Bonds and Interest Rates: Understand Price Changes
Separate the coupon from yield to maturity and understand why a bond’s price responds to market rates.
Published 2026-07-21
The first loss on a bond fund always surprises: how can a supposedly prudent investment fall seven percent? The answer lives in price mechanics. The coupon, the interest promised at issue, never moves; it is the security's price that adjusts when market rates change, nobody paying full price for a 2.5% bond when new issues offer 4.5%. Sensitivity is measured by duration: nine years of duration, roughly nine percent of decline per point of rate increase; a short duration barely flinches. On top sit credit risk, a separate matter, and the difference between holding a bond to maturity and holding a fund that rolls its positions continuously. This article dismantles the mystery piece by piece, shows how to align duration with horizon and how to read a paper loss without obeying it — because the mismatch between product and calendar, not the market, is usually the fixable part.
Separate the coupon from the yield to maturity
A bond carries two numbers conversation confuses: the coupon, the interest promised at issue, fixed forever; and the yield to maturity, what the security actually returns when bought at its current price. When prices move, the coupon stays and the yield to maturity adjusts: a bond with a two-and-a-half percent coupon bought below par returns more than its coupon, the difference coming from the gain at maturity. That distinction illuminates the bond market's paradoxes: a fund that just lost seven percent suddenly displays a higher yield to maturity, falling prices having raised the return for buyers. The coupon describes the security's past; the yield to maturity describes its future — and the future is what gets compared.
Understand the price-rate seesaw
A bond's price and market rates move in opposite directions, by pure arithmetic: nobody pays full price for a two-and-a-half percent coupon when new issues offer four and a half, and the old bond's price falls until the yields equalize. The seesaw works both ways, rate cuts inflating existing bonds' prices. This mechanism is neither a flaw nor a danger signal: it is the normal operation of a market where payment promises trade at current prices. The loss displayed on a bond fund after a rate rise reflects that adjustment, not borrower defaults: the coupons keep landing and the securities keep repaying at maturity.
Read duration as the sensitivity gauge
A security's or fund's sensitivity to rate moves is summarized in one published number: duration. The practical approximation: a nine-year-duration fund loses about nine percent per point of rate increase, and gains as much per point of decrease; a two-year duration barely flinches. Duration is read in the fund facts and guides the match with your horizon: short duration for the coming years' money, where stability rules; long duration for distant horizons, where the sensitivity is tolerable and the higher yields compound. Mismatch is the true source of bond surprises: a long-duration fund housed in short-term savings does exactly what it promises, at the wrong moment.
Distinguish held bond from fund, credit from rates
Two distinctions complete the reading. Holding a bond to maturity eliminates price risk, repayment at par erasing the interim fluctuations; a bond fund, by contrast, has no single maturity, its holdings rolling continuously, and its value tracks the market permanently — which changes how its declines should be read. And rate risk is not credit risk: the first moves prices without threatening repayment; the second, the issuer's solvency, threatens repayment itself and is compensated through the yield spread on lower-quality securities. A bond portfolio is chosen on both axes: duration by horizon, credit quality by function — stability cushion or return engine.
Quebec scenario: compare before confirming
A machinist in Sorel-Tracy buys a bond ETF believing it as stable as a GIC. Six months and two rate hikes later, the position shows -7% and he cannot understand how bonds lose value. His advisor takes the mystery apart. The coupon, the interest promised at issue, never moved; the price fell, because nobody pays full price for a 2.5% bond when new issues offer 4.5%. The yield to maturity of his fund has actually risen. Sensitivity depends on duration: his ETF, at nine years, loses roughly nine percent per point of rate increase, while a short-duration bond fund would barely have flinched. On top sit credit risk, a separate matter, and the difference between holding a bond to maturity and having to sell it early. He reorganizes around his real horizons: short duration for money needed within five years, long duration only for the retirement slice. The paper loss stops being a riddle; it was the price of a product mismatched to his calendar, and the mismatch, not the market, was the part he could actually fix.
Checklist
- Separate the coupon from yield to maturity
- Understand the price's reaction to rates
- Record each held fund's duration
- Estimate the sensitivity per rate point
- Align duration with the money's horizon
- Check credit risk separately
- Distinguish a bond held to maturity from a fund
- Read a paper loss without obeying it
- Reorganize by horizon rather than by fear
Frequently asked questions
How can a bond lose value?
Through its price: when rates rise, nobody pays full price for a coupon now below new issues, and the price falls to compensate. The coupon itself does not change, and the same security's yield to maturity rises. The displayed loss reflects the market, not a default.
What does duration tell me about my fund?
Its sensitivity: as an approximation, a nine-year-duration fund loses about nine percent per point of rate increase, a short-duration fund barely anything. Align duration with your horizon: short for money needed in the next few years, long only for the long term.
Does holding to maturity eliminate the risk?
It eliminates price risk, not credit risk: the issuer must stay solvent until repayment. And a bond fund has no single maturity — its holdings roll continuously. The distinction between holding a bond and holding a fund changes how declines should be read.