What is compound interest?

Compounding is the quiet force behind almost all lasting wealth. Understand this one idea and the rest of investing makes sense.

The simple definition

Compound interest is returns earning returns. You invest money; it grows; and then that growth also starts to grow. Each gain sits on top of the last, so your money doesn't grow in a straight line — it grows on itself, faster and faster, the longer it is left alone.

A simple example

Invest £1,000 at a 7% average annual return. After year one you have £1,070. In year two you earn 7% not on £1,000 but on £1,070 — so you gain more than the year before, without adding a penny. Repeat that for decades and the effect becomes enormous. The interest earns interest, which earns interest.

Why time matters more than the amount

Human intuition is linear — we expect small inputs to give small outputs. Compounding breaks that. The curve stays almost flat for a long, boring while, then bends sharply upward. This is why the early years of investing feel like nothing is happening, and why most people quit right before the curve does its work. The ones who win simply didn't stop during the flat part.

The cost of waiting

Because compounding rewards time so heavily, your earliest invested pounds are worth far more than your last — they have the longest to grow. Someone who starts at 25 and someone who starts at 35, investing identically, can end up decades later with a difference measured in six figures. Same discipline, same fund; one difference: a ten-year head start. Waiting is the quietest, most expensive mistake in investing.

It works against you, too

Compounding has a dark side: debt compounds as well, just in the wrong direction. A credit card at 20-something percent grows the lender's wealth the same way an index fund grows yours — faster. So before you invest, clear high-interest debt. Clearing a 20% debt is a guaranteed 20% return, better than almost any investment.

How to put it to work

You don't need to be clever with compounding — you need to be patient and consistent. Invest a manageable amount regularly, keep costs low, and leave it alone for years. That's it. For the practical steps, read how to start investing in the UK. The math always wins.