What does it take to become a millionaire?
Well, one thing is certain: You don't have to make a million dollars to have saved a million. (Tweet This)
The number of workers who have $1 million or more saved in 401(k) or other workplace retirement plans provided by Fidelity Investments nearly doubled from 2012 to 2014, according to the firm. In a separate study, Fidelity analyzed over 5,500 plan participants who made less than $150,000 a year, yet had amassed over $1 million in 401(k) assets by the end of 2012.
The success of these retirement savers wasn't purely based on the stock market's gains. Financial advisors say to become a 401(k) millionaire, you need to become a serious saver by doing three things:
Start early
If you're in your 20s and you just started your first job, put money away now for retirement. Fidelity has found that to become a 401(k) millionaire, workers generally started saving at age 25 and plan to retire at 67. That doesn't mean you're too late if you jump start your retirement savings in your 40s or 50s, but you'll have to contribute a lot more money every month to make up for lost time.
Read MoreWhen to retire? The answer may surprise you
"It's the discipline that can make a difference, not the amount you earn. Keep in mind that the amounts that can be contributed to a 401(k) or similar retirement plan are the same regardless of your salary," said senior financial advisor Mary Ryan at Vanguard Asset Management Services. You can contribute up to $18,000 in a 401(k) plan this year. If you're in your 50s, you can put away an extra $6,000 "catch-up contribution" for a total of $24,000 in 2015.
Save at least 10 percent of your salary
You should aim to contribute a minimum of 10 to 15 percent of your pay to your 401(k) every year—increasing your contributions by 1 percent a year until you reach the 15 percent mark or the maximum contribution limits. If you're not saving 10 to 15 percent, at least contribute enough to meet your employer match. Most employers will offer some matching contribution. That's free money that you don't want to leave on the table.
Read MoreHow much do you really need to retire well?
Fidelity found the savings rate for the average 401(k) millionaire is actually a little higher at 16 percent, including employer contributions.
Stick to your plan
This is most important. Start saving early and save as much as you can–and make a vow to stay the course.
"It takes a combination of actions to reach any financial goal. Starting early can give you a powerful advantage, thanks to the power of compounding. But having the discipline to keep investing through the plan is also important," Ryan said.
