An author who studied over 10,000 millionaires says those planning to retire early often overlook a crucial step

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If you want to retire early as a millionaire, you need to plan for "the bridge period."

• Future millionaires who want to retire early often forget to plan for "the bridge period," according to an author who surveyed 10,000 American millionaires.
• The bridge period is the time between leaving work and when you're able to withdraw funds from retirement accounts.
• If you withdraw from a retirement account before age 59 1/2, you could incur a penalty.
• Experts say you can avoid this by investing in a growth stock mutual fund or creating a Roth IRA conversion ladder.

If you think you're on track to retiring early as a millionaire, make sure you don't make the same mistake that some aspiring early retirees do.

"One thing that often gets overlooked by a lot of future millionaires is the bridge period," wrote Chris Hogan in his book,"Everyday Millionaires: How Ordinary People Built Extraordinary Wealth - and How You Can Too."

The bridge period, he said, refers to the time between leaving work and when you're able to withdraw funds from retirement accounts.

In partnership with the Dave Ramsey research team, Hogan studied 10,000 American millionaires (defined as those with a net worth of at least $1 million) for seven months, and found that those who decide they want to retire early need to plan a little more for the extra money they'll need to cover their bridge years.

"Let's say you want to retire at 55. That's great - but only if you've planned for it," Hogan wrote. "You can't withdraw funds from your retirement accounts without incurring a penalty until you're age 59 1/2, so, in this example, you'll need about four-and-a-half years of investment savings outside of your retirement accounts to live on."

"If all your money is in IRAs and 401(k)s, not only will you pay state and federal income tax when you take it out to pay your bills - after all, it has never been taxed - but you may also pay a 10% penalty for premature withdrawals," Mari Adam, a certified financial planner based in Florida who founded Adam Financial Associates, previously told Business Insider.

Read more: 7 things no one tells you about early retirement

To avoid these penalties, Hogan recommends setting money aside outside of your retirement accounts in a growth stock mutual fund.

Mutual funds pool together money from multiple investors and are managed by fund managers. The investor buys shares in a mutual fund - the percentage they own is the percentage of gains they're entitled to. It's a good option if you don't have the time or knowledge to choose your own investments, Business Insider previously reported.

Saving and investing outside of tax-sheltered retirement plans is one of two ways you can access penalty-free money in early retirement, according to financial planner Jeff Rose. The other, he said, is creating a Roth IRA conversion ladder.

You can avoid penalties by implementing a Roth IRA conversion ladder

You may be able to access retirement funds without penalty before age 59 1/2 by transferring money from a 401(k) or traditional IRA to a Roth IRA, a type of individual retirement account that's funded with after-tax money. The strategy only works if you've had the Roth IRA for five years.

By transferring sums of money to your Roth IRA year after year - referred to as "climbing the ladder" - you'll have early access to your 401(k) savings throughout retirement. You do have to pay taxes on the money when you transfer it to the Roth IRA, but there are no taxes upon withdrawal. You also have to wait five years after the first conversion to access the money, so you must begin the process at least five years before your desired retirement age.

"They will have to make annual Roth IRA conversions equal to the amount of money they believe they will need in order to live comfortably in retirement," Rose said.

The standard retirement withdraw rate is 4% a year, meaning: a retiree should be able to withdraw 4% of their nest egg in the first year and live comfortably, adjusting for inflation each year after that. Those living by the 4% rule should be set for at least 30 years, but the strategy isn't foolproof. And some early retirees learn to live on less. Chris Reining, who retired early as a millionaire at age 37, lives off 2% of his nest egg.

Read more: How much money you need to retire early depends almost entirely on 2 factors

"As you plan your retirement dream, simply set a retirement age target, figure out how much money you'll need to live on, and make sure you're on track to have that much set aside for each year of your early retirement until you can access your retirement accounts without penalty," Hogan wrote.

He recommends working with both a financial and a tax professional since it can be tricky to figure out.