Bond duration is a number, expressed in years, that estimates how much a bond’s price will move when interest rates change. A bond with a duration of five years will lose roughly 5% of its market value if rates rise by one percentage point, and gain roughly 5% if rates fall by the same amount. The unit is years, but the figure works as a volatility gauge more than a timeline. You’ll see it quoted on brokerage statements, fund fact sheets, and the standardized prospectuses that mutual funds file with the SEC on Form N-1A.1Securities and Exchange Commission. Form N-1A
The Rate-Move Rule in Plain Terms
Duration’s practical value comes down to one relationship: for every one-percentage-point change in interest rates, a bond’s price moves in the opposite direction by roughly the percentage equal to its duration.2FINRA. Brush Up on Bonds: Interest Rate Changes and Duration A bond with a duration of seven years drops about 7% if rates climb one point, and gains about 7% if rates fall by the same amount.
The inverse relationship exists because new bonds entering the market after a rate hike offer higher yields, making existing lower-yield bonds less attractive. Buyers won’t pay full price for an old bond paying 4% when they can buy a new one paying 5%. The existing bond’s price falls until its effective yield matches the new market rate. Duration tells you how steep that price drop will be.
The approximation holds well for small rate changes. The quarter-point and half-point shifts that dominate most rate cycles are exactly where the rule works cleanly.
Duration Is Not the Same as Maturity
Maturity marks only the final payment date. Duration accounts for every payment along the way. Every bond runs on an indenture agreement that spells out how often the issuer pays interest, how much it pays, and when the full principal returns.3Internal Revenue Service. Understanding Bond Documents Duration takes all of those scheduled payments, weights each one by when it arrives and how large it is relative to the bond’s price, and produces a single number: the weighted average time you wait to collect the bond’s total cash flows.
Picture a balanced seesaw where each weight represents a scheduled payment. A bond that pays generous coupons early places heavy weights near the front, pulling the balance point closer to today. A bond that pays little or nothing until the end pushes the balance point far into the future. The closer that balance point sits to today, the less the bond’s price swings when rates move.
Two bonds maturing in ten years can have very different durations if one pays a 6% coupon and the other pays nothing until the end. Same maturity, different risk.
What Pushes a Bond’s Duration Up or Down
Time to Maturity
Longer maturities produce higher durations, because more of the bond’s value depends on payments far in the future. Those distant payments are more vulnerable to rate changes because a small shift in the discount rate compounds over many years. A thirty-year Treasury bond swings far more violently than a two-year note for the same rate move. This is the single biggest driver of duration differences.
Coupon Rate
Higher coupons pull duration down. When a bond pays substantial interest along the way, you’re getting more of your money back sooner, which shifts the weighted average closer to today. The longer the maturity, the higher the duration; the higher the coupon, the lower the duration.2FINRA. Brush Up on Bonds: Interest Rate Changes and Duration Zero-coupon bonds sit at the extreme: since they make no payments until maturity, their duration equals their maturity exactly. A ten-year zero has a duration of ten years, with no coupon cushion at all.
Market Yield
When prevailing yields are high, future cash flows get discounted more steeply, which makes distant payments matter less to the current price. The result is a lower duration. When yields are low, those future payments loom larger in the price calculation, stretching duration out and making the bond more rate-sensitive. Ultra-low-rate environments tend to amplify bond price volatility across the board for this reason.
Call Features
A call provision gives the issuer the right to redeem the bond before maturity, typically when rates have fallen enough to make refinancing attractive.4FINRA. Callable Bonds: Be Aware That Your Issuer May Come Calling As rates drop and the probability of a call rises, the market treats the bond as though it will mature sooner, compressing its duration. As rates rise and the call becomes unlikely, duration extends back out toward what you’d expect from a non-callable bond of the same maturity. Mortgage-backed securities behave similarly, because homeowners refinance when rates fall.
The Three Versions You’ll See Quoted
Macaulay Duration
Macaulay duration, the original version developed in the 1930s, is the weighted average number of years you need to hold a bond before the present value of all its cash flows equals the price you paid. It’s used mainly by pension funds and insurers matching assets to future obligations. Individual investors rarely see it quoted directly.
Modified Duration
Modified duration converts the Macaulay figure into a direct measure of price sensitivity: the approximate percentage price change for a one-percentage-point move in yield. When a brokerage or news outlet says “this bond has a duration of 4.5 years,” they almost always mean modified duration. It answers the question investors actually ask: how much will I lose if rates go up?
Effective Duration
Modified duration assumes a bond’s cash flows are fixed. That assumption breaks for callable bonds, putable bonds, and mortgage-backed securities, where the issuer or borrower can change the payment schedule. Effective duration handles this by modeling how the bond’s price actually moves when rates shift up and down by a small amount, rather than relying on a formula that assumes fixed payments. If you own a bond fund holding corporate or mortgage-backed debt, the duration figure on your statement is almost certainly effective duration, because modified duration would overstate the fund’s real sensitivity.
Where the Duration Rule Starts to Miss
Duration draws a straight line through a curved relationship. Bond prices don’t move in perfectly proportional steps when rates change; the actual price path curves. For small rate moves the straight line and the curve stay close. For larger moves the gap widens, and that gap is what convexity measures.
For ordinary bonds without call features, convexity works in your favor. When rates fall, the bond gains slightly more than duration alone predicts. When rates rise, it loses slightly less. The FDIC’s examination guidance notes that convexity-adjusted duration should be used for rate changes exceeding 100 basis points to get more accurate price estimates.5FDIC. Section 7.1 Sensitivity to Market Risk
Callable bonds and mortgage-backed securities flip this. They exhibit negative convexity: when rates drop, you don’t capture the full upside because the issuer is likely to call the bond or the borrower is likely to refinance. Duration shortens right when you’d want it to be long. When rates rise, the call becomes unlikely and duration extends, amplifying losses. That’s one reason mortgage bond funds can underperform during volatile rate periods even when they look conservative on paper.
Using Duration If You Own a Bond Fund
Most people don’t buy individual bonds; they buy bond funds. A fund’s duration is the weighted average of the durations of all the bonds it holds. FINRA describes duration risk as the sensitivity of a bond investment’s price to a one-percentage-point change in rates, noting that the higher the duration number, the more sensitive the investment will be.6FINRA. Bonds Fund managers disclose the figure on the fact sheet or in the prospectus.
Match it to your timeline. If you’re five years from needing the money, a fund with a duration of eight years carries more rate risk than your horizon warrants. A fund with a duration closer to when you’ll spend the money gives you a better balance between yield and volatility. Duration doesn’t eliminate risk. It lets you size it.