Bond Duration Explained

Executive Summary

Duration is one of the most widely used, and most widely misunderstood, concepts in fixed income analysis. In simple terms, duration provides an approximate measure of how sensitive a bond's price is to changes in interest rates. This article explains, in general terms, what duration measures, how it differs from a bond's simple time to maturity, and works through a clearly labelled illustrative example of how duration can be used to estimate a bond's likely price response to a hypothetical change in yields.

What Duration Actually Measures

Duration was originally conceived (as 'Macaulay duration') as the weighted average time until an investor receives a bond's cash flows, factoring in both coupon payments and the final repayment of face value. In practice, most investors encounter 'modified duration', which is derived from Macaulay duration and is used more directly as an estimate of price sensitivity: modified duration approximates the percentage change in a bond's price for a 1% (100 basis point) change in yield, in the opposite direction. For example, a hypothetical bond with a modified duration of 6 years would be expected to see its price fall by approximately 6% if yields rose by 1%, and rise by approximately 6% if yields fell by 1%, all else equal — this is an approximation and actual price changes can differ, particularly for larger yield movements.

It is worth emphasising that duration is expressed in years but is fundamentally a measure of price sensitivity, not simply a restatement of a bond's time to maturity, even though the two are related.

Duration vs Time to Maturity

While a bond's time to maturity is one input into its duration, duration is generally somewhat shorter than the bond's stated maturity for any bond that pays periodic coupons, because coupon payments return some of the investor's capital before the maturity date, effectively shortening the weighted-average time to receive the bond's total cash flows. A hypothetical zero-coupon bond — one that pays no periodic interest and simply repays a lump sum at maturity — would have a duration exactly equal to its time to maturity, since all of its value is received on a single date. By contrast, a hypothetical coupon-paying bond of the same maturity would generally have a somewhat lower duration, because part of its value is returned earlier via coupon payments.

• Longer time to maturity generally increases duration, all else equal.
• Higher coupon rates generally decrease duration relative to a lower-coupon bond of the same maturity.
• Zero-coupon bonds have duration equal to their time to maturity.
• Higher prevailing yields generally slightly reduce duration for a given bond, all else equal.

Duration compresses a bond's entire cash flow schedule and yield environment into a single number that approximates price sensitivity to rate changes.

A Worked Illustrative Example

Consider a purely hypothetical portfolio holding a bond with a modified duration of 7 years. If prevailing market yields for comparable bonds were to rise by 0.5% (50 basis points), the approximate expected price impact, using the duration estimate, would be a decline of around 3.5% (7 × 0.5%). If yields instead fell by 0.5%, the approximate expected price impact would be a gain of around 3.5%, all else equal. These figures are illustrative approximations only; actual price movements can differ due to convexity and other market factors, and this example does not represent any actual bond, portfolio or expected return.

Key Considerations for Investors

• Use duration as a general guide to relative interest rate sensitivity, not a precise predictor of price movements.
• Compare duration across bonds or portfolios to understand relative exposure to interest rate risk.
• Remember that duration estimates are less accurate for larger yield changes, where convexity becomes more relevant.
• Consider your own time horizon and risk tolerance when assessing whether a higher or lower duration exposure is appropriate.
• Reassess duration exposure periodically, as it will change as a bond approaches maturity and as yields move.

Conclusion

Duration provides a practical, if approximate, way to gauge how sensitive a bond's price is likely to be to changes in interest rates. Understanding the relationship between coupon, maturity and duration can help investors make more informed comparisons across different fixed income securities and better anticipate how their holdings might respond to a range of hypothetical rate scenarios. This article is general information only and does not constitute personal financial advice.

Related topics

Information contained within these insights is provided for general information purposes only and does not constitute personal financial advice, an offer or recommendation to acquire or dispose of any financial product. Investors should consider their individual circumstances and obtain appropriate professional advice before making investment decisions.