In the past 12 years, the share of women with $1 million or more invested in their 401(k) has doubled, according to a Fidelity Investments report cited by the New York Times.

Fidelity oversees 401(k) accounts for around 15 million people, and 133,000 of those accounts have $1 million or more in assets. Around 20% of Fidelity's 401(k) millionaires were women in the 2017 report, compared with just under 10% in 2005.

While saving $1 million may seem like a stretch, it is possible even for people on an average salary who start young or who save aggressively. To get an idea of what it might take for you to become a 401(k) millionaire, let's look at what can be learned from all these new 401(k) millionaire women.

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Having a good job helps

Fidelity revealed that the average women who are 401(k) millionaires earn $287,700 annually, which is far above the median household income. However, while earning a ton of money definitely makes it much easier to become a 401(k) millionaire, Fidelity's data showed there were plenty of men and women with $1 million or more in their 401(k)s who had incomes below $150,000.

With a typical income of around $46,000, if you started saving around 15% of your income in a 401(k) and put away $6,900 each year for 35-years, you'd end up with $1.19 million, assuming an 8% return on investment.

If you think you need more income to become a millionaire, you can always take on a side gig or take steps to increase your income, such as advancing your education or making sure to negotiate salary and raises when you get hired at a new job.

It takes time to become a 401(k) millionaire

The millionaires with 401(k) accounts at Fidelity didn't build up their 401(k) balances overnight. In fact, the average age of 401(k) millionaires is 58.5 for women, and 59.3 for men.

Unless you inherit, earn a ton of money, or get very lucky with your investments, slow and steady progress -- and making smart money decisions over time -- is the best way to build up a nest egg of $1 million or more.

As you get older and your salary goes up, you can also afford to invest more money. Fidelity data shows savings rates increase as retirement approaches: Millennials save 10.2% on average, compared with 11.7% for Gen Xers.

The good news is, when you reach age 50, you're allowed to make catch-up contributions, which means you can invest $6,000 more in a 401(k) and $1,000 more in an IRA than the standard annual limits. If you're reaching retirement and aren't yet a millionaire, being able to save more money pre-tax can help you get there.