Editing Interest (section)

===Compound interest===
{{main|Compound interest}}
{{see also|Rate of return}}
Compound interest includes interest earned on the interest that was previously accumulated.

Compare, for example, a bond paying 6 percent semiannually (that is, coupons of 3 percent twice a year) with a certificate of deposit ([[Guaranteed Investment Certificate|GIC]]) that pays 6 percent interest once a year. The total interest payment is $6 per $100 par value in both cases, but the holder of the semiannual bond receives half the $6 per year after only 6 months ([[time preference]]), and so has the opportunity to reinvest the first $3 coupon payment after the first 6 months, and earn additional interest.

For example, suppose an investor buys $10,000 par value of a US dollar bond, which pays coupons twice a year, and that the bond's simple annual coupon rate is 6 percent per year. This means that every 6 months, the issuer pays the holder of the bond a coupon of 3 dollars per 100 dollars par value. At the end of 6 months, the issuer pays the holder:

:<math>\frac {r \cdot B \cdot m}{n} = \frac {6\% \times \$10\,000 \times 1}{2} = \$300</math>

Assuming the market price of the bond is 100, so it is trading at par value, suppose further that the holder immediately reinvests the coupon by spending it on another $300 par value of the bond. In total, the investor therefore now holds:

:<math>\$10\,000 + \$300 = \left(1 + \frac{r}{n}\right) \cdot B = \left(1 + \frac{6\%}{2}\right) \times \$10\,000</math>

and so earns a coupon at the end of the next 6 months of:

:<math>\begin{align}\frac {r \cdot B \cdot m}{n} 
&= \frac {6\% \times \left(\$10\,000 + \$300\right)}{2}\\
&= \frac {6\% \times \left(1 + \frac{6\%}{2}\right) \times \$10\,000}{2}\\
&=\$309\end{align}</math>

Assuming the bond remains priced at par, the investor accumulates at the end of a full 12 months a total value of:

:<math>\begin{align}\$10,000 + \$300 + \$309 
&= \$10\,000 + \frac {6\% \times \$10,000}{2} + \frac {6\% \times \left( 1 + \frac {6\%}{2}\right) \times \$10\,000}{2}\\
&= \$10\,000 \times \left(1 + \frac{6\%}{2}\right)^2\end{align}</math>

and the investor earned in total:

:<math>\begin{align}\$10\,000 \times \left(1 + \frac {6\%}{2}\right)^2 - \$10\,000\\
= \$10\,000 \times \left( \left( 1 + \frac {6\%}{2}\right)^2 - 1\right)\end{align}</math>

The formula for the '''annual equivalent compound interest rate''' is:

:<math>\left(1 + \frac{r}{n}\right)^n - 1</math>

where

:r is the simple annual rate of interest
:n is the frequency of applying interest

For example, in the case of a 6% simple annual rate, the annual equivalent compound rate is:

:<math>\left(1 + \frac{6\%}{2}\right)^2 - 1 = 1.03^2 - 1 = 6.09\%</math>