Almost half of America's millionaires do not feel rich enough despite their out-sized savings, while almost two-thirds fret over the chance that rising taxes might further erode their enviable nest eggs, according to a new survey.

Fidelity Investments, a Boston-based financial services company, polled US households with assets to invest - excluding workplace retirement accounts and real estate - of at least $US1 million.

Of the more than 1,000 millionaires who responded, 42 per cent said they do not feel all that well off.

Fidelity then asked the unsatisfied cohort of millionaires what it would take to make them feel better about their net worth. The answer: assets to invest of at least $US7.5 million.

For comparison's sake, the average US household has a net worth - including retirement assets and real estate holdings - of $US86,000, according to the Federal Reserve.

Sixty-four per cent of the millionaires who participated in the Fidelity survey said they were "extremely or very concerned" about the impact of potential tax changes on their investments, and were talking with investment advisers to minimise the impact.

- Reuters