Bonds, Duration and Interest-Rate Risk

A bond is a loan, but its market price can move

A bond is a debt security issued by a government, company, or other borrower. The investor lends money in exchange for promised interest and repayment terms. If a fixed-rate bond is held to maturity and the issuer meets its obligations, the contractual cash flows may be known. But before maturity, the bond can trade above or below its face value.

That market price matters to anyone who may sell before maturity and to funds that continuously hold portfolios of bonds. A bond fund does not have one maturity date at which the investor is guaranteed to receive a particular face value.

A useful way to study bonds, duration and interest-rate risk is to separate the calculation from the decision. The calculation answers a narrow question using stated inputs; the decision also depends on timing, liquidity, uncertainty, fees, taxes, contractual terms and what happens if an assumption is wrong. In this lesson, the central idea is understand why bond prices and yields move in opposite directions, what duration measures, and how maturity and credit risk differ. Treat that statement as a framework to test rather than a one-off rule to memorise.

Debt should be modelled as a stream of future cash flows, not only as a balance or an interest rate. Record the outstanding principal, annual or periodic rate, minimum or scheduled payment, remaining term, fees and whether the rate can change. Then compare alternatives over the same time horizon. A lower monthly payment can be useful for cash flow while still producing a higher total cost if the debt is extended for many more months or if new fees are added.

For repayment decisions, distinguish interest saved from liquidity given up. Paying principal earlier can reduce future interest, but the cash used for an overpayment is no longer available for an emergency or other obligation. That trade-off becomes especially important when the debt is low-cost, when income is uncertain or when the product charges a prepayment fee. A sound comparison therefore includes both the mathematical saving and the household's remaining cash buffer.

Variable-rate and promotional products need a second scenario. Model the cost after an introductory rate expires or after a plausible rate increase. For revolving credit, check whether the planned payment still reduces principal meaningfully at the higher rate. If the payment barely covers interest, the payoff period can become extremely long, so the first useful intervention may be changing the payment or stopping new borrowing rather than searching for a slightly better headline rate.

When comparing consolidation or refinancing, include every cost that changes: setup fees, settlement charges, transfer fees, security over an asset, term length and the consequences of missing payments. Replacing several debts with one facility can simplify administration, but it does not erase the principal. The comparison is strongest when the old and new paths are placed side by side with the same starting date and a clear assumption about future spending.

A practical exercise is to build a baseline using today's best-known numbers, then change one important input at a time. Keep the other assumptions fixed so the effect is visible. After that, combine two adverse changes to see whether the conclusion is still robust. This method is deliberately simple: it does not predict the future, but it shows which variable has the greatest leverage and where a small amount of extra margin could materially improve resilience.

Finish by writing a short decision note: what was assumed, what evidence supports those assumptions, what could invalidate them, and when the calculation should be reviewed. That habit is especially useful for bonds, duration and interest-rate risk because the inputs can change while the original reasoning is easily forgotten. A model becomes more valuable when someone can return later, update the changed facts, and understand why the earlier conclusion moved.

Why rates and fixed-rate bond prices usually move in opposite directions

Imagine an existing bond pays a fixed coupon based on a 3% market environment. If new comparable bonds become available at 5%, the old bond's fixed payments are less attractive. Its market price generally has to fall enough for a buyer to earn a return that is competitive with current alternatives. If market rates fall, the opposite effect can make the older higher-coupon bond more valuable.

This is interest-rate risk. It exists even when the issuer remains financially healthy. Credit risk is different: it is the risk that the issuer cannot make promised payments.

Duration measures sensitivity, not simply time to maturity

Duration is a measure related to the timing of a bond's cash flows and is commonly used to estimate how sensitive its price is to a change in yield. As a rough approximation, a bond or bond fund with a duration of five years might move about 5% in the opposite direction for a one-percentage-point parallel move in yields, before allowing for convexity and other effects.

This is an approximation, not a guarantee. Real yield curves do not always shift uniformly, and credit spreads can move at the same time. Duration remains useful because it translates an abstract rate move into an approximate price sensitivity.

Longer duration generally means more rate sensitivity

Longer-maturity fixed-rate bonds tend to have greater interest-rate sensitivity than otherwise similar shorter-maturity bonds because more of their value comes from cash flows further in the future. Lower coupons can also increase duration. Floating-rate instruments behave differently because their coupons can reset, though they still carry other risks.

A bond portfolio's duration can change over time as securities mature, new bonds are purchased, or the manager changes exposures.

Yield is not the same as guaranteed return

A quoted yield is based on price and expected cash flows under particular assumptions. Realised return can differ because the investor sells early, coupons are reinvested at different rates, the issuer defaults, a bond is called, or a fund changes holdings. High yield often reflects higher perceived risk rather than a free increase in return.

Compare yield with credit quality, duration, liquidity, currency exposure, call features, fees, and the reason the bond is in the portfolio.

Bond funds add portfolio mechanics

A bond fund owns many securities and may receive coupons, trade holdings, and distribute income. Its net asset value can rise or fall. Because the portfolio continually changes, "holding the fund to maturity" does not work the same way as holding one individual bond with a specific maturity date.

Funds can improve diversification across issuers, but they also introduce management fees and can still be concentrated by sector, country, credit quality, or duration.

Credit risk and spread risk

Corporate and other non-government bonds can trade at a yield spread over a lower-risk benchmark. If investors become more concerned about the issuer or sector, that spread can widen, pushing the bond price down even if benchmark interest rates do not change. A rate-risk model alone therefore cannot explain every bond-price movement.

Credit ratings are one input, not a guarantee. Investors should understand the issuer, seniority, security, covenants where relevant, and what happens in default.

Liquidity matters during stress

Some bonds trade less frequently than large public equities. In stressed markets, bid-ask spreads can widen and the price available for an immediate sale can be worse than a model value. Funds holding less-liquid assets can also face pressure when many investors redeem at once.

Liquidity risk is one reason a bond with a higher quoted yield may not be directly comparable with a more liquid security.

Authoritative references

Investor.gov: Bonds FAQ

Investor.gov: When interest rates go up, prices of fixed-rate bonds fall

Investor.gov: Bond funds and income funds