The rule of 72 is a simple and helpful math trick to help you calculate the number of years it will take to double your invested money. All it requires is a basic division problem that you can probably do in your head.

The equation is:

Number of years = 72 / compound annual interest rate

Time out—what's the compound annual interest rate? It's the annual rate of return on your investment, applied to the original principal and to the accumulated interest from previous periods. Compound interest is the engine that will boost your investment to its goal.

Here's an example:

You want to double your investment in a mutual fund that you estimate will return 8% annually. Plug the interest rate (as "8") into the equation:

# of years = 72 / 8

The answer is 9. This means that your money will double in 9 years with an average return of 8%.

The equation works in the other direction, too. If you want to calculate the interest rate needed to double your money in 12 years, you plug 12 into the left side of the equation:

12 = 72 / x

Divide 72 by 12 and the answer is 6. You'll need an annual return of 6% to double your money in 12 years.

The rule of 72 provides an estimate that is extremely close to the exact answer for rates under about 11%. A few percentage points above that, it's better to use 73; a few above that, 74; and so on.

The key to the equation and your returns is annually compounding interest. It works because the interest gained in the previous year grows your investment, boosting the return for the following year. The cycle continues to grow your investment, increasing returns proportionally.

In the first example above, it takes you 9 years to double your investment at an 8% average annual return rate. If your investment is $1,000, you will have $2,000 after 9 years with compounding interest.

Let's say that your investment really did earn 8% every year (an average return rate doesn't necessarily mean that this will happen, but that the end result will be the same as if it did). After 1 year, your investment has earned 8%, or $80.

Now, your investment is $1,080. You earn 8% the next year, too, but this time you're earning 8% on $1,080, not $1,000. So your 8% return for the second year equals $86.40. Your investment has grown to $1,166.40 in two years. This is compound interest in action.

Its effect is drastic. If you'd decided to withdraw the return each year and spend it, instead of letting it compound, your returns would be much different. You still earn $80 after one year, but you take it home and spend it. Your investment stays at $1,000 for the second year, so you earn another $80 that you take home and spend.

You earn the same $80 every year for the nine years it should have taken you to double your money and what is your total return? $80 * 9 = $720. That's $280 dollars less than you would've made with compounding interest. If your investment was $10,000, you would have lost $2,800.

That's a significant difference. The rule of 72 uses the compound annual return rate to help you estimate length of time and rates of return for you to double your money. It's a valuable tool to help you set investing goals that you can achieve.

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The Federal Reserve sets the guardrails for the federal funds rate, and through that helps control the money supply for the nation.

When you take out a loan for a car, charge something to your credit card, or get a personal line of credit, there is going to be an interest rate that applies to your loan.

A lot of different factors go into what you will be charged, including your own personal credit score. But even those with flawless credit still see a minimum charge that they can't get around. That all goes back to the Federal Funds Rate.

One thing consumers rarely realize is that all of our banks are lending money to each other every night. Banks are legally required to maintain a certain percentage of their deposits in non-interest-bearing accounts at the Federal Reserve to ensure they have enough money to cover any withdrawals that may unexpectedly come up. However, deposits can fluctuate and it's very common for some banks to exceed the requirement on certain days while some fall short. In cases like this, banks actually lend each other money to ensure they meet the minimum balance. It's a bit hard to imagine these multibillion-dollar financial institutions needing to borrow money to tide them over for a bit, but it happens every single night at the Federal Reserve. It's also a nice deal for those with balances above the reserve balance requirement to earn a bit of money with cash that would normally just be sitting there.

The Federal Reserve The Federal Reserve


The exact interest rate the banks will charge each other is a matter of negotiation between them, but the Federal Open Market Committee (FOMC) (the arm of the Federal Reserve that sets monetary policy) meets eight times a year to set a target rate. They evaluate a multitude of economic indicators including unemployment, inflation, and consumer confidence to decide the best rate to keep the country in business. The weighted average of all interest rates across these interbank loans is the effective federal funds rate.

This rate has a huge impact on the economy overall as well as your personal finances. The federal funds rate is essentially the cheapest money available to a bank and that feeds into all of the other loans they make. Banks will add a slight upcharge to the rate set by the Fed to determine what is the lowest interest that they will announce for their most creditworthy customers, also known as the prime rate. If you have a variable interest rate loan (very common with credit cards and some student loans), it's likely that the interest rate you pay is a set percentage on top of that prime rate that your lender is paying. That's why in times of low interest rates (it was set at 0% during the Great Recession), a lot of borrowers should go for fixed interest rate loans that won't increase. However, if the federal funds rate was relatively high (it went up to 20% in the early 1980's), a variable interest rate loan may be a better decision as you would be charged less interest should the rate drop without the need to refinance.

The federal funds rate also has a major impact on your investment portfolio. The stock market reacts very strongly to any changes in interest rates from the Federal Reserve, as a lower rate makes it cheaper for companies to borrow and reinvest while a higher rate may restrict capital and slow short-term growth. If you have a significant portion of your investments in equities, a small change in the federal funds rate can have a large impact on your net worth.

Getty Images/Maria Stavreva

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