Rate-Shock Stress Test: What the Next Fed Move Does to Your Interest Bill
A Fed move reaches your credit cards within a statement, your HELOC within a billing cycle, your adjustable-rate mortgage only at its reset, and your savings whenever your bank decides. Enter what you owe and what you hold to see the net effect, when each piece arrives and what to pay first.
Loading the Fed’s current range…
The Fed move to test (percentage points)
What you owe (variable rates only)
What you earn
| Item | Rate | Per month | Per year | Starts |
|---|
When each rate changes (next 12 months)
Balances, margins and yields are examples; enter your own. Interest changes are on today’s balances, before tax. Nothing you type leaves your browser.
How the Stress Test Works
- Credit cards and HELOCs charge the prime rate plus a margin. Banks set prime at the top of the Fed’s target range plus 3 points, so it moves one for one with the Fed, and the new rate usually applies from your next statement or billing cycle.
- Variable private student loans usually follow SOFR, the overnight rate that tracks the Fed, and change at their monthly or quarterly reset.
- Adjustable-rate mortgages change only at scheduled resets: the new rate is the index (usually SOFR) plus the margin, held within the periodic and lifetime caps. The calculator compares the reset rate with and without the Fed move, so a binding cap shows up as a smaller change.
- Savings accounts move when your bank decides. You set how much of the Fed’s move it passes on; the calculator assumes about two months. Money market funds and T-bills follow within about two months, because money funds must keep their average maturity at 60 days or less.
Changes are interest on today’s balances: rate change × balance, per year and per month. “Net, once rates catch up” is the full-year change in interest paid minus interest earned; the timeline adds up the next 12 months as each rate reaches you. Fixed-rate debts, including federal student loans and fixed mortgages, don’t change, so leave them out.
Reading the Result
Two numbers matter most. The first is any payment jump inside a year, usually an ARM reset or a HELOC draw period ending: a few tenths of a point on a large mortgage balance outweighs a quarter point on everything else combined. The second is your highest rate, where every extra dollar earns the most. Our variable-rate debt triage guide ranks the moves in that order, and what the September hike changes covers everything else a Fed move touches. Savings can offset part of the cost, but less than you might think: from early 2022 to early 2024, while the Fed raised its range by 5.25 points, the FDIC’s national average savings rate went from 0.06% to 0.47%.
What the Stress Test Leaves Out
- Payments versus interest. It measures interest cost. An amortizing ARM’s new required payment also depends on the remaining term; your reset notice shows the exact figure.
- Changing balances and promotions. Paydowns, new charges, 0% transfer offers and penalty APRs aren’t modeled.
- Taxes. Interest you earn is taxable; mortgage and HELOC interest may be deductible if you itemize. The Cash Yield Showdown handles the tax side of savings.
- Market spreads. SOFR and bill yields don’t move exactly one for one with the Fed, especially before a decision, when markets move early.
For the bigger picture, the Rate-Shock Index shows what each Fed hike adds to card interest per person in every state, and what every prime-rate hiking cycle since 1979 would cost at today's balances.
Frequently Asked Questions
Sources: FOMC statement, Sep 16, 2026 · Federal Reserve H.15 (prime) · New York Fed: SOFR · Regulation Z §1026.9 · SEC Rule 2a-7 · FDIC national savings rate (FRED). Checked Sep 28, 2026.