Same duration, different fund: BND and PIMCO Income under the hood

Pick two of the largest bond funds in the country and look up one number for each. BND — the Vanguard Total Bond Market Index Fund — has an effective duration of about 5.6 years. PIMCO Income has an effective duration of about 6.1 years. On the one figure most people check first, they are twins.

They are not twins. One is a passive, fully-invested, almost-entirely-investment-grade index fund; the other is a leveraged, multi-currency, credit-and-structured fund that holds defaulted paper and runs positions in eighteen currencies. The shared duration is a coincidence of arithmetic — two funds arriving at the same place by completely different roads — and it is a good illustration of why a single number, read on its own, can tell you almost nothing. This is a look under the hood at what that one number hides.

Everything below comes from each fund's own Form N-PORT filing for the quarter ending in early 2026 — the standardized portfolio report every SEC-registered fund files. (For the mechanics of duration and credit-spread duration, see the companion piece, Duration and credit-spread risk; this article is the case study it points at.)

The two funds at a glance

The sleeve figures are shares of each fund's assets. PIMCO Income's can exceed 100% because the fund is levered — its long positions total 152% of assets — while BND, fully invested, sums to 100%.

Read that column by column and the "same duration" framing falls apart. The duration is the only line on which they resemble each other.

One fund is fully invested; the other is leveraged

Start with the simplest structural fact: how much does each fund actually own, relative to its size?

BND owns bonds worth 100% of its assets and essentially nothing else — no meaningful short positions, no borrowing, no derivatives. What you see is what you get: a dollar in the fund buys a dollar of bonds.

PIMCO Income owns long positions worth 152% of its assets. It gets there the way an active fixed-income fund does — with short positions, repo financing, and a derivatives book layered on top of its cash bonds. That extra half-turn of leverage is not a footnote; it is central to how the fund earns its return and how it will behave in a stress. A 6.1 on this levered book and a 6.1 on a fully-invested one describe the same sensitivity to rates — which is exactly why the number can't tell them apart. Duration already accounts for the leverage, and in doing so hides it: "safe and fully invested" and "levered and active" collapse into the same figure, and how each behaves under stress sits entirely outside it.

What they hold: index Treasuries vs. a securitized-and-credit engine

BND is built to track the Bloomberg U.S. Aggregate index by sampling it: Treasuries, investment-grade corporates, and agency mortgage-backed securities, held long, in roughly the proportions of the U.S. investment-grade bond market. Government and corporate bonds are 78% of the fund; securitized paper (agency MBS, some ABS) is another 21%; cash is the last 1%. It is deliberately unremarkable — the point of the fund is to be the market, not to beat it.

PIMCO Income is the opposite by design. Its single largest sleeve is securitized credit worth 110% of the fund's assets — mortgage-backed, asset-backed, and collateralized debt — more than the entire fund, before you count anything else. On top of that sit straight bonds (34% of assets), bank loans (3%), a slice of equity, a derivatives book, and offsetting shorts. Where BND holds the bond market, PIMCO Income holds a curated, levered bet on the parts of the credit and securitized markets its managers favor.

The tell: maturity that is long, duration that is not

Here is the detail that shows the two funds are doing genuinely different things, and it is invisible on the fact sheet.

The bonds in PIMCO Income have an average stated maturity of 22.8 years — nearly three quarters of its debt is dated beyond ten years. The bonds in BND average 11.7 years. By stated maturity, PIMCO holds far the longer-dated portfolio. Yet the two funds end up at almost the same duration.

That gap — very long maturities, ordinary duration — is the signature of a securitized, actively-hedged book. Mortgage-backed securities carry 30-year stated maturities but pay down and prepay long before then, so their effective duration is a fraction of their stated life. And a fund that uses derivatives can dial its duration to a target independent of what its cash bonds would imply. PIMCO Income does both: it holds long-dated, prepaying, structured paper and then steers the portfolio's rate sensitivity down to about six years. BND has no such gap because it has no such machinery — its duration is simply the weighted average of the plain bonds it holds.

So the identical "6" means two different things. For BND it is a description: the fund's rate sensitivity is what its holdings are. For PIMCO Income it is an outcome — the result of long assets pulled down by prepayment and hedges to a number the manager chose.

Where the risk actually sits: credit, and distress

Duration measures sensitivity to interest rates. It says nothing about the other big risk in a bond fund — credit — and this is where the two funds diverge most sharply.

Split each fund's credit-spread risk into investment-grade and high-yield, and BND's high-yield share is about 1%. It is, for practical purposes, an all-investment-grade fund; a widening in junk-bond spreads barely touches it. PIMCO Income's high-yield share is about 16% — a real, deliberate slug of below-investment-grade credit — risk the duration number can't see.

The difference at the distressed end is smaller, but it is one of kind. BND holds no defaulted bonds, no bonds in arrears, and no pay-in-kind debt — none. PIMCO Income holds all three, in modest size: about a tenth of a percent of its ~$330bn bond book is in default or arrears, and roughly 0.4% — about $1.3 billion — is paying interest in more bonds rather than cash (that is what "pay-in-kind" means, and it can mark stressed or aggressively-structured credit — though several of these positions trade near par). This is not a criticism of PIMCO Income; taking considered credit risk is the fund's job. It is simply a category of risk that BND does not carry at all, and that the shared duration number is completely silent about.

One fund is domestic; the other is global

There is a last dimension the headline hides. BND reports its interest-rate sensitivity in a single currency — the US dollar. It is a domestic fund end to end.

PIMCO Income reports rate exposure across eighteen currencies — the euro, yen, sterling, Australian and Canadian dollars, and a long tail of emerging-market currencies from Brazilian real to Mexican peso to South African rand. And it is not uniformly long: in at least one currency it runs its rate exposure the opposite way to the rest of the book, which is the fingerprint of relative-value and curve positioning rather than a single directional bet. "Duration 6.1" quietly rolls up a global, multi-currency rates book into one US-dollar-equivalent figure. BND's "5.6" is just US dollars.

What you are paid for the difference

None of this makes PIMCO Income a worse fund or BND a better one — they are built for different jobs, and the market prices the difference. BND's portfolio yields about 3.9%; PIMCO Income's yields about 5.0%. That extra roughly one percentage point is the compensation for everything above: the leverage, the high-yield and distressed credit, the structured paper, the currency risk. Two caveats keep that honest: these are portfolio yields, measured before fees, and PIMCO Income — an active fund — charges far more than BND, so an investor keeps less of the gap than the headline figures show; and part of the extra is simply the carry on the leverage, not a pure premium for risk. Whether what's left is enough compensation is the actual investment question — and it is a question you can only ask once you have looked past the duration to what generates the yield.

How to read a duration number after this

The lesson generalizes past these two funds. A duration is a real, useful, standardized figure, but it is a summary — and a summary of a bond fund can be identical for portfolios that share almost nothing. Before you treat two funds as comparable because their durations match, it is worth asking a few of the questions this comparison turned on:

• Is the fund leveraged? Compare its total long exposure to its size. Fully invested and 1.5x levered are different animals wearing the same duration.
• What does it actually hold? Index Treasuries and a securitized-credit book can carry the same duration and behave nothing alike.
• How much of the risk is credit, not rates? The investment-grade / high-yield split, and any defaulted or pay-in-kind holdings, sit entirely outside the duration figure.
• Is it domestic or global? A single duration can hide a rates book spread across eighteen currencies.
• What is the yield, and what earns it? The yield gap between two same-duration funds is usually the price of the risks the duration doesn't show.

A note on the numbers: these come from regulatory filings for a quarter that has already closed, so they describe each fund's recent posture, not its position this morning — though the broad shape (leverage, credit mix, currency breadth) is stable from quarter to quarter. Where the two funds report figures under different sign conventions, we compared magnitudes rather than raw signs. And filings occasionally contain scale errors, so anything extreme is worth sanity-checking against what the fund actually holds. Used with those caveats, this is a read on two funds that a fact sheet's duration line cannot give you — and it has been sitting in the filings the whole time.