Play · what interest is
The Interest-Rate Seesaw
Updated 2026-07-30 · Sources: 4 · Concept: one price, two faces
People talk about interest rates as though a high one were good news or bad news. It is neither, because it is not one thing happening to one person. Every rate is a price paid by somebody to somebody else. The identical number that fills a retiree's grocery budget empties a young family's. There is one lever below. Move it, and watch both ends at once.
Predict first
Rates rise one percentage point. Who feels it first?
Borrowers, but only the ones whose rate can move. A credit card or a home equity line is usually priced at the prime rate plus a margin, and prime tracks the federal funds rate, so a policy change reaches those statements within a billing cycle or two. A thirty-year fixed mortgage already signed feels nothing at all, ever. Savers wait longest: deposit rates are each bank's own decision, and they have long been observed to lag policy moves on the way up and to pass through less than the full change. That last point is a general, well-documented pattern rather than one published figure; you can watch it happen yourself by reading the Fed's weekly H.15 release (federal funds and prime, side by side) against the FDIC's national deposit rates.
Bar heights are one year's interest on each side, on a shared scale (the taller bar at 10% is $31,000). The plank shows who the lever is favouring; the bars show how nearly equal the two amounts are, because they are the same money.
Savers: paid for waiting
$10,000 emergency fund
$400
a year ($33.33 a month)
Retiree living on $300,000 in cash
$12,000
a year of income, $1,000 a month, without touching the principal
Borrowers: charged for hurrying
$300,000, 30-year fixed mortgage
$1,432
a month, $11,904 of it is interest in year one, $215,609 over the full thirty years
$5,000 left on a credit card
$16.67
of interest this month, $200 a year, and that is before a single new purchase
At 4.00%, the retiree's cash pays $12,000 a year and the mortgage charges $11,904 of interest in its first year. Two households, opposite feelings, one number.
One rate, applied honestly to everything. This tool deliberately prices every product off the same number, because that is the concept: interest is one price. Real products do not sit on top of each other: a savings account pays below the policy rate, a thirty-year mortgage is priced off long-term Treasury yields (Freddie Mac's Primary Mortgage Market Survey tracks the average), and a credit card sits far above both; card APRs in the twenties are ordinary, so the card figure here is much gentler than a real statement. The spreads and the different speeds are the subject of the Fed lesson; the weekly H.15 release publishes federal funds, prime and Treasury yields side by side, and the Fed's own explainer describes how a change in the funds rate spreads to other rates. Method: standard fixed-rate amortization, M = P·r(1+r)n/((1+r)n−1), monthly; first-year interest is summed month by month on the declining balance; savings interest is simple annual interest, so compounding would make the saver's side slightly larger. No taxes, no fees, no inflation. Teaching only, not advice.
Thou oughtest therefore to have put my money to the exchangers, and then at my coming I should have received mine own with usury.Matthew 25:27
KJV. 'Usury' here is simply interest, and 'the exchangers' are bankers; the master's complaint is that the frightened servant would not even take the safe, boring rate. Read alongside the Law's protection of the poor from interest (Exodus 22:25, Leviticus 25:35-37): Scripture treats the price of money as real and as morally weighted on both sides of the seesaw.
Interest is one price with two faces
A rate is not a fact about the economy the way rainfall is a fact about the weather. It is the price of using someone else's money for a while, and like every price it has a payer and a receiver. When the number rises, the retiree above gets a raise and the family with the mortgage gets a pay cut, in the same week, from the same decision. Neither of them did anything.
Which is why "are rates good right now?" is an unanswerable question, and why the useful one is personal: which side of the plank am I sitting on, and which of my own rates can move? Fixed debt is a promise nobody can reprice; variable debt is an open position. Cash is a position too. Take the inventory; see where "the" interest rate comes from for how the machinery works and how fast each pipe carries a change.