Key Takeaways
The Rule of 72 is a simple way to estimate how long it will take your investments to double by dividing 72 by your expected annual return rate. Higher-risk investments like stocks have historically doubled money faster (around seven years) compared with lower-risk options like bonds (around 12 years).
“By the time you hit 33 years old, you should have $100,000 saved somewhere,” he said, urging viewers that they can accomplish this goal. “Save 20 percent of your paycheck and let the market grow at 5% to 7% per year,” O'Leary said in the video.
If you invest $300 each month, that comes out to $3,600 over the course of a full year. And after 30 years of investing, that would total $108,000. But with the power of compounding, your portfolio's value could rise far higher than that.
Buy $4000 worth of goods at wholesale, resell them with a 150% markup. Pay your taxes. Done. Invest some of the money in tools and supplies and provide a service.
Years It Takes to Double
So, to use this formula for the $100,000 investment mentioned above, with a 6% rate of return, you can determine that your money will double in 11.9 years, which is close to the 12 years you'd get if you simply divided 72 by 6.
There is only one way in which you can double your money in 5 years and that is through mutual funds. Despite the market risks, mutual funds can earn significant returns in 5 to 6 years. This is because mutual funds offer higher returns than any other investment option and higher risk.
If you take your $100,000 and put it in an S&P 500 index fund, you could end up with over $1 million within 24 years if the index produces returns in line with its historical average. If you keep saving, you can get there even faster.
The table below shows the present value (PV) of $1,000 in 20 years for interest rates from 2% to 30%. As you will see, the future value of $1,000 over 20 years can range from $1,485.95 to $190,049.64.
Automate your savings
Consistency is key when it comes to saving your first $100,000. One of the best ways to remain consistent when you have a savings goal is to take on a set-it-and-forget-it approach with automated savings.
Investing $500 a month can lead to significant long-term growth, thanks to the power of compounding returns. Whether you are just starting out or adding to an existing portfolio, consistently investing $500 each month can help you build substantial savings for future goals, like retirement or a down payment on a house.
“By the time you're 40, you should have three times your annual salary saved. Based on the median income for Americans in this age bracket, $100K between 25-30 years old is pretty good; but you would need to increase your savings to reach your age 40 benchmark.”
One of those tools is known as the Rule 72. For example, let's say you have saved $50,000 and your 401(k) holdings historically has a rate of return of 8%. 72 divided by 8 equals 9 years until your investment is estimated to double to $100,000.
Here are the best low-risk investments in 2025:
High-yield savings accounts. Money market funds. Short-term certificates of deposit. Cash management accounts.
If you start with 1 dollar and double it every day for 30 days, you would have approximately $1,073,741,824. This shows the concept of exponential growth. Like the penny example, this is not typically possible in real-world investing scenarios.
There are two approaches you could take. The first is increasing the amount you invest monthly. Bumping up your monthly contributions to $200 would put you over the $1 million mark. The other option would be to try to exceed a 7% annual return with your investments.
Calculate how much you need to save each month to reach $10,000 in three months. That's approximately $3,333 per month, which should fit into your spending plan. This likely means you'll have to prioritize your needs over wants and make some tough sacrifices, at least in the short term.
Those will become part of your budget. The 50-30-20 rule recommends putting 50% of your money toward needs, 30% toward wants, and 20% toward savings. The savings category also includes money you will need to realize your future goals.
If you invest $50 per week, that's the equivalent of $200 per month, or approximately $2,400 per year. Over a 30-year period, that would result in more than $72,000 in savings. It's a good chunk of savings, but it isn't a life-changing amount.