How to Estimate a Bond ETF's Response to Interest Rates Using Duration
Duration is a starting point for understanding a bond ETF's rate sensitivity. Estimate the price impact of a 0.5-percentage-point rate move and distinguish maturity, credit spreads, and income.
Modified or effective duration estimates a bond price's sensitivity to interest-rate changes. For small rate moves, the approximate percentage price change is duration multiplied by the rate change, with the opposite sign. Duration is not the same as average maturity, and this is not a formula that determines total investment return.
Align the units of the interest-rate change first
When yield rises from 4% to 4.5%, the change is 0.5 percentage points, or 0.005 in decimal form. With duration of 6, the approximate price change is −6 × 0.005 = −3%. Multiplying directly by 0.5 or using the 12.5% increase relative to the initial 4% gets the units wrong. Also check which type of duration the product reports. The first step is to avoid mixing a cash-flow-weighted maturity concept with a measure that directly expresses price sensitivity.
Check when the idea of waiting until maturity applies
An individual bond's contractual maturity differs from the holding period of a typical bond ETF. If an ETF replaces bonds to maintain index criteria, holding it longer does not automatically drive its interest-rate sensitivity toward zero. Target-maturity products have a separate structure, so read their intended termination dates and liquidation methods. In either case, credit events and expenses remain possible, so the purchase price cannot be regarded as guaranteed. Listing average maturity separately from the ETF's own intended operating period reduces confusion.
A single government yield cannot explain every change
Corporate bond ETFs are affected by credit spreads as well as government bond yields. Even if government yields fall amid economic concerns, widening credit spreads can offset the positive price effect. Actual yield curves move differently at different maturities, and nonlinear effects such as convexity matter for large moves. Holding-period returns can also include interest income, reinvestment, expenses, and currency movements. Label duration calculations as sensitivity analysis holding other factors constant, and distinguish them from final return forecasts. Separate columns for price effects and interest income in a rate-scenario table help explain how interest receipts can coexist with unrealized losses.
Worked example
Hypothetical example: You invest KRW 10 million in an ETF with duration of 7. If relevant rates rise in parallel by 0.5 percentage points, the first-order estimated price loss is about 3.5%, or KRW 350,000. If interest and similar income over the same period totals KRW 200,000, simple addition gives approximately −KRW 150,000. This is also simplified because income timing and the price path differ in practice. In a separate scenario where rates rise 1 percentage point, the approximate price change is −7%.
What to check
- Have you checked the duration type and measurement date?
- Have you converted the rate change from percentage points to a decimal?
- Have you distinguished average maturity from the ETF's termination date?
- Have you treated spreads, currency movements, and interest income as separate factors?
Sources and primary materials
This is educational material about research methods. For an actual company decision, verify the filings and primary sources for the relevant period.