Compound Interest Explained: How to Make Money Work for You

Albert Einstein reportedly called compound interest "the eighth wonder of the world." Warren Buffett built one of the world's greatest fortunes on it. So what actually is it, and how can you use it?

Simple vs Compound Interest

Say you invest £10,000 at 7% annual return.

After 30 years, compound interest gives you 2.5x more money than simple interest on the same investment.

The Rule of 72

Want to know how long it takes to double your money? Divide 72 by your annual return rate.

• At 6% return: 72 ÷ 6 = 12 years to double
• At 8% return: 72 ÷ 8 = 9 years to double
• At 10% return: 72 ÷ 10 = 7.2 years to double

Why Starting Early is Everything

This is the most important lesson in personal finance. Consider two investors:

• Sarah invests £200/month from age 25 to 65 at 7% return → £525,000
• James invests £200/month from age 35 to 65 at 7% return → £243,000

Sarah invests only £24,000 more total but ends up with £282,000 more. That extra decade of compounding makes the difference.

Where to Put Your Money (UK & US)

UK — use your ISA allowance first:

• Stocks & Shares ISA — £20,000/year, all growth tax-free forever
• Pension — contributions get tax relief at your marginal rate
• Lifetime ISA — 25% government bonus (for first home or retirement)

US — tax-advantaged accounts first:

• 401(k) — up to $23,500/year (2026), especially if employer matches
• Roth IRA — $7,000/year, tax-free growth and withdrawals
• HSA — triple tax advantage if you have a high-deductible health plan

The Biggest Enemy of Compound Interest: Fees

A 1% annual fund fee doesn't sound much. But on £100,000 over 30 years at 7% growth:

• 0% fee: £761,000
• 1% fee: £574,000
• The 1% fee costs you £187,000

Always choose low-cost index funds (Vanguard, iShares) over actively managed funds. The evidence consistently shows they outperform after fees.