Record average account balances in the nation’s 401(k)s have translated into a surge in retirement savers who can now consider themselves millionaires. 

New data from Fidelity Investments shows about 72,000 workers have $1 million or more parked in 401(k) plans, twice as many in 2012 and five times as many as a decade ago. 

Half of 401(k) millionaires had balances between $1 million and $1.25 million at the end of 2014, 39 percent held between $1.25 million and $2 million, and 9 percent of accounts held more than $2 million. 

The record amount of millionaires, and record account averages for all savers of more than $91,000, is explained in part by strong market returns.

But year-over-year contribution rates also increased by 4 percent, to an average of $9,670. When factoring employer matches, the average savings rate for all participants was 12.2 percent of salary in 2014. 

The millionaires, meanwhile, are investing for growth. About 72 percent of their portfolios are held in equities or equity funds, with only 12 percent of that being in company stock. 

And they contribute aggressively over the long haul. The average millionaire’s contribution was 16 percent of income in 2014, or $21,400. After factoring employers’ matches, the average contribution was $35,700 last year. 

The wealthiest savers are, as might be expected, older, almost 60 on average. That suggests they didn’t become 401(k)-rich overnight, but rather have been building their nest egg over decades. 

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401(k) contribution rates rising faster than ever