Money

Why does compound interest make you rich slowly, then suddenly fast?

You put money in. It earns a little. Then that little also starts earning. That is the whole trick — and it is sneakier than it sounds.

Interest on interest

Simple interest pays you only on what you put in. Compound interest pays you on what you put in plus every bit of interest you've already earned. Each year the pile you earn from gets bigger, so each year's growth is bigger than the last.

Year one looks boring. Year twenty looks like magic. Nothing changed except time.

Why it feels slow, then sudden

Growth that builds on itself doesn't move in a straight line — it curves. For a long stretch the curve hugs the ground and feels disappointing. Then it bends sharply upward, like a hockey stick.

Say you earn about 10% a year. Your money roughly doubles every 7 years — a shortcut called the Rule of 72 (divide 72 by your interest rate to get the doubling time). Watch the doublings stack up:

• Years 0–7: 1x → 2x
• Years 7–14: 2x → 4x
• Years 14–21: 4x → 8x
• Years 21–28: 8x → 16x

The jump from 8x to 16x is eight times bigger than the first jump from 1x to 2x — and it happens in the same seven years. Each step you wait for is more dramatic than the last.

Time beats amount

This is why "start early" is the most boring and most powerful money advice there is. A small sum given decades to compound usually beats a big sum that starts late, because the early money gets the most doublings. You're not buying returns — you're buying time for the curve to bend.

The same engine, reversed

Compound interest also works against you. A credit-card balance compounds in the bank's favour: the interest you owe starts earning interest for them. Same machine, pointed the other way — which is why small balances can balloon.

The takeaway

You don't get rich from compounding by being clever. You get rich by being patient and letting the curve do the bending. The first years feel like nothing is happening. They are the most important ones.