Learning About Interest Rate Risk

Watching what rising rates have done to bank securities portfolios over the past year got me looking into the broader question. Unrealized losses on bonds are one thing. The bigger question is how rate movements affect an entire balance sheet, not just the securities line. That's what interest rate risk actually is, and I've been spending time trying to understand the framework bankers use to manage it.

Duration

The concept that keeps coming up is duration. Not in the everyday sense of how long a loan lasts, but as a measure of how sensitive a financial instrument is to changes in interest rates.

The intuition is straightforward. If you hold a bond that pays you back entirely in two years, a rate change affects you for two years. If you hold one that pays over thirty years, you are exposed to that rate environment for much longer. Duration captures this. A higher duration means more sensitivity to rate changes. That is why long-dated bonds got hammered when the Fed raised rates. They had the most years of below-market payments baked into their price.

What made this click for me is that duration isn't just about bonds. Loans have duration. Deposits have duration, although that gets complicated because clients can move their money whenever they want. The whole balance sheet has a duration profile, and managing that profile is a big part of what risk management in banking is actually about.

The Mismatch

Here is where it gets interesting. Banks make money by borrowing short and lending long. Deposits, which are a bank's main source of funding, tend to reprice quickly. When rates go up, clients eventually demand higher rates on their savings. But loans, especially fixed-rate mortgages and commercial real estate, reprice slowly or not at all until they mature.

This is the asset-liability mismatch. The bank's liabilities (deposits) adjust to new rates faster than its assets (loans). In a rising rate environment, the cost of funding goes up while the income from loans stays flat. That compresses margins.

The Asset Liability Committee, often called ALCO, is the function at any bank responsible for monitoring this mismatch and deciding how to manage it. How much duration should the balance sheet take on? How fast are deposits likely to reprice? What happens to net interest margin if rates go up another 100 basis points? These are the kinds of questions ALCO works through, and the answers shape how a bank positions itself for the rate environment.

Repricing

The repricing side is where the mechanics get more concrete. Every asset and liability has a repricing date, the point at which the rate resets or the instrument matures.

A variable-rate loan reprices when the index it is tied to changes. A fixed-rate loan does not reprice until it matures or the borrower pays it off. On the liability side, checking accounts can theoretically reprice immediately but practically tend to be sticky. CDs reprice at maturity. Savings accounts fall somewhere in between.

What risk managers do is lay out all the assets and liabilities by when they reprice and look for gaps. If significantly more liabilities reprice in the next twelve months than assets, the bank is exposed to rising rates. Funding cost goes up but income does not follow. That gap is what they are trying to manage.

What I'm Taking Away

None of this was on my radar before. Now I understand enough to follow the conversation when it comes up. The framework isn't complicated once you see the pieces. Duration measures sensitivity. The mismatch between short-term funding and long-term lending is the fundamental risk. Repricing gaps show where the exposure sits.

What strikes me is how much of banking comes down to managing timing. A bank earns a spread between what it pays and what it charges, and the risk is that the timing of those cash flows does not line up the way the bank planned. Rates have moved fast over the past year. The Fed just raised again, and the rate is at 4.50 to 4.75 percent now. I'm glad I understand the mechanics better. It makes any conversation about portfolio positioning and deposit strategy feel a lot less abstract.