WASHINGTON – Social Security recipients will get a raise in January, their first increase in benefits since 2009. It’s expected to be about 3.5 percent.

Some 55 million beneficiaries will find out for sure today when a government inflation measure that determines the annual cost-of-living adjustment, or COLA, is released.

Congress adopted the measure in the 1970s, and since then it has resulted in annual benefit increases averaging 4.2 percent. But there was no COLA in 2010 or 2011 because inflation was too low.

That was small comfort to the millions of retirees and disabled people who have seen retirement accounts dwindle and home values drop during the period of economic weakness, said David Certner, legislative policy director for the AARP.

“People certainly feel like they are falling behind, and these are modest-income folks to begin with, so every dollar counts,” Certner said. “I think sometimes people forget what seniors’ incomes are.”

Some of the increase in January will be lost to higher Medicare premiums, which are deducted from Social Security payments. Medicare Part B premiums for 2012 are expected to be announced next week, and the trustees who oversee the program are projecting an increase.

Monthly Social Security payments average $1,082, or about $13,000 a year. A 3.5 percent increase would amount to an additional $38 a month, or about $455 a year.

Most retirees rely on Social Security for a majority of their income, according to the Social Security Administration. Many rely on it for more than 90 percent of their income.

Mark Zandi, chief economist at Moody’s Analytics, said the COLA would give a boost to consumer spending next year, amounting to about $25 billion in government support, or 0.2 percent more economic growth, if beneficiaries spend it all. For comparison, the 2-percentage-point cut in Social Security payroll taxes in 2011 was worth $115 billion to U.S. households.

“It is not a magic bullet for the economy, but it will certainly be a positive for households on fixed incomes,” Zandi said.

Only twice since 1975 — the past two years — has there been no COLA.