Compound interest is the closest thing personal finance has to magic, and yet it is nothing more than interest earning interest. Grasp it once and a lot of money advice suddenly makes sense.
The snowball, in numbers
Say you put £1,000 in an account paying 5 per cent. After a year you have £1,050. The next year, you earn 5 per cent on the whole £1,050, not just your original grand. The interest itself starts earning interest. Early on the difference is small, but left for decades the curve bends sharply upward.
Why time beats timing
Because the effect builds on itself, the years at the start matter most. Someone who saves modestly from 25 often ends up ahead of someone who saves far more from 40, simply because their money had longer to compound. You cannot get those early years back, which is why "start now" is the most repeated bit of money advice there is.
It cuts both ways
The same force works against you when you owe. Credit card debt at a high rate compounds in the lender's favour, so an unpaid balance grows alarmingly fast. That is why clearing expensive debt usually beats investing — you are guaranteed to "earn" the interest rate you avoid.
Putting it to work
- Start saving and investing as early as you can manage, even small amounts.
- Leave returns to reinvest rather than spending them.
- Attack high-interest debt first, because compounding is your enemy there.
Understand this one idea and you are most of the way to handling money well.