but for well-heeled buyers there is no shortage of discounts and sweeteners to be had.

The analysis, a compilation of both public and proprietary listing and building data, is one of the most sobering looks yet at the city’s flagging condo market, which peaked about three years ago amid a glut of inventory. Now the market could face new obstacles, from growing fears of a recession, to changes in tax law and political instability heading into an election year.

For an industry accustomed to selling apartments years ahead of completion and skilled at concealing the pace of sales when the market falters, further headwinds could force more drastic measures.