The Magic of Compound Interest
“Compound interest is the eighth wonder of the world. He who understands it, earns it; he who doesn’t, pays it.” — attributed to Einstein
Simple vs Compound Interest
Simple interestis calculated on your original amount only. Invest £1,000 at 5% and you earn £50 every year. After 10 years: £1,500.
Compound interest is calculated on your original amount plusall the interest you’ve already earned. After 10 years at 5%: £1,629. The difference grows dramatically over time.
Key Concept
Compounding means your money earns money, and then that money earns money. It’s a snowball rolling downhill — the longer it rolls, the faster it grows.
The Rule of 72
Want to know how long it takes your money to double? Divide 72 by the annual return:
- At 4%: 72 ÷ 4 = 18 years to double
- At 7%: 72 ÷ 7 = ~10 years to double
- At 10%: 72 ÷ 10 = ~7 years to double
Why Starting Early Matters More Than Amount
This is the most important table in personal finance. Three people invest £100/month at 7% annual return:
| Person | Starts At | Years Investing | Total Contributed | Value at 60 |
|---|---|---|---|---|
| Alice | Age 20 | 40 years | £48,000 | £262,481 |
| Bob | Age 30 | 30 years | £36,000 | £121,997 |
| Carol | Age 40 | 20 years | £24,000 | £52,093 |
Real-World Example
Alice contributed only £12,000 more than Bob but ended up with £140,484 more. That’s not because she invested more — it’s because her money had an extra 10 years to compound. Time is the most powerful ingredient.
The Compounding Curve
Compounding is not a straight line — it’s exponential. The first £100,000 is the hardest. After that, your money is earning so much on its own that the growth accelerates. Many investors find their second £100,000 arrives in half the time.
How to Harness It
- Start now — even £25/month is better than waiting for the “perfect” time.
- Reinvest everything — choose “accumulation” fund units, not “income.”
- Don’t interrupt it — resist the urge to cash out during dips.
- Use a Stocks & Shares ISA — so tax doesn’t eat your compound growth.
- Keep fees low — even 0.5% in extra fees compounds against you.
Warning
Compound interest works against you on debt. A credit card charging 22% APR means your debt doubles in just over 3 years if you don’t pay it off. Always clear expensive debt before investing.