Compound Interest, Explained Like You're 12

Published April 14, 2026 · 1 min read

The maths behind why $1 today is worth $10 in 30 years — and the 3 levers you control.

Albert Einstein supposedly called compound interest "the eighth wonder of the world." He probably didn't, but the maths is wonderful anyway.

The three numbers that matter

When you invest, three things determine how much money you end up with:

1. What you start with (principal)
2. How much you add each month (contributions)
3. The annual return (rate)

The fourth — time — is the most powerful and the cheapest to "buy". Every year you wait costs you.

Compound Interest Calculator — See how your money grows with the power of compounding — visualised with charts and AI explanations.

A real example

Sarah invests $200/month from age 25 to 35. Then she stops, never adds another dollar, and lets it sit until age 65. Total contributed: $24,000.

Mark waits until 35 to start, then invests $200/month every month until age 65. Total contributed: $72,000.

At a 7% return, who has more at 65?

Sarah: $361,000. Mark: $244,000.

Sarah contributed a third as much and finished with 50% more. That's compounding.

The three levers you control

You can't change the return rate (the market does). But you can:

• Start now — the most expensive year is always the next one
• Automate contributions — willpower fails; auto-debit doesn't
• Avoid fees — a 1% annual fee = ~25% less money at retirement

Try our compound interest calculator with your numbers. The result will surprise you.