How interest rates affect the value of bonds

Bond prices generally fall when market interest rates rise, and rise when rates fall. The reason is that a bond's payments are fixed when it is issued, so an existing bond becomes less attractive when newer bonds pay more and more attractive when they pay less. The relationship is inverse, and it applies even when the issuer has done nothing wrong. The bond's price adjusts until its return for a new buyer is roughly in line with what the market offers for bonds of similar quality and maturity.

This mechanism explains a pattern that surprises many people. A bond bought at face value can be worth less a year later even though the issuer has paid every coupon on schedule. Nothing about the company or government has to change for the price to move. The shift comes from the comparison between the bond's fixed payments and the rates available on new bonds.

A worked example with a 3 percent bond

Consider a bond with a face value of 1,000 dollars, a 3 percent coupon and ten years left to maturity. It pays 30 dollars a year. For simplicity, assume one payment a year. Suppose new bonds of the same quality and maturity offer 4 percent. A buyer would not pay full price for an older bond that pays 30 dollars when a new one pays 40 dollars on the same amount. The older bond has to become cheaper until its effective return matches 4 percent.

Working through the arithmetic, the present value of ten 30 dollar payments plus the 1,000 dollars due at maturity, discounted at 4 percent, comes to about 919 dollars. The bond would trade near 919 dollars, below its face value. A buyer who paid that price and held to maturity would earn about 4 percent overall, because the lower price makes up for the lower coupon. Reverse the example: if market rates fell to 2 percent, the same bond would be worth roughly 1,090 dollars. These figures are a hypothetical illustration of the mechanism, not a prediction about any real bond.

What duration measures in plain terms

Duration is a way to estimate how sensitive a bond's price is to changes in interest rates. It is measured in years and works as a rule of thumb: for a small change in rates, the price tends to move by about the duration multiplied by the change, in the opposite direction. A bond with a duration of 5 would be expected to fall by roughly 5 percent if market rates rose by 1 percentage point, and to rise by roughly 5 percent if rates fell by the same amount. Real moves differ, especially when rate changes are large, so treat the figure as an approximation.

Duration is usually shorter than maturity for coupon-paying bonds, because coupons return part of the money before the final date. A zero-coupon bond, which pays nothing until maturity, has a duration equal to its time to maturity. You do not need to calculate duration yourself. Bond funds often report it in their fact sheets, and comparing those figures is a practical way to see how sensitive two holdings are likely to be.

Why longer bonds usually react more

Longer-dated bonds tend to have longer durations and therefore larger price swings. A long bond locks in its coupon for many years, so the gap between its payments and new market yields persists for longer, and buyers demand a bigger price adjustment to compensate. A short-term bond reaches face value sooner, which returns the money earlier so it can be reinvested at prevailing rates. Bonds with lower coupons also tend to be more sensitive than higher-coupon bonds of the same maturity, because more of their value sits in the final repayment.

This is why people who want to limit price swings often look at shorter maturities, while those who accept larger moves in exchange for the possibility of larger gains when rates fall may look further out. Both choices involve trade-offs, and neither removes the risk that rising rates reduce market value.

Rate moves, holding to maturity and reinvestment

A rise in rates lowers the market value of a bond you already own. If you hold it to maturity and the issuer pays in full, however, you still receive every coupon and the face value. The loss is only realized if you sell before maturity. The trade-off is that your money stays tied to the older, lower rate. The opposite is also true: falling rates raise the market value of a bond you hold, but coupons received then may need to be reinvested at lower yields. That reinvestment risk is easy to overlook.

Some bonds respond to rate changes differently. Floating-rate bonds have coupons linked to a reference rate that resets periodically, and inflation-linked bonds adjust their principal to changes in consumer prices. Those features change the pattern of price movement but do not remove it altogether, and the details differ by issue. The bond's offering documents or a fund's fact sheet show how each one behaves. If the question is how much of your savings belongs in bonds at all, a licensed financial adviser can help you think through your own timeline and goals.

The core idea is simple even when the arithmetic gets detailed: a fixed payment is worth more when alternatives pay less, and worth less when they pay more.