The Philadelphia School District will save $15.9 million on charter school payments because the state has recalculated charter rates 10 months into the fiscal year that ends June 30.

But the new amounts set by the Department of Education are bad news for the city's 83 charters. They'll see their total revenue decline by that amount.

Under the new rates, city charters will be paid $23,720 for every special education student - $401 less than they had been told earlier. The rate for a student in regular classes is now $7,745, a $209 reduction.

The state charter law contains a formula that determines the charter rate for every district in the state based on how much the district spent to educate its own students the prior year.

Philadelphia officials said the new amount reflects the fact that the district spent less on its own students than expected in 2014-15. The charters receive a portion of the total monthly payments.

Uri Monson, the district's chief financial officer, sent letters to charter operators across the city on Monday to tell them the new rates.

Timothy Eller, executive director of the Keystone Alliance for Public Charter Schools in Harrisburg, said the changes could be disruptive.

"The affected charter schools in and outside Philadelphia have to compensate for this lost revenue before the end of June - less than three months," Eller said. "This doesn't only affect the current school year, but charter schools are now forced to plan and budget for less funding in the 2016-17 school year."

Eller, whose group represents 50 of the state's 160 brick-and-mortar charter schools, said several city schools stand to lose between $250,000 and $500,000 as a result of the rate changes. He declined to name the charters.

Expecting charters "or any organization to alter their budget nine months into the school year is unreasonable and forces them to make decisions that could disrupt their students' education," he said.

DawnLynne Kacer, executive director of the district's charter school office, said the impact on individual schools will depend on their total enrollment and the number of special education students they have.

"The notice came out relatively late," she said. "It's unfortunate that there is a short amount of time to settle the books."

With only May and June payments left in the current fiscal year, Kacer said, the district would reduce the amounts of those months' checks to recover 10 months of overpayments.

In his letter, Monson said the district realized that the changes would have an impact on charters' finances and could reduce payments for some by more than 30 percent.

A spokeswoman for the Education Department said the new rates were released later than normal.

"There are other schools impacted by the revision, however, Philadelphia would see the largest impact due to the number of charter schools in the district," Nicole Reigelman said.

According to district officials, city charters can expect to see higher payment rates after July 1 because the district spent more on its own schools this school year.

The preliminary district budget released last month lists spending $61 million more on charters because of an anticipated increase in per-student rates.