The city is again using fuzzy math to give municipal unions credit for health-care savings that are largely created by inflating projected costs, budget watchdogs said Thursday.

Between 2019 and 2021, city officials estimated health-care cost increases of 6 to 7 percent each year.

But the Citizens Budget Commission pointed out that the city has already reached deals capping costs for one of its health-care providers at 3.5 percent in 2020 and 3 percent in 2021.

Without lifting a finger, the unions are being credited for the difference between the projected increases and the actual ones.

That tactic gives them $658 million of the $1.1 billion in savings they’re supposed to achieve over the next three years, according to the government watchdog.

“Conservative budget assumptions are meant to ensure fiscal flexibility, and any savings against high projections should be used for general operating needs, rather than claimed as a savings against labor costs,” said CBC vice president Maria Doulis.

“Attributing these savings to the health agreement essentially is claiming credit for work not done.”

The latest health-care deal with the Municipal Labor Committee requires savings of $200 million in fiscal 2019, $300 million in 2020, and $600 million in 2021 — on top of $1.3 billion in recurring savings that were generated under the prior agreement.

The earlier deal, which ran from 2014 to 2017, also relied on what critics derided as budget gimmicks.

City officials insist their projections for health-care cost increases are based on sound finances.

“I think it’s perfectly legitimate and responsible what we did,” Labor Commissioner Bob Linn said.

“I believe we fairly accounted for the projections based on sophisticated actuarial advice from the last administration and this administration, and that savings from those projections are absolutely real savings in the budget and financial plan.”