A million dollars ain't what it used to be.

More than four out of ten American millionaires say they do not feel rich. Indeed many would need to have at least $7.5 million in order to feel they were truly rich, according to a Fidelity Investments survey.

Some 42 percent of the more than 1,000 millionaires surveyed by Fidelity said they did not feel wealthy. Respondents had at least $1 million in investable assets, excluding any real estate or retirement accounts.

Every person in the survey is wealthy, said Sanjiv Mirchandani, president of National Financial, a unit of Fidelity. But they are still worried about outliving their assets.

The average age of respondents was 56 years old with a mean of $3.5 million of investable assets. The threshold for rich rose with age.

They compare themselves to their peer group ... and they are also thinking about the long period they will have in retirement and want more assets to fund their lifestyle, said Michael Durbin, president of Fidelity Institutional Wealth Services.

Still, millionaires are slightly more optimistic now than they were in 2009, when 46 percent did not feel wealthy.

Respondents were also more optimistic about the U.S. economy. While they thought the current U.S. economy remained very weak, they think it will improve by the end of this year.

Fidelity noted the wealthiest 5 percent of Americans hold more than 55 percent of the nation's wealth.

(Reporting by Helen Kearney)