What Is Bond Duration?
Bond investing looks calm on the surface, but the prices of bonds and bond funds do move, sometimes a lot, when interest rates shift. Duration is the measure that tries to capture how much.
The short answer
Duration is a measure of how sensitive a bond’s price is to changes in interest rates, expressed roughly in years. A higher duration means the bond’s price will tend to move more, in either direction, for a given change in rates, while a lower duration means less price movement. It’s related to, but not the same as, a bond’s actual time until maturity.
Why bond prices move with rates at all
Bond prices and interest rates generally move in opposite directions. If new bonds are being issued at higher rates than an existing bond pays, that existing bond becomes relatively less attractive, so its price tends to fall to compensate a buyer for the lower rate. If new rates fall below what an existing bond pays, that older bond becomes relatively more attractive, and its price tends to rise. Duration measures how strongly a given bond reacts to this dynamic.
A simplified illustration
As a hypothetical example, imagine two bonds, both currently priced the same, but one has a duration of 2 years and the other a duration of 10 years. If interest rates rise by a percentage point, the bond with a duration of 10 would generally be expected to lose roughly five times as much value as the bond with a duration of 2, all else equal. This illustrates the core relationship: longer duration means greater sensitivity to rate changes, not just a longer wait until maturity.
What drives duration higher or lower
A few factors push duration up or down. Bonds with longer times until maturity generally have higher duration, since there’s more time for rate changes to affect the bond’s value. Bonds with higher coupon, or interest, payments generally have lower duration than similar bonds with lower payments, because more of the bond’s value is returned to the investor sooner, in the form of interest, rather than waiting until maturity. This is why duration and maturity, while related, aren’t identical measures.
Why it matters for a portfolio
Understanding duration helps an investor gauge how much a bond holding, or a bond fund, might swing in value if rates move. Someone who expects to need the money soon, or who wants to minimize price swings, might lean toward shorter-duration holdings. Someone with a longer time horizon might be more willing to accept the larger swings that come with longer duration, particularly if it’s paired with a higher yield. This connects to a bond’s yield to maturity as well, since yield and duration together give a fuller picture of what a bond might offer and how much it might move along the way.
A practical habit
Duration isn’t something most people need to calculate by hand, since it’s typically published for bond funds and available for individual bonds through a broker. The practical habit is simply to check it before assuming any bond holding is automatically low-risk — a long-duration bond can experience meaningful price swings, even though bonds as a category are often described as more stable than stocks. Checking duration alongside diversification across a portfolio gives a clearer sense of how exposed your overall holdings are to interest rate changes.