The interest rate acceptable to any potential lender is based on time-preference and a risk component. Based on the interest rates acceptable to the different potential lender, and the rate that potential borrowers are willing to pay, one ends up with a nominal market rate of interest.
The risk-component can be broken into the risk of inflation, the risk of loss of partial or full default (“credit risk”), the risk that the market rate will change while one is tied into the term of this particular bond (“interest rate risk”).There is also the risk of a bond being “called” (where a borrower can effectively cancel the loan after a certain duration). In today’s world, there could also be currency-risk if one has the option of dealing in bonds that are denominated in various currencies. One can probably list other risks too.
The price of the bond reflects this aggregate market interest rate. Typically, a bond also has a face value and pays a rate based on that face value. These face-value (or “par value”) figures explain the inverse relationship of a bond and its market interest rate.
Example 1: Consider a bond of face value $1000 which pays $100 each year (10% of face value).
Suppose the market interest rate for this type of bond (for this “bundle of time-preference and risks”) is 5%, i.e. people who lend $1000 to a bond of similar risk are paid an interest $50 a year. Clearly, this bond, paying $100 a year, is worth more than $1000, since the lender gets $50 over the typical market return each year. So, it will sell at a premium to its face value.
Example 2: Consider a bond of identical risk and identical face value of $1000, but which pays $25 each year. This bond, paying just half of what the market expects, is worth less than its face-value of $1000. Though the lender will get back $1,000 at the end of the term of this bond, he is giving up $25 each year while he holds it.
If one knows the face value, the interest payment, term of the bond, and the acceptable interest rate it is a mechanical calculation to compute the acceptable price. The really difficult part is the judge the various risks and arrive at the correct acceptable interest rate. This is where opinions will differ.
Example 3: Finally, imagine that the market valuation of time-preference and risks changes. Imagine that now the market rate for these types of bonds is 15% ($150 each year, on $1000). Now, the bond in Example 1 is worth less than its face value. The market interest rose, and that bond therefore became less attractive. This is the sense in which the price of a bond is inversely related to moves in the interest rate.