Inflation, the silent leak

Ask an older relative what a loaf of bread cost when they were young, and you’ll get a small gasp of a number. “A few pence.” The loaf hasn’t changed — same flour, same size, still feeds the same family. What changed is the money. Each pound buys less bread than it used to. Nobody announced it; no single day did it happen. The money just quietly got weaker, year after year, while you weren’t looking. That slow leak has a name — inflation — and it is the headwind that every other force in this course runs against.

Prices rise, so money shrinks

Inflation means prices, on average, drift upward over time. A 3% inflation rate means the same basket of shopping that costs £100 this year costs about £103 next year. The change feels small and forgettable, which is exactly why it’s dangerous.

Here’s the mental flip that matters: when prices rise, money’s value falls. They are two descriptions of one event. If everything costs 3% more, then each pound buys 3% less. The number on your banknote is fixed — a £20 note is always “£20” — but what that £20 can do shrinks a little every year. The honest measure of money isn’t the number. It’s the purchasing power — how much real stuff it commands.

So when you ask “how much money do I have?”, the deeper question is “how much can it buy?” — and that second number is always quietly eroding.

The cash under the mattress isn’t safe

People think of cash as the safe choice. No ups and downs, no risk of a crash — just a solid number that can’t fall. But that solidity is a trap, because the number staying the same is not the same as the value staying the same.

Picture £10,000 sealed in a drawer, untouched, while prices rise 3% a year. The number never moves. But follow what it can buy:

• After 10 years, your £10,000 still says £10,000 — but it buys what about £7,400 buys today.
• Keep waiting, and after roughly 23 years, it buys what £5,000 does now. Half of it has evaporated, without a single penny being spent or stolen.

Nothing was taken. The drawer was never opened. Yet a quarter of the money’s real worth vanished in a decade, and half within a working lifetime. “Safe” cash is perfectly safe from crashes and perfectly exposed to inflation. Doing nothing is itself a slow, guaranteed loss.

The rule of 70

You don’t need a calculator to feel the speed of the leak. There’s a shortcut called the rule of 70: divide 70 by the inflation rate, and you get roughly the number of years for prices to double — which is the same as the years for your idle cash to halve in real worth.

Walk it. At 3.5% inflation, 70 ÷ 3.5 = 20 years for prices to double. At a punchy 7%, 70 ÷ 7 = just 10 years to double. At a calm 2%, 70 ÷ 2 = 35 years. The rule shows something compounding always hides from intuition: small differences in the rate are huge over time. Going from 2% to 7% doesn’t make the leak a bit faster — it makes your money halve three and a half times sooner.

This is also why a “guaranteed” return matters less than it sounds. If a savings account pays 1% while inflation runs 3%, your money is growing in number but shrinking in worth — a guaranteed loss of about 2% a year, dressed up as a gain.

On the whole

Inflation reframes the whole game. It’s why money usually has to be put to work just to stand still, and why “I’ll keep it all in cash where it’s safe” is quietly one of the riskiest things you can do with a long horizon. Compounding, from the last lesson, is the engine that can outrun the leak. Inflation is the leak it has to outrun.

There’s a deeper point hiding here. Money was never the real thing — it’s a claim on bread, rent, heat, time. Inflation is the slow reminder that the claim is written in disappearing ink. Understanding it doesn’t make you anxious; it makes you clear-eyed. You stop measuring your money by the comforting number on the note, and start measuring it by what it can still do for the life you’re actually living — which is the only measure that was ever real.