What is duration risk in debt funds and how does it affect your NAV? 4 things investors should know

Debt mutual funds may appear less volatile than equity funds, but their NAV is not immune to market movements. A change in interest rates can affect the market value of the bonds held by a fund, with the impact generally larger when the portfolio has a longer duration. This means investors in debt funds can face interim gains or losses even when they are not taking equity-like exposure.

Understanding duration risk is therefore important, particularly for investors considering medium- or long-duration debt funds. Sanjiv Bajaj, Joint Chairman & Managing Director, BajajCapital Ltd, explains how changes in bond yields affect a debt fund’s NAV, what modified duration tells investors and which other factors they should check before investing.

Bond prices and yields generally move in opposite directions. When yields rise, prices of existing bonds tend to fall, while bond prices generally rise when yields decline. Since a debt mutual fund’s NAV reflects the market value of its underlying securities, these price movements can affect the NAV.

“Duration risk essentially tells us how sensitive a bond or debt-fund portfolio is to these interest-rate movements,” Bajaj said.

If new bonds start offering higher yields, existing bonds with lower yields become relatively less attractive. Their market prices then adjust, affecting the NAV of funds that hold them.

The longer the portfolio’s duration, the greater its sensitivity to interest-rate movements. A longer-duration fund can therefore see larger NAV movements, in either direction, than a fund holding shorter-maturity instruments.

However, longer duration is not inherently better or worse. “It depends on the investor’s objective, time horizon and comfort with interim volatility,” Bajaj said.

Modified duration is a useful starting point for understanding how sensitive a debt fund is to changes in interest rates.

For example, a fund with a modified duration of four years could see roughly a 4% change in its portfolio value for a 1 percentage-point movement in yields, in the opposite direction, assuming other factors remain unchanged.

But this should be treated as a rule of thumb rather than a precise forecast. Yield movements may not be uniform across the yield curve, while factors such as convexity can also influence the actual outcome.

“Duration only tells you about interest-rate sensitivity. Credit quality, liquidity and portfolio composition matter as well,” Bajaj said.

Therefore, investors should consider modified duration along with the fund’s overall portfolio, mandate and their own investment horizon rather than viewing the metric in isolation.

Investors should first look at the fund category, Bajaj said. Liquid, ultra-short, short-duration, medium-duration, dynamic bond and gilt funds can have very different interest-rate exposures because their mandates and portfolios differ.

Average maturity and modified duration can help investors understand the portfolio’s sensitivity to interest-rate movements. Yield-to-maturity is another useful indicator of the yield of the underlying portfolio, but it should not be treated as a promised return.