Interest rates: the economy's thermostat

A family sits at the kitchen table with two figures in front of them. The first is what they earn. The second is what their mortgage costs each month. One day, far away, a committee nudges a single number — the rate at which money is lent. A few months later, the family’s second figure has changed, and so has how much is left for everything else. They never met the committee. They never signed anything new. Yet their month got tighter, or looser, because of one small move. How does pulling one lever reach a kitchen table that’s nowhere near it?

A rate is the price of borrowing

An interest rate is a price. It’s the price of borrowing money — and, looked at from the other side, the reward for saving it.

Borrow £100 for a year at 5%, and you pay back £105. The extra £5 is the rent on the money. Save £100 at 5%, and you end the year with £105. Same number, opposite seat. Lending and saving are the same act seen from two sides, and the rate is what sits between them.

Now the key move. There isn’t one interest rate — there are thousands, on mortgages, car loans, business loans, savings accounts. But a country’s central bank sets one key rate: roughly, the rate at which banks themselves borrow. Every other rate is built on top of that. Nudge the key rate and the whole stack shifts, the way a tide lifts every boat in the harbour without touching any of them directly.

Which way the ripple runs

Start with a cut. The central bank lowers the key rate. Borrowing gets cheaper everywhere. Cheaper loans mean more people borrow — to buy a house, a car, to expand a business. More borrowing means more spending and more investment. Money moves faster. The economy heats up.

Now raise it. The central bank lifts the key rate. Borrowing gets dearer. A loan that looked worth it at a low rate no longer pays off. Fewer people borrow; some who’d have spent now save instead, because saving pays more. Spending cools. The economy slows.

That is the whole mechanism, and it’s why people call the key rate a thermostat. Turn it down and the economy warms. Turn it up and it cools. It isn’t run for its own sake — it’s leaned warmer or cooler depending on what the economy needs.

This is the main tool against the inflation you met earlier. Too much spending chasing too few goods pushes prices up. Raise rates, cool the spending, and the upward pressure on prices eases. It’s also the lever that leans against the cycle of booms and busts from the last lesson — tightening when things run hot, loosening when they stall.

A worked example: a £200,000 mortgage

Watch the lever reach the table. A household takes a £200,000 mortgage. Keep it simple and look just at the interest in the early years.

1. At 3%, the interest on £200,000 is about £6,000 a year — £500 a month.
2. At 6%, the interest is about £12,000 a year — £1,000 a month.

The rate doubled, and the monthly cost roughly doubled too. That’s £500 more, every month, gone before the family buys a thing.

Where does that £500 come from? Not from thin air. It comes out of everything else — the meals out, the new clothes, the saved-up holiday. So a rate rise doesn’t only change a mortgage. It quietly reaches into restaurants, shops, and travel firms the family would have spent at. Multiply that across millions of households and you can see how one number cools a whole economy: it shrinks what ordinary people have left to spend.

Run it the other way and a cut frees up that £500 — more to spend, more flowing to those same shops. The lever pushes both ways.

It works slowly, and it cuts both ways

Two honest complications.

First, lags. The thermostat is slow. A rate change takes months — often a year or more — to fully work through borrowing, spending, and prices. The committee is steering a heavy ship by a rudder that responds long after the wheel is turned. They must act on where the economy is heading, not where it is.

Second, the trade-off. Cooling inflation by raising rates also cools jobs. Dearer borrowing means firms expand less and hire less; some trim staff. So the same move that tames prices can lift unemployment — the subject of the next lesson. There’s rarely a setting that’s gentle on everyone at once.

The whole, not the lever

Step back from the kitchen table. One rate touches everyone, but not in the same way. A young family with a big mortgage feels a rise as pain. A retiree living off savings feels the same rise as relief — their interest income goes up. A firm weighing a loan feels it as a go or no-go on hiring. A saver and a borrower sit on opposite sides of the very same number.

That’s worth holding onto. There is no single rate that is simply right. Every setting helps some people and presses on others, and the committee is choosing between them with slow tools and partial sight. You are not above that choice, watching it land on other people. You hold a mortgage, or a savings account, or a job at a firm deciding whether to borrow — so the lever reaches you too. Seeing the whole web of who it touches is what makes it possible to judge any rate decision with a little humility, instead of certainty.