The Power of Starting Early — Compound Interest in the UK

Compound interest is often called the eighth wonder of the world — and the numbers bear it out. The single biggest financial advantage most people can give themselves is time. Here's why starting early is so powerful, with real UK figures.

What is compound interest?

Compound interest means earning returns not just on your original investment, but on the returns themselves. Each year, your gains are added to the pot and start generating their own gains. The effect is exponential — slow at first, then dramatic.

The Rule of 72

Divide 72 by your annual return to estimate how long it takes to double your money:

• At 4% (cash savings): doubles every 18 years
• At 6% (cautious investment): doubles every 12 years
• At 8% (global equities historical avg): doubles every 9 years
• At 10% (higher growth): doubles every 7.2 years

Starting early vs starting late

Consider two investors both contributing £300/month into a Stocks & Shares ISA earning 7% annually:

The early starter contributes only £36,000 more than the late starter but ends up with over twice as much. The first 10 years of contributions are worth more than all subsequent years combined.

ISAs vs pensions for long-term growth

Both benefit from compound growth, but in different ways:

• Pension: Tax relief going in (20–45%), tax-free growth, taxed on withdrawal. Best for money you won't need before 57.
• Stocks & Shares ISA: No tax relief going in, but completely tax-free growth and withdrawals. £20,000/year allowance. Best for flexibility.

For most people under 40, maximising pension contributions (especially with employer matching) then topping up ISAs is the optimal order.

What about inflation?

At 3% inflation, prices double roughly every 24 years. If your investments aren't growing above inflation, you're losing purchasing power. UK cash savings rates (currently 4–5%) barely beat inflation once tax is considered. This is why long-term savings benefit from investment exposure.