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Bond Duration: Why Interest Rates Affect Bond Prices

Duration estimates how strongly a bond or bond fund price may respond to an interest-rate change. A higher duration means greater rate sensitivity, but the estimate is approximate and does not measure credit, liquidity, inflation or every other bond risk.

Timeline

  1. Before purchase: Read the bond offering document or fund prospectus and identify duration, maturity, credit quality, call features, fees and liquidity.
  2. When market yields change: Use duration as an approximate first estimate of price sensitivity, recognizing that larger moves and embedded options can change the result.
  3. Before selling: Check the current market price, transaction costs and tax consequences rather than assuming face value is available before maturity.

Bond prices generally move in the opposite direction from market interest rates. If newly issued bonds offer higher yields, an older fixed-rate bond becomes less attractive unless its price falls; when prevailing yields fall, the older coupon may command a higher price. This market-price effect matters when a bond is sold before maturity and is reflected continually in a bond fund’s net asset value. Holding an individual bond to maturity can avoid selling at an unfavorable market price, but repayment still depends on the issuer meeting its obligations. [1][2][3]

Duration expresses interest-rate sensitivity in years, although it is not simply the number of years until repayment. FINRA describes duration risk as the sensitivity of a bond’s price to a one-percentage-point rate change. As a first-order estimate, a modified duration of five suggests that a parallel one-point rise in yields could reduce price by roughly 5%, while a one-point fall could increase it by roughly 5%. The estimate describes direction and scale, not a promised return. [1][2]

Maturity influences duration but does not determine it alone. The timing and size of every promised cash flow matter: a higher coupon returns more cash earlier and will generally shorten duration relative to an otherwise similar lower-coupon bond. Yield and call or redemption features also affect the calculation. That is why two bonds with the same maturity date can have different duration figures, and why a fund’s reported duration is more informative about rate sensitivity than its name alone. [1][2]

The percentage estimate works best for relatively small, parallel shifts in the yield curve. Price changes are curved rather than perfectly linear, a relationship called convexity, so the simple duration calculation becomes less exact as rates move farther. Rates at different maturities can also move by different amounts. A duration-based scenario is therefore a useful stress estimate, not a forecast of the next market price or a guarantee that gains and losses will be symmetrical. [2]

An individual bond and a bond fund use the same pricing mechanics but have different cash-flow structures. A conventional individual bond has a stated maturity and, absent default or an early call, returns face value then. A fund usually replaces maturing or sold holdings and has no single date when the shareholder’s original principal must return. Its duration can change as the portfolio changes, so investors should use the current fact sheet, shareholder report or prospectus rather than an old number. [1][2][4]

Embedded options complicate the calculation. When rates fall, homeowners may refinance mortgages and issuers may call eligible bonds, returning principal just when reinvestment yields are lower; when rates rise, expected cash flows can extend. Effective duration is commonly used for securities whose cash flows can change as rates change. Even then, models rely on assumptions about future behavior, so mortgage-backed, callable and other option-bearing securities require more than a single duration figure. [1][2][3]

Duration should be compared with the investor’s time horizon and ability to tolerate price movement, but a low number does not make a bond safe. Bonds and bond funds can also carry credit, liquidity, inflation, call and reinvestment risks, while funds add operating expenses. Review current official disclosures and the specific security’s terms; for a sale, obtain an actual price or quote. Duration is best used as one common measure for rate exposure alongside—not instead of—the rest of the risk review. [1][2][3][4]

Sources

  1. FINRA — Bonds
  2. FINRA — Duration: What an Interest Rate Hike Could Do to Your Bond Portfolio
  3. Investor.gov — Bonds FAQs
  4. FINRA — Mutual Funds

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