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Compound Interest Explained

Simple guide to compound interest with real examples and calculations. Learn how "interest on interest" builds wealth over time.

Simple Interest vs Compound Interest

The key difference between simple and compound interest is that compound interest earns "interest on interest," leading to exponential growth.

The Magic of Compounding

Simple Interest

£1,000 at 5% for 3 years

Year 1: £1,000 × 5% = £50
Year 2: £1,000 × 5% = £50
Year 3: £1,000 × 5% = £50
Total: £1,150

Compound Interest

£1,000 at 5% for 3 years

Year 1: £1,000 × 5% = £50
Year 2: £1,050 × 5% = £52.50
Year 3: £1,102.50 × 5% = £55.13
Total: £1,157.63

How Compound Interest Works

The Basic Formula

Future Value = Present Value × (1 + rate)time
For £100 at 5% for 2 years:
£100 × (1.05)² = £110.25

Real World Example

Initial investment:£10,000
Annual return:7%
Time period:20 years
Final amount:£38,697
£28,697 from compounding!

The Power of Time

Same Investment, Different Start Times

Starting AgeMonthly InvestmentBy Age 65Total Invested
25£200£500,000+£96,000
35£400£400,000£144,000
45£800£250,000£144,000

*Assuming 7% annual return. The key insight: starting early with smaller amounts often beats starting late with larger amounts.

Compounding in Different Scenarios

Savings Accounts

High-interest savings accounts compound interest, usually monthly. Even modest rates add up significantly over time.

Example: £5,000 at 4% AER for 5 years = £6,083 (vs £6,000 simple interest - £83 from compounding).

Investment Portfolios

Stocks and funds historically return 7-10% annually with compounding. This is how millionaires are made from regular investing.

Warren Buffett: Started with £10,000 at age 11. At 7% compounding, that would be worth £5.4 million by age 65.

Debt and Credit Cards

Compound interest works against you with debt. Credit cards compound daily, making minimum payments very expensive.

Credit card example: £2,000 balance at 20% APR compounded daily costs £1,200+ in interest over 2 years if only minimum payments are made.

Practical Ways to Use Compound Interest

Start Small, Be Consistent

Even £50-100 per month invested regularly can grow to substantial amounts through compounding and dollar-cost averaging.

Reality: Most millionaires built wealth through consistent, long-term investing rather than large lump sums.

Reinvest Everything

Never spend investment returns. Always reinvest dividends, interest, and capital gains to maximize compounding.

Power: Reinvesting £500 in annual dividends at 7% adds £18,000 to your portfolio over 20 years.

Give Time Time

Compounding needs time to work its magic. The longer your investment horizon, the more powerful the effect.

Key insight: An investment returning 7% annually doesn't just double every 10 years - it grows exponentially faster over time.

Avoid High-Interest Debt

Compound interest in debt destroys wealth. Pay off credit cards and high-interest loans before focusing on investing.

Priority: Emergency fund first, then debt payoff, then investing. Don't let compounding work against you.

The Rule of 72

Quick Doubling Calculator

Divide 72 by your expected return rate to estimate how long it takes for your money to double through compound interest.

Years to Double = 72 ÷ Annual Rate
3% Rate
24 years
6% Rate
12 years
8% Rate
9 years

See Compound Interest in Action

Use our free compound interest calculator to experiment with different rates, amounts, and time periods. See how small changes make big differences.

Compound CalculatorStart Investing

Frequently Asked Questions

Common questions about compound interest and calculations

Ready to See Compound Interest in Action?

Use our free compound interest calculator to experiment with different rates, time periods, and contribution amounts.

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