How Compound Interest Works — The Most Powerful Force in Finance
Compound interest is the process of earning interest on both your original deposit and the interest you have already earned. It sounds simple, but its effects over time are remarkable — and understanding it is arguably the most important thing you can do for your financial future.
Simple Interest vs Compound Interest
With simple interest, you earn interest only on your original deposit. If you put £1,000 in an account at 5% per year, you earn £50 every year. After 10 years you have £1,500.
With compound interest, you earn interest on your original deposit plus all the interest you have already accumulated. After year one you have £1,050. In year two you earn 5% of £1,050 — not just £1,000. After 10 years you have £1,629. That extra £129 came from doing nothing differently — just letting interest compound.
The formula: A = P × (1 + r/n)^(nt) — where P is your principal, r is the annual interest rate, n is how many times interest compounds per year, and t is time in years.
Why Time Matters More Than Amount
This is the counterintuitive truth that most people miss. Consider two people:
- Anna saves £200/month from age 20 to 30 (10 years), then stops completely. Total contributed: £24,000.
- Ben saves £200/month from age 30 to 60 (30 years). Total contributed: £72,000.
At 7% annual return, Anna ends up with more money at 60 — despite contributing three times less. Starting 10 years earlier outweighed three times the contributions. That is the power of compounding time.
Compounding Frequency
Interest can compound annually, monthly, weekly or daily. The more frequently it compounds, the faster it grows. Daily compounding is slightly better than annual, but the difference is smaller than most people think — the rate and time are far more important.
The Rule of 72
A useful mental shortcut: divide 72 by your annual interest rate to find roughly how many years it takes to double your money. At 6%, your money doubles every 12 years. At 9%, every 8 years. At 4%, every 18 years.
Compound Interest Working Against You
The same force that grows savings also grows debt. Credit card debt at 20% APR compounds just as relentlessly. A £3,000 balance with no payments becomes £18,000 in 10 years. Understanding compounding is equally important for knowing why high-interest debt should be paid off before investing.
The Practical Takeaway
Start early, stay consistent, and let time do the heavy lifting. Even small amounts invested in your twenties will likely outperform larger amounts invested in your forties. The best time to start was yesterday. The second best time is today.