Macaulay and Modified Duration Explained

Duration is the key measure of how sensitive a bond (or debt fund) is to interest-rate changes. There are two related versions worth knowing.

Macaulay duration — the time measure

Macaulay duration is the weighted average time (in years) until you receive a bond’s cash flows (coupons + principal). A zero-coupon bond’s Macaulay duration equals its maturity; a coupon-paying bond’s is shorter, since you get some cash earlier. It’s expressed in years.

Modified duration — the price-sensitivity measure

Modified duration converts that into price sensitivity: it estimates the % change in a bond’s price for a 1% change in interest rates. A modified duration of 4 means a 1% rate rise would cut the price ~4%, and a 1% fall would raise it ~4%. Higher duration = more interest-rate risk, both ways.

Why it matters for debt funds

Debt funds publish their portfolio duration:

• Low-duration funds (liquid, ultra-short) barely move with rates — safer, lower return.
• Long-duration / gilt funds swing a lot with rates — more risk and more potential gain when rates fall.

So in a falling-rate environment, longer-duration funds can benefit; in a rising-rate one, they can lose. Match a fund’s duration to your horizon and rate view.

A refinement: convexity

Convexity captures that the price-yield relationship isn’t perfectly linear — important for large rate moves, but duration is the first-order measure most investors use.

Duration is the single most useful number for judging a debt investment’s rate risk. Education only, not a recommendation.