Duration: The Number That Tells You How Rate Changes Will Hit Your Bonds
Bonds are marketed as the safe half of a portfolio, yet a long-duration bond fund can lose more in a single bad quarter than a diversified stock portfolio. Duration is the single number that tells you, before it happens, roughly how large that move will be.
The core principle
Duration measures how sensitive a bond's price is to a change in interest rates, expressed in years. The name is misleading: duration is not simply how long until the bond matures, though the two are related. Duration is a weighted average of the time until each of a bond's cash flows arrives, with the weights determined by the present value of each payment. A zero-coupon bond that pays nothing until maturity has a duration exactly equal to its maturity, because all its value arrives on one date. A coupon-paying bond has a shorter duration than its maturity, because some value arrives earlier, in the form of periodic interest payments, and money received sooner is less exposed to what happens to rates later.
The practical shortcut that matters for an investor is this: a bond or bond fund with a duration of D years will see its price move by approximately D percent in the opposite direction of a one percentage point change in interest rates. Duration of 6 means a one point rate increase produces roughly a 6% price decline, and a one point rate decrease produces roughly a 6% price gain. This relationship is called interest rate risk, and duration is simply the standard unit for measuring it. The relationship is approximate rather than exact because bond prices do not move in a perfectly straight line as rates change; the true price curve bends slightly, a property called convexity, but for rate moves of a percentage point or two, the straight-line duration estimate is close enough to be useful.
Two structural facts drive duration higher or lower. Longer maturity raises duration, because more of the bond's value sits further out in time, exposed to rate changes for longer. Lower coupon rates also raise duration, because less value arrives early as interest and more is concentrated in the final principal repayment. A 30 year Treasury bond, with a low coupon relative to its price and a distant maturity, commonly carries a duration above 15 years. A 2 year Treasury note, by contrast, typically has a duration close to 2, because nearly all its value returns to the holder within two years regardless of rate moves.
The math
Example 1: estimating a fund loss from a rate move. Suppose you hold $80,000 in an intermediate-term bond fund with a published effective duration of 6.2. The Federal Reserve raises its target rate path and market yields across the curve rise by 0.75 percentage points over the following month. The estimated price impact is duration × rate change = 6.2 × 0.75% = 4.65%. Applied to the balance, the expected decline is $80,000 × 4.65% = $3,720, leaving an estimated value of roughly $80,000 − $3,720 = $76,280, before accounting for any interest income the fund paid out over that period, which would partially offset the price decline in your total return.
Example 2: comparing two bonds with the same maturity. Bond A is a 10 year Treasury note carrying a coupon of 4.5%, priced near par with a duration of approximately 8.1 years. Bond B is a 10 year zero-coupon Treasury, which by definition has a duration equal to its full 10 year maturity. If rates rise by 1 percentage point, Bond A is estimated to fall by roughly 8.1 × 1% = 8.1%, while Bond B is estimated to fall by roughly 10 × 1% = 10%. Both bonds mature on the same date and carry the same government guarantee of repayment at par, yet the zero-coupon bond is meaningfully more volatile along the way, purely because none of its value returns to the holder early as coupon income.
How it shows up in real portfolios
The most common real-world mistake is treating "bond fund" as a single risk category. An investor who moved from a money market fund into what she assumed was a conservative bond fund, only to discover it was a long-term Treasury fund with a duration near 17, watched her supposedly safe holding fall more than 15% during a fast rate-hike cycle, a decline comparable to a moderate stock market correction. The fund did nothing wrong and eventually recovered as it rolled into higher-yielding bonds, but the investor's time horizon and risk tolerance were mismatched to the fund's actual duration, and nobody had pointed that out to her before she bought it.
Retirees drawing income face the opposite version of the same problem: a retiree relying on a bond ladder or bond fund for near-term spending needs generally wants low duration, so that a spike in rates does not force selling depreciated bonds to cover living expenses. A rule of thumb many practitioners use is to roughly match a bond holding's duration to the number of years until the money is needed, which is also the logic behind a laddered portfolio of individual bonds held to maturity, where interim price swings do not matter if you never sell before the bond matures.
A high-earning professional building a 401(k) often defaults into a target-date fund's bond sleeve without examining its duration at all. A 45-year-old physician with 20 years to retirement, holding a target-date fund whose bond allocation carries a duration near 6, is taking on a moderate, appropriate level of rate risk for that time horizon. The same physician, if she instead built a separate fixed income sleeve using long-term corporate bond funds for their higher stated yield without checking duration, could unknowingly double her rate sensitivity relative to what the target-date fund would have delivered on its own, for a yield pickup that may not compensate for the added volatility.
A family saving for a child's education through a 529 plan faces a related, often overlooked version of this problem inside the plan's own bond allocation. A 529 plan's age-based portfolio typically shifts from stocks toward bonds as college approaches, but the specific bond fund used inside that glide path still carries its own duration, and a fund with duration near 6 held in the two years immediately before tuition is due can still see a meaningful drawdown from a rate spike at exactly the wrong moment, the same mismatch that catches unprepared retirees. Checking the underlying bond fund's duration inside an age-based plan, not just assuming "near college age means safe," is worth the five minutes it takes.
Actionable breakdown
- Finding a fund's duration:
- Check the fund fact sheet for "effective duration."
- Compare it against your actual time horizon for the money.
- Using duration to estimate risk:
- Multiply duration by an expected rate move in points.
- Treat the result as an approximate, not exact, price move.
- Remember convexity makes large moves slightly asymmetric.
- Managing duration deliberately:
- Shorten duration when rates seem likely to rise, or cash is needed soon.
- Lengthen duration to lock in yield if rates seem likely to fall.
- Consider a ladder to sidestep timing the direction entirely.
Common pitfalls
- Assuming "bond fund" automatically means low volatility; long-duration funds can swing as much as stocks over short periods.
- Confusing duration with maturity, which understates how much a coupon-paying bond's price can move relative to its stated term.
- Ignoring duration risk during periods of very low rates, when the next large move is more likely to be upward, hitting long-duration holdings hardest.
- Chasing a slightly higher yield in a longer-duration fund without weighing the added price volatility that comes with it.
Related concepts
For the underlying instrument, see bond and bond fund. For the specific mechanics of rate exposure, see interest rate risk and credit spread. For an alternative approach to managing this risk without forecasting rates, see bond ladder. For portfolio-level context, see the guide on bonds and risk.
The bottom line
Duration converts an abstract interest rate forecast into a concrete price estimate, which is exactly the number to check before assuming any bond holding is automatically safe.