Mayor Zohran Mamdani’s vision for a free child care system for all New York City children under 5 could cost much more than he estimated, according to a new report set to be released on Monday.

A new model developed by the Center for New York City Affairs, a nonpartisan research organization affiliated with the New School, estimated that the proposed program would cost about $9 billion annually, with an average cost per child of around $27,000.

That’s about $5 billion more than the city spent on child care in 2025.

City Hall has not yet released its own cost model. As a candidate, Mr. Mamdani said he thought it would cost about $6 billion per year to create a free, universal child care system for children as young as 6 weeks, regardless of family income. The city currently offers free pre-K for 3-year-olds and 4-year-olds, with limited seats for 2-year-olds starting this fall.

The mayor used the $6 billion figure to argue that the state would need to raise taxes on New York’s highest earners to pay for more seats.

Free child care is by far the most ambitious, complex and expensive piece of Mr. Mamdani’s push to expand the city’s safety net. The new projection underscores just how difficult it could be to find the money to fund even part of his plan at a moment when families across the city have cited the soaring cost of child care as a primary reason they are struggling to afford life in New York.

But the center’s research suggests that it would be next to impossible for Mr. Mamdani to make good on his promise without receiving billions of dollars in new revenue from the state or the federal government, or perhaps both, said Lauren Melodia, the director of the center’s economic and fiscal policy work and one of the authors of the report.

The roughly $3 billion gap between Mr. Mamdani’s estimate and the center’s projection is largely attributable to teacher pay. The center’s model builds in higher rates that researchers recommend the city pay the child care centers it contracts with. Researchers also found that the city will have to hire about 23,000 new child care workers, mostly to staff classrooms of infants and children under 3 years old.

Child care providers in family care centers — small programs run out of private homes — have a median take home pay of just $6 an hour, according to a separate report by the center.

Jenna Lyle, a spokeswoman for the mayor, did not directly comment on the center’s $9 billion estimate, but said the city is committed to getting child care workers raises while also meeting parent demand for more free care.

“Delivering on those promises will require additional public investment, and this administration has been clear about that from the start,” she said in a statement.

The researchers projected that the Mamdani administration would need to create about 10,000 new seats to accommodate the infants and toddlers who are too young to enroll in pre-K right now.

The city would most likely be able to meet that demand by relying on New York’s patchwork system of family child care centers. But many of those facilities serve families who use vouchers, which are not yet a significant part of City Hall’s child care plan.

Mr. Mamdani has said he wants to offer free access without any income cutoffs, but the center’s report argues that it could be simpler — and at least marginally less expensive — to incorporate child care vouchers for low-income families, which are already funded mostly by the state and federal government. (City Hall could, for example, offer subsidized vouchers to all families in some low-income ZIP codes, Ms. Melodia suggested.)

Gov. Kathy Hochul has given gave the Mamdani administration $1.2 billion to jump-start its program for 2-year-olds in some neighborhoods. City Hall currently has no plan to fund the expansion after next year, besides asking the state for more money. The governor has said she plans to continue to fund the program.

“We are grateful to Governor Hochul for her partnership and initial investment in this work,” said Ms. Lyle, the spokeswoman.

Mr. Mamdani and his allies have already promised to renew pressure on the governor to pass a tax increase in Albany on New York City residents making more than $1 million a year. The center’s new estimate might make that task even more urgent.

To make its case, City Hall would most likely have to prove that parents are clamoring for more child care spots. So far, that picture is mixed: Despite Mr. Mamdani’s active encouragement, roughly the same number of parents applied this year as last year, under former Mayor Eric Adams, who barely advertised the programs.

City officials said they were encouraged by parent demand for seats for 2-year-olds; enrollment data will be released on Tuesday.

“The extraordinary response to our first 2-K application season is proof the demand is there,” Ms. Lyle said. “Now our responsibility is to meet it.”