New York has plenty of charter schools — 225 just serving elementary-school students at the most recent count.

But advocates for the schools, which are publicly funded but privately managed, say some parts of the city have disproportionately few, considering that they are home to the poor students the schools are intended to serve.

In a new report, the Thomas B. Fordham Institute calls attention to “charter school deserts,” which it defines as “three or more contiguous census tracts that have poverty rates greater than 20 percent but that have no charter schools.”

In New York City, according to the report, parts of Harlem, the Bronx, and central Brooklyn meet the desert criteria. So do parts of Buffalo, Albany, and Rochester upstate.

The report makes the case that geography constrains many families’ ability to choose schools, so having relatively few charter schools could mean that families do not have access to school choice — one of the Fordham Institute’s top priorities.

In New York City, the dynamic is different because transportation is widely available, and many families choose to travel some distance to access the schools they want. In addition, because of the city’s large and dense population, census tracts are relatively small, meaning that a student without a charter school in his or her own tract might in fact live near a charter school anyway.

The report scanned all 42 states that allow charter schools for deserts and found them in 39, particularly in areas just outside of city boundaries where low-income families have moved because of gentrification. Its authors say the analysis is meant to help policy makers, charter operators, and parents alike.

“Policymakers and parents can use this information to better understand the supply of schooling options in their states and cities — and to press for changes that would improve that supply,” the report reads. “Charter operators and authorizers may also find this analysis helpful as they consider where to establish new schools.”

Whether New York City and state need more charter schools is a contested topic in Albany, where lawmakers have set limits about how many of the schools can operate. Last year, Gov. Andrew Cuomo proposed changing the charter school cap so that more schools could open in New York City, but that proposal did not advance, meaning that the city can add only about 30 more schools without bumping up against the limit.

New York City Mayor Bill de Blasio, on the other hand, has long said the city has “a good dynamic right now with the cap the way it is.”

Having charter schools nearby can strain city-run schools for space, enrollment, and resources — a dynamic that charter advocates say is essential to ensuring that schools face competitive pressure to serve students effectively.