What duration tells you about a bond fund—and what it misses

You may own a bond fund to balance your stock investments or provide income. But its value can still fall. Understanding what could cause that fall is a useful part of deciding whether the fund fits your needs.

Two measures help. Interest-rate duration estimates how sensitive the fund is to changing interest rates. Credit-spread duration estimates how sensitive it is when investors demand more compensation for holding debt with credit risk.

Read together, they help explain why two funds with similar durations can respond differently to the same market conditions. Neither is a complete measure of risk.

What happens when interest rates change?

Suppose a fund has an interest-rate duration of six years. If rates rise by one percentage point—from 4% to 5%, for example—its value would fall by roughly 6%, assuming other conditions stay unchanged. A fall in rates of the same size would imply a gain of roughly 6%.

This is an illustration, not a forecast for a particular fund. It estimates a change in value, excluding income earned over time and other changes affecting the portfolio.

“Six years” describes sensitivity. It does not mean you must hold the fund for six years, or that all its bonds mature in six years.

For the same small rate move, a larger positive duration implies a larger estimated price change. A duration close to zero suggests little sensitivity to that move—but says nothing by itself about the fund's other risks.

You may also see basis points. One basis point is 0.01 percentage point; 100 basis points is one percentage point. A rise from 4% to 5% is therefore a rise of 100 basis points, not a 1% relative increase.

What happens when investors become more worried about credit?

Investors generally require extra yield to hold debt that carries more credit risk than a comparable government bond. That additional yield is called a credit spread. It can reflect concerns about repayment, difficulty selling the bond, and investors' willingness to take risk.

When investors demand a larger spread, the price of an existing bond generally falls, assuming other conditions stay unchanged. The borrower does not have to miss a payment for its bonds to lose market value.

Credit-spread duration estimates sensitivity to that change. In a hypothetical fund with a spread duration of four years, a one-percentage-point widening in spreads would imply a decline of roughly 4%, before other effects.

The two measures answer different questions:

Rates and spreads can move at the same time, sometimes in opposite directions. A fund may gain from falling benchmark rates while losing value as credit spreads widen. Neither number alone tells you the combined result.

Why a floating-rate fund can still lose value

Floating-rate bonds adjust their interest payments as a reference rate changes. This can make their prices less sensitive to benchmark interest-rate moves than those of otherwise similar fixed-rate bonds.

It does not remove credit risk. Their market prices can still fall if investors become less willing to lend to the borrowers or demand a larger credit spread.

Consider a hypothetical fund with rate duration close to zero and spread duration of four years. Its price might barely respond to a small benchmark-rate move, yet remain sensitive to a change in credit conditions. Reading only its rate duration would miss that distinction.

This is why “low duration” should prompt a second question: low sensitivity to what?

How to read the two numbers together

Start with what the fund owns, then use the measures to put that portfolio in context.

Check how much of the fund is in bonds. The measures shown here express sensitivity relative to the whole fund's net assets—its assets minus liabilities. The same headline number can describe different underlying exposures in a dedicated bond fund and a balanced fund that also owns stocks. A small number does not make the rest of the portfolio insensitive to market changes.

Look at where the sensitivity sits. Short-term and long-term rates do not always move by the same amount. A breakdown by maturity helps distinguish funds with similar total durations but different exposure along the yield curve—the range of interest rates at different maturities.

Separate credit quality from credit sensitivity. Investment-grade debt has higher credit ratings; high-yield debt has lower ratings and generally greater repayment risk. The split of spread sensitivity between these categories adds context. But a category's share of measured sensitivity is not the same as its share of the fund's holdings.

Check the reporting date. A filing describes the fund at a particular point in time. Holdings, hedges, and market conditions may have changed since then. When comparing funds, use matching reporting dates where possible and identify any difference.

Where the estimates have limits

These estimates work best for small market changes. Larger moves can change how bonds behave, especially when borrowers can repay early or issuers can redeem their bonds. Use a duration-based scenario to understand the scale of a possible move, rather than as a precise forecast.

Three other distinctions are worth keeping in mind:

• Blank is not zero. A blank means no usable figure is available here. It might reflect the reporting requirements or a gap in the available data. A reported zero means the measured sensitivity was zero at the precision shown; it does not mean the fund has no risk.
• Direction matters. After the filing's sign convention has been established, a negative duration can indicate exposure that benefits from rising rates or widening spreads. Do not turn every negative into a positive: that can erase the effect of a hedge. If direction cannot be established, the interpretation should say so.
• One scenario cannot describe every market move. Applying the same change at every maturity is a simplifying assumption. Actual changes can be larger at one end of the curve than the other. Sensitivity measures also do not fully describe liquidity, financing, or the consequences of a borrower defaulting.

A closer look: why larger moves change the estimate

The relationship between a bond's price and yield is curved. Duration approximates it with a straight line over a small change; convexity describes that curvature.

For an ordinary bond with positive convexity, the straight-line estimate tends to slightly overstate losses from rising yields and understate gains from falling yields. Bonds with repayment options, including mortgage-backed and callable bonds, can behave differently because the timing of their payments can change.

That is why a large-move estimate can be wrong in either direction. “Roughly 5%” is a more useful way to read an illustrative scenario than treating “5.07%” as a prediction.

Where the numbers come from

The fund pages use interest-rate and credit-spread sensitivity disclosures in Form N-PORT, a portfolio report filed with the SEC. The dollar sensitivities are converted into measures relative to net assets so funds of different sizes can be compared.

Not every fund is required to report these measures. The requirement depends on its debt exposure, including certain derivatives—not just the bonds it holds. A missing figure therefore does not mean the fund has no sensitivity to rates or credit spreads.

Under the reporting instructions linked below, the report for the third month of a fund's fiscal quarter becomes public when it is filed. The filing deadline is generally 60 days after the quarter ends; publication does not have to wait until that deadline. Some fields remain non-public, and reporting rules can change.

The useful habit is simple: look at rate sensitivity, look at spread sensitivity, and then ask what holdings and financing produce them. For a worked example, read Similar duration, different risks: comparing BND and PIMCO Income.

Source note: the reporting threshold

Item B.3 of Form N-PORT sets a 25% of net assets reporting threshold based on average debt exposure over the previous three months. For this calculation, exposure is the sum of absolute values: debt securities' values, the notional amounts of swaps and futures referencing debt securities or interest rates, and the delta-adjusted notional amounts of options on those instruments. Delta adjustment accounts for an option's sensitivity to its underlying instrument. This is a reporting test, not a measure of the fund's overall risk.

The form's General Instructions A and F describe the filing deadline and public availability. The SEC's February 2026 proposal provides context on changes to the reporting schedule; proposed rules are not final rules.