Bond Duration Explained: Estimate Price Sensitivity to Yield Changes

Bond Duration Explained: Estimate Price Sensitivity to Yield Changes

বাংলায় পড়ুন

Learning objective

Learn what bond duration measures, how to estimate the price impact of a yield change, and why a bond or bond fund with low credit risk can still experience a material market-value loss.

Fixed-rate bond prices and market yields generally move in opposite directions. When new bonds offer higher yields, an existing fixed coupon becomes less attractive unless its market price falls. Duration translates this relationship into a practical sensitivity estimate.

FINRA describes duration as an indication of how much a bond investment's price is likely to change when interest rates move. It is stated in years, but it is not simply the time until maturity.

Step 1: separate maturity from duration

Maturity is the date when the issuer is scheduled to repay principal. Duration combines the timing of all coupon and principal cash flows into a measure of interest-rate sensitivity.

In general:

• longer maturity increases duration;
• a lower coupon increases duration;
• a lower yield increases duration;
• earlier or variable cash flows can reduce effective duration.

Two bonds with the same maturity can have different durations because their coupons, yields or embedded options differ.

Step 2: know which duration is quoted

Common measures include:

• Macaulay duration: the weighted average time to receive the bond's cash flows;
• modified duration: an estimate of percentage price sensitivity to a change in yield;
• effective duration: an estimate that allows expected cash flows to change when yields move, useful for callable or mortgage-related securities.

A fund factsheet may quote one measure without displaying the formula. Confirm the definition and date because portfolio duration changes as holdings, yields and time to maturity change.

Worked example

Assume a hypothetical bond fund has:

• market value: £10,000;
• modified duration: 6.5;
• parallel yield increase: 0.75 percentage point, or 0.0075 in decimal form.

The first-order estimate is:

Estimated price change ≈ −modified duration × yield change

−6.5 × 0.0075 = −0.04875, or approximately −4.88%

On £10,000, that is an estimated market-value decline of about:

£10,000 × 4.875% = £487.50

This is a price estimate before coupon income, fees, tax, credit-spread changes and currency effects. If yields fell by 0.75 percentage point, the simple duration estimate would suggest a gain of about 4.88%, but the actual gain would not normally be exactly symmetrical because the price-yield relationship is curved.

Step 3: treat duration as an approximation

The duration formula works best for small, parallel movements in the relevant yield curve. It becomes less precise when:

• yields move by a large amount;
• short- and long-term yields move differently;
• credit spreads change independently of government yields;
• a bond can be called, prepaid or extended;
• liquidity deteriorates;
• the portfolio holds derivatives or foreign-currency exposure.

Convexity measures part of the curvature that duration leaves out. Positive convexity can make gains from falling yields larger and losses from rising yields smaller than the simple linear estimate. Callable and mortgage-related securities can behave differently because cash-flow timing changes as yields move.

Step 4: separate interest-rate risk from credit risk

A government bond can have low default risk and high duration risk. A short high-yield bond can have lower duration but substantial credit and liquidity risk. Low duration does not mean low total risk.

Track at least:

• government-yield duration;
• credit-spread sensitivity;
• issuer and sector concentration;
• liquidity and bid-offer costs;
• currency exposure;
• call, prepayment or extension features.

Investor.gov notes that even government-guaranteed bonds can lose market value if sold before maturity because the guarantee applies to scheduled payments, not the secondary-market sale price.

Step 5: understand a bond fund's rolling exposure

An individual bond that does not default can repay face value at maturity, subject to its terms. A bond fund usually has no single maturity date: it continually receives cash flows, buys new securities and replaces maturing holdings. Its net asset value therefore remains exposed to the portfolio's duration.

Rising yields can create an immediate price loss but allow future coupons and reinvestment at higher yields. Falling yields can create a price gain while reducing the yield available on reinvestment. The relevant outcome depends on the investor's holding period as well as duration.

A common rule of thumb is that if the investment horizon is close to the portfolio's duration, higher reinvestment income may offset an initial rate-driven price loss over time. This is not a guarantee: credit events, fund flows, changing holdings and non-parallel yield moves can alter the result.

Step 6: stress the curve, not one rate

Test more than a single central-bank rate move:

• all yields rise 0.50 or 1.00 percentage point;
• short yields rise while long yields remain stable;
• long yields rise more than short yields;
• government yields fall but credit spreads widen;
• the home currency moves against foreign bond holdings;
• liquidity costs increase during a forced sale.

For a portfolio, calculate the contribution from each maturity bucket and credit sector. One headline duration can hide a concentrated position at a particular point on the curve.

Common mistakes

• Treating duration as the same thing as maturity.
• Reading 6.5 duration as a 6.5% guaranteed loss.
• Forgetting to convert a percentage-point yield move into decimal form.
• Assuming price response is perfectly linear for large yield changes.
• Using government-yield duration while ignoring credit spreads.
• Calling a low-duration bond fund low risk without checking credit and liquidity.
• Assuming an individual bond and a bond fund have the same maturity behaviour.
• Comparing duration figures calculated with different methods or dates.

Practical checklist

• Identify whether the figure is Macaulay, modified or effective duration.
• Record the calculation date and current portfolio duration.
• Estimate price changes for several yield shocks.
• Add coupon income, fees and expected holding period separately.
• Stress non-parallel curve and credit-spread moves.
• Review currency, liquidity, call and prepayment risks.
• Compare fund duration with the time when the money may be needed.
• Recheck duration after major yield moves or portfolio changes.

Sources

• FINRA: Duration—What an Interest Rate Hike Could Do to Your Bond Portfolio
• Investor.gov: When Interest Rates Go Up, Prices of Fixed-Rate Bonds Fall
• Investor.gov: Bonds—FAQs

This tutorial was prepared with AI-assisted research and writing under The Money Man editorial workflow. It is educational information, not personalised investment, tax or financial advice. The figures are hypothetical estimates; bond prices, yields, credit quality and liquidity can change, and investors can lose money.