How Bond Duration Measures Interest-Rate Risk

The one number on the factsheet worth reading

Ramesh is comparing two debt funds. They hold similar-sounding things, they have produced similar returns over the past couple of years, and the names give away nothing. One of them will barely move if interest rates change and the other could fall by several per cent.

The number that separates them is on both factsheets. It is called duration, it is quoted in years, and it is almost universally misread as a maturity date.

What it actually tells you

Duration's purpose is entirely practical: it estimates how much the price moves for a given change in interest rates.

The working rule is simple. A bond with a duration of five will fall roughly five per cent in price if rates rise by one percentage point, and rise roughly five per cent if they fall by one. Duration two moves about two per cent, duration ten about ten.

That is the whole of the useful content. Duration converts an abstract statement — rates might rise — into an estimate of what happens to Ramesh's money.

The reason it is measured in years is historical: it began as a weighted average of when you receive the bond's cash flows, and that average happens to determine price sensitivity. The interpretation as sensitivity is the one to keep.

Why waiting longer means moving more

The intuition is worth having, because it turns duration into something you can reason about rather than merely look up.

If rates rise, an existing bond's fixed coupon is now below what new bonds pay, and how badly that hurts depends on how long you are stuck with it. A bond maturing next year is disadvantaged briefly: it repays soon and the money is reinvested at the new, higher rate, so a buyer will not demand much of a discount. A bond maturing in twenty years leaves you receiving a below-market income for two decades, so a buyer demands a large discount to accept it, and the price falls a long way.

Same rate change, very different price moves, and the difference is simply how long the money is committed for. That is duration.

Duration is not maturity

This is the distinction causing most of the confusion.

Maturity is when the principal comes back. Duration is the sensitivity measure, and it is almost always shorter than maturity, because you receive coupons along the way and those earlier payments pull the weighted average of your cash flows forward.

A bond paying no coupon at all has duration equal to its maturity, since all the money arrives at the end. Every coupon-paying bond has duration below its maturity, and the larger the coupon the bigger the gap.

For a bond fund there is no single maturity at all, because it holds many bonds and keeps replacing them. Duration is the only meaningful sensitivity number, and it is the one to look for.

Reading a debt fund by its duration

This is where duration becomes genuinely practical, since most Indian investors hold bonds through funds rather than directly.

Debt fund categories differ mainly in how long they lend for, and therefore in duration. A fund holding very short paper barely moves when rates change; a long-duration or gilt fund can move sharply. That single number tells Ramesh more about how a fund will behave than its name or its past returns do — and two funds with similar recent returns and very different durations are not similar investments. One of them simply has not been tested by a rate move yet.

The practical use follows directly: match duration to how long the money is committed. Money that might be needed within a year does not belong in a fund whose price can fall several per cent on a rate move. Money with a long horizon can accept that volatility in exchange for the higher yield that longer lending usually pays.

What duration does not tell you

Duration measures one risk, and treating it as a measure of risk in general is a mistake.

It says nothing about credit risk. A short-duration fund holding poor-quality corporate paper can lose money permanently through defaults while its duration number stays reassuringly low. Duration and credit are independent, and confusing them is how "low risk" debt funds surprise people.

It is an approximation, and it degrades for large moves, because the relationship between price and yield is curved rather than straight. For small rate changes duration is accurate; for large ones it understates gains and overstates losses. That curvature has its own measure, convexity, which matters mostly for long bonds and large moves.

It assumes all rates move together, when in practice short and long rates can move by different amounts or in opposite directions — a duration estimate assumes a uniform shift, which is a simplification.

And it changes over time. As a bond ages its duration falls, and a fund's duration changes as the manager buys and sells. A fund permitted to vary it may not have the duration it had when Ramesh invested, which is the subject of its own page.

Using it

Look it up before buying a debt fund, since it is on the factsheet and it is the most informative number there. Then translate it into money: duration six on the amount you plan to invest, with a one-point rate rise, is a specific rupee figure, and working it out before investing is a considerably better experience than working it out after a fall.

Do not take duration risk with short-term money, which is the single most common avoidable error in debt investing. And remember it cuts both ways — long duration is why a bond fund can produce an unexpectedly good year when rates fall. It is not a defect; it is the exposure you chose.

What to take away

Duration estimates how much a bond or a bond fund moves when rates change: roughly its value in per cent, for each percentage point of rate change. Longer means more, in both directions.

It is not maturity, and it is not a general measure of risk, since a low-duration fund can still lose money to defaults. Check it before buying, convert it into an amount of money on your own investment, and match it to how long you can leave the money alone.