2.2.4 Duration

A measure of interest-rate sensitivity

Duration is expressed in years, but it is not simply the time left until maturity. It combines the time to each cash flow with its size, giving a single number that estimates how much a bond's price will move for a given change in interest rates. A bond with a duration of 7 will typically fall in price by roughly 7% if rates rise by one percentage point, and rise by roughly the same amount if rates fall.

Duration measures how sensitive a bond's price is to changes in interest rates. It is expressed in years, but its practical meaning is about price change, not time. A bond with a duration of 5 years will lose approximately 5% of its value for every 1 percentage point rise in interest rates, and gain approximately 5% for every 1 percentage point fall. This is the most important number for understanding how a debt fund will behave when the RBI changes rates.

There are two common measures: Macaulay duration (the weighted average time to receive all cash flows, measured in years) and modified duration (Macaulay duration adjusted for the yield level, which gives a direct price sensitivity estimate). For practical investment decisions, modified duration is the more useful number, and it is the one most commonly reported in mutual fund factsheets.

What drives duration higher or lower

Longer time to maturity generally means higher duration, since more of the bond's value sits further in the future. Lower coupons also raise duration, because a larger share of the total return comes from the final repayment at maturity rather than from steady interim payments. A long-maturity, low-coupon bond will typically have meaningfully higher duration, and therefore higher price sensitivity, than a short-maturity, high-coupon one.

Three factors drive duration higher: longer maturity (cash flows are further away), lower coupon rate (less of the total return comes from near-term coupons), and lower yield to maturity (future cash flows are discounted less, making them relatively more important). A 30-year zero-coupon government bond has the highest possible duration for a given maturity because 100% of its cash flow arrives at the end. A short-term bond with a high coupon has low duration because most of the cash flow arrives quickly.

In India, short-duration debt funds typically maintain a portfolio duration of 1-3 years, medium-duration funds 3-5 years, and long-duration or gilt funds 7-12 years or more. The choice between them is fundamentally a bet on the direction of interest rates. If you expect rates to fall, longer duration amplifies the capital gain. If you expect rates to rise, shorter duration protects capital. If you have no view on rates (which is the honest position for most investors), a short to medium duration fund provides a reasonable balance of yield and stability.

Why this matters when choosing a debt fund

A debt fund's category name usually signals its typical duration range, and that duration is the single biggest driver of how much its price will move when interest rates shift. Matching a fund's duration to your own time horizon and comfort with price swings is one of the more important decisions in debt investing.