Compound Interest Explained

Albert Einstein allegedly called compound interest the "eighth wonder of the world." Whether the quote is genuine is debatable — but the effect behind it is very real. Understanding compound interest leads to better decisions about saving, investing, and retirement planning.

What is compound interest?

With simple interest, you earn interest only on your original principal. With compound interest, the interest earned is added to your principal and then earns interest itself in the following period. In short: you earn interest on your interest.

The compound interest formula

The formula is simpler than it looks:

Final amount = Principal × (1 + interest rate)^years

Or in short: K = K₀ × (1 + p)ⁿ

• K₀ = initial principal (e.g. €1,000)
• p = interest rate as a decimal (5% = 0.05)
• n = number of years
• K = final amount after n years

The formula reveals one key truth: growth is exponential, not linear. The longer you stay invested, the more powerful the effect becomes.

The compounding effect over 10, 20, and 30 years

The table below shows how €10,000 grows at different interest rates — with no additional contributions, purely through compounding:

Interest rate	After 10 years	After 20 years	After 30 years
3%	€13,439	€18,061	€24,273
5%	€16,289	€26,533	€43,219
7%	€19,672	€38,697	€76,123
10%	€25,937	€67,275	€174,494

At 7% per year — a realistic long-term equity return — your capital grows sevenfold in 30 years. Half of that growth happens in the final 10 years. The takeaway is clear: time is the most important factor.

How savers can use compound interest

ETF savings plans

Broadly diversified ETFs tracking the MSCI World have historically returned around 7–8% per year. Investing monthly via a savings plan and reinvesting distributions — or choosing an accumulating ETF — puts the full power of compounding to work.

Fixed-term and overnight deposits

Compound interest works in safer savings products too, just more slowly. At 3% on a fixed-term deposit, €10,000 still grows to €18,061 over 20 years.

Savings plans with regular contributions

Compounding is most powerful when you contribute regularly. €200 per month over 30 years at 7% grows to roughly €243,000 — from just €72,000 of your own money. More than €170,000 comes entirely from the compounding effect.

Common mistakes when building wealth

1. Starting too late

Compound interest needs time. Someone who starts at 25 has a massive advantage over someone who starts at 35 — even with the same monthly savings rate. Ten extra years don't add 30% more wealth; they often double it.

2. Rebalancing too often

Every sale interrupts the compounding effect. Transaction costs and taxes on realised gains eat into your returns. In most cases, buy-and-hold outperforms frequent trading.

3. Ignoring inflation

At 2% inflation, purchasing power halves in 35 years. Real return is always nominal return minus inflation. Use our inflation calculator to see what that means for you.

4. Withdrawing gains instead of reinvesting

Taking out dividends or interest instead of reinvesting them effectively switches compounding off. During the accumulation phase, returns should always stay in the pot.

Conclusion

Compound interest is no secret — but it is consistently underestimated. The core rules are simple:

1. Start early. Every year counts.
2. Contribute regularly. Even small amounts add up.
3. Reinvest your returns. That's the only way the effect kicks in.
4. Be patient. The most powerful phase comes at the end.

The best time to invest was 20 years ago. The second best time is now.

More useful calculators: Savings Plan Calculator · ROI Calculator · Rule of 72 Calculator