Rule of 72 - First Florida Credit Union (2024)

FAQs

Is the Rule of 72 accurate? ›

The Rule of 72 is a simplified formula that calculates how long it'll take for an investment to double in value, based on its rate of return. The Rule of 72 applies to compounded interest rates and is reasonably accurate for interest rates that fall in the range of 6% and 10%.

Do you know the Rule of 72? It's an easy way to calculate just how long it's going to take for your money to double. Just take the number 72 and divide it by the interest rate you hope to earn. That number gives you the approximate number of years it will take for your investment to double.

You can also apply the Rule of 72 to debt for a sobering look at the impact of carrying a credit card balance. Assume a credit card balance of $10,000 at an interest rate of 17%. If you don't pay down the balance, the debt will double to $20,000 in approximately 4 years and 3 months.

Number of years to double the money = 72 / Interest Rate

It is a reasonably accurate formula and more so while using lower interest rates than higher ones. If your money is kept in a savings account that earns just 4%, it will take 18 years to double your money.

Errors and Adjustments

The rule of 72 is only an approximation that is accurate for a range of interest rate (from 6% to 10%). Outside that range the error will vary from 2.4% to 14.0%. It turns out that for every three percentage points away from 8% the value 72 could be adjusted by 1.

It's worth noting, the “rule of 72” definition isn't necessarily perfectly accurate because past market results do not predict future market behavior. However, it's a “back of the napkin” way to determine where your portfolio might potentially be in the years ahead.

Expert-Verified Answer

It will take approximately 24.04 years for a $2,200 investment to increase to $10,000 with a compound annual interest rate of 6.5%.

If you need to double your financial investment in 10 years, a savings account with a 5% interest rate, for instance, wouldn't help achieve your goals. You'd need something with a higher rate of return (at least 7.2%) to make that 10-year milestone happen.

Here's how the Rule of 72 works

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.

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To turn $5,000 into more money, explore various investment avenues like the stock market, real estate or a high-yield savings account for lower-risk growth. Investing in a small business or startup could also provide significant returns if the business is successful.

This means that the investment will take about 12 years to double with a 6% fixed annual interest rate. This calculator flips the 72 rule and shows what interest rate you would need to double your investment in a set number of years.

For example, the Rule of 72 states that $1 invested at an annual fixed interest rate of 10% would take 7.2 years ((72 ÷ 10) = 7.2) to grow to $2. In reality, a 10% investment will take 7.3 years to double (1.10^7.3 = 2). The Rule of 72 is reasonably accurate for low rates of return.

Key Takeaways:

The rule of 72 can help you forecast how long it will take for your investments to double. Divide 72 by the annual fixed interest rate to determine the rate at which the money would double. Historical returns on your investment type can help choose a realistic expected return rate, in some cases.

Choice of rule

Since daily compounding is close enough to continuous compounding, for most purposes 69, 69.3 or 70 are better than 72 for daily compounding. For lower annual rates than those above, 69.3 would also be more accurate than 72. For higher annual rates, 78 is more accurate.

The Rule of 72 is not precise, but is a quick way to get a useful ballpark figure. For investments without a fixed rate of return, you can instead divide 72 by the number of years you hope it will take to double your money. This will give you an estimate of the annual rate of return you'll need to achieve that goal.

The rule of 72 can help you get a rough estimate of how long it will take you to double your money at a fixed annual interest rate. If you have an average rate of return and a current balance, you can project how long your investments will take to double.

It is not an exact value and can only provide a general estimate of the time required to double the investment. If the interest rate changes due to some factor, the Rule of 72 becomes null and void. The Rule of 72 does not apply to changing interest rate investments or basic interest investments.