Custom Search

Effective Annual Interest Rates

Any present value calculation done in nominal terms can also be done in real terms, and vice versa. Most financial analysts forecast in nominal terms and discount at nominal rates. However, in some cases real cash flows are easier to deal with. In our example of Bill Gates, the real expenditures were fixed. In this case, it was easiest to use real quantities. On the other hand, if the cash-flow stream is fixed in nominal terms (for example, the payments on a loan), it is easiest to use all nominal quantities.

Thus far we have used annual interest rates to value a series of annual cash flows. But interest rates may be quoted for days, months, years, or any convenient interval. How should we compare rates when they are quoted for different periods, such as monthly versus annually?

Consider your credit card. Suppose you have to pay interest on any unpaid balances at the rate of 1 percent per month. What is it going to cost you if you neglect to pay off your unpaid balance for a year?

Don`t be put off because the interest rate is quoted per month rather than per year. The important thing is to maintain consistency between the interest rate and the number of periods. If the interest rate is quoted as a percent per month, then we must define the number of periods in our future value calculation as the number of months. So if you borrow $100 from the credit card company at 1 percent per month for 12 months, you will need to repay $100 (1.01)12 = $112.68. Thus your debt grows after 1 year to $112.68. Therefore, we can say that the interest rate of 1 percent a month is equivalent to an effective annual interest rate, or annually compounded rate of 12.68 percent. In general, the effective annual interest rate is defined as the annual growth rate allowing for the effect of compounding. Therefore,

(1 + annual rate) = (1 + monthly rate)12

When comparing interest rates, it is best to use effective annual rates. This compares interest paid or received over a common period (1 year) and allows for possible compounding during the period. Unfortunately, short-term rates are sometimes annualized by multiplying the rate per period by the number of periods in a year. In fact, truth-inlending laws in the United States require that rates be annualized in this manner. Such rates are called annual percentage rates (APRs).9 The interest rate on your credit card loan was 1 percent per month. Since there are 12 months in a year, the APR on the loan is 12 1% = 12%.

If the credit card company quotes an APR of 12 percent, how can you find the effective annual interest rate? The solution is simple:

Step 1. Take the quoted APR and divide by the number of compounding periods in a year to recover the rate per period actually charged. In our example, the interest was calculated monthly. So we divide the APR by 12 to obtain the interest rate per month:

Monthly interest rate = APR = 12% = 1% 12 12

9 The truth-in-lending laws apply to credit card loans, auto loans, home improvement loans, and some loans to small businesses. APRs are not commonly used or quoted in the big leagues of finance.

Step 2. Now convert to an annually compounded interest rate:

(1 + annual rate) = (1 + monthly rate)12 = (1 + .01)12 = 1.1268

The annual interest rate is .1268, or 12.68 percent.

In general, if an investment of $1 is worth $(1 + r) after one period and there are m periods in a year, the investment will grow after one year to $(1 + r)m and the effective annual interest rate is (1 + r)m Ј 1. For example, a credit card loan that charges a monthly interest rate of 1 percent has an APR of 12 percent but an effective annual interest rate of (1.01)12 Ј 1 = .1268, or 12.68 percent. To summarize:

The effective annual rate is the rate at which invested funds will grow over the course of a year. It equals the rate of interest per period compounded for the number of periods in a year.

EFFECTIVE ANNUAL INTEREST RATE

Interest rate that is annualized using compound interest.

ANNUAL PERCENTAGE RATE (APR) Interest rate that is annualized using simple interest.

The Effective Interest Rates on Bank Accounts

Back in the 1960s and 1970s federal regulation limited the (APR) interest rates banks could pay on savings accounts. Banks were hungry for depositors, and they searched for ways to increase the effective rate of interest that could be paid within the rules. Their solution was to keep the same APR but to calculate the interest on deposits more frequently. As interest is calculated at shorter and shorter intervals, less time passes before interest can be earned on interest. Therefore, the effective annually compounded rate of interest increases. Table 1.10 shows the calculations assuming that the maximum APR that banks could pay was 6 percent. (Actually, it was a bit less than this, but 6 percent is a nice round number to use for illustration.)

You can see from Table 1.10 how banks were able to increase the effective interest rate simply by calculating interest at more frequent intervals. The ultimate step was to assume that interest was paid in a continuous stream rather than at fixed intervals. With one year`s continuous compounding, $1 grows to eAPR, where e = 2.718 (a figure that may be familiar to you as the base for natural logarithms). Thus if you deposited $1 with a bank that offered a continuously compounded rate of 6 percent, your investment would grow by the end of the year to (2.718).06 = $1.061837, just a hair`s breadth more than if interest were compounded daily.