First, the plain-English version
A bond ETF is a basket of loans. When you own BND or SGOV, you own tiny slices of thousands of IOUs — mostly from the U.S. Treasury and other high-quality borrowers — and you collect the interest they pay. That's the income part.
The part almost nobody explains is duration. Duration is a weighted-average measure of how long it takes to get your money back in present-value terms — and, more usefully, it doubles as a rate-sensitivity dial: the longer the duration, the more the fund's price moves when interest rates change. Short duration, small moves. Long duration, big moves.
That single idea is why two funds that both get filed under "safe money" can behave nothing alike.
The tension: "safe" is not one thing
Today the Federal Reserve announces its rate decision, and a rate hike is widely expected — which would be its first increase since 2023, reversing the cuts that followed. To be clear up front: we are not predicting what the Fed will do, and this piece does not depend on it. The lesson below holds whether they hike as anticipated or sit still.
Here's the friction. A total-market bond fund like BND carries a duration of about six years. A rule of thumb: for every 1% move in interest rates, a bond fund's price moves in the opposite direction by roughly its duration in percent. Six-year duration, ~6% price swing per 1% rate move. Rates up, price down.
An ultra-short T-bill fund like SGOV has a duration near 0.1 years. Same 1% rate move, and its price barely registers it — on the order of a tenth of a percent.
Both get called "safe." They are safe in different ways, from different risks. Conflating the two is where portfolios get surprised.
Performance and yield figures are historical and may change. Total return includes price movement and distributions where available. Past performance does not guarantee future results. Yield is not the same as total return.
The mental model, in four steps
You don't need bond math. You need one number and a direction. Here's how to read any bond fund you hold.
Step 1 — Find the duration
It's on every fund's page. BND sits around six years; SGOV sits near zero. That number is your sensitivity dial. Everything else follows from it.
