Retirement is becoming more and more expensive, and there's a good chance you may need at least $1 million to retire comfortably. If you're falling behind on your savings, though, it may seem impossible to become a retirement millionaire.

While it does take work to build a robust nest egg, with the right strategy, a million-dollar portfolio may be within reach. Here are three steps that can take you from $100,000 to $1 million or more in savings.

Young person putting dollar bill into a piggy bank.

Image source: Getty Images.

1. Stick to a long-term strategy

There are plenty of stories about people making a lot of money in the stock market by buying the trendiest new investment or risking everything on dangerous stocks. While it is possible to make money with those strategies, you're more likely to lose more than you gain.

A safer bet, then, is to invest for the long term. With this approach, you're buying solid stocks from healthy companies and holding them for several years or, ideally, decades.

The key to long-term investing is to leave your money alone no matter what is going on with the market. It can be tempting to try to time the market and pull your investments out before a downturn, but that can be a risky move.

The stock market may experience downturns between now and when you retire, but as long as you're investing in strong stocks, it's very likely they'll recover and continue growing.

2. Give your money time to grow

The more time you have to invest, the easier it will be to accumulate a significant amount in savings. Even if you can't afford to save much each month, you're better off investing whatever you can afford rather than waiting.

Say, for example, you're currently 40 years old with $100,000 in retirement savings. If you wanted to reach $1 million by, say, age 65, you'd need to invest just over $350 per month, assuming you're earning a modest 8% average annual rate of return on your investments.

On the other hand, say you decided to temporarily put off saving until you were 45 years old. Assuming you were still earning an 8% average annual return, you'd need to invest around $1,000 per month to become a millionaire by age 65.

3. Invest aggressively enough

While it is important to avoid investing in extremely risky stocks, you also don't want to play it too safe. When the stock market is volatile, it can be tempting to put your money behind bonds or other conservative investments. Some people may choose to keep their money out of the market altogether and stash it in a savings account.

It's perfectly normal to be concerned about market volatility, but if you're not investing the majority of your savings in stocks, your money won't grow as much as it could.

The S&P 500 stock market index has earned an average rate of return of around 10% per year, historically. Bonds, on the other hand, generally earn returns of around 5% per year, on average.

Say you have $100,000 in savings right now, and you're investing $400 per month. Here's approximately what you'd have over time depending on the returns you earn on your investments:

Number of Years Total Savings When Earning a 10% Average Annual Return Total Savings When Earning a 5% Average Annual Return
5 $190,000 $154,000
10 $336,000 $223,000
20 $948,000 $424,000
30 $2,535,000 $751,000

Author's calculations via

As you get closer to retirement, you will want to adjust your asset allocation so that you're investing more conservatively, because that will better protect your money when you need it the most. But if you still have many years until you retire, it's wise to put the majority of your savings behind stocks to help your money grow faster.

Saving $1 million by retirement age isn't easy, but it is possible. With the right strategy, you can earn more than you may think and enjoy a more financially secure retirement.