I didn't really understand compound interest until I ran my own numbers for the first time. Not the formula, not the concept. The actual number. When I saw what 200 euros a month could become over 30 years, I understood why people say time in the market is the most important variable.

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Simple Interest vs Compound Interest

With simple interest, you earn interest only on your original capital, the same amount every year. With compound interest, the interest you earn is added to your principal, and in the next period, you earn interest on that larger amount too. The result is exponential growth instead of linear growth.

Example: £10,000 at 5% over 10 years

Simple interest: £10,000 × 5% × 10 = £5,000 → Final: £15,000

Compound interest: £10,000 × 1.05¹⁰ = £16,289 → £1,289 extra from compounding

Over 30 years, the gap becomes dramatic: simple interest yields £25,000, compound interest delivers £43,219. That's nearly double, without investing a single penny more.

The Compound Interest Formula

Aₙ = A₀ × (1 + r)ⁿ

A₀ = Starting capital · r = Annual interest rate (decimal, e.g. 0.05 for 5%) · n = Years

The Rule of 72: How Long to Double Your Money

Divide 72 by your annual return to get the approximate number of years until your money doubles:

Compound Interest in Practice: Monthly Savings

£200/month at 7% annual return over 30 years

Total invested: 200 × 360 = £72,000

Final value: approx. £226,000

Compound interest earned: £154,000, more than double your contributions

The Compound Interest Tipping Point

There's a moment when your cumulative investment gains overtake your total contributions. At 7% return, this typically happens around year 12–15. After that point, your wealth grows primarily through compounding, not through what you put in.

Time Beats Everything

Starting 10 years earlier (at 25 vs 35) with the same monthly contribution and same return can result in twice the final wealth, purely from those 10 extra years of compounding. The best time to start is today.

Note: Historical returns are not a guarantee of future results. Compound interest applies to any investment, but capital loss risk always exists, especially in the short term with equity ETFs.