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.