When I started thinking seriously about what to do with my savings, compound interest was one of those terms I'd heard a hundred times without really understanding why it mattered so much. I knew it was "a good thing", but I couldn't quite picture how it worked in practice.
The idea itself is simple: it's interest earning interest. Your money grows, and the growth also grows. But the moment the numbers actually show you what that means — especially over the long term — it changes how you think about time and money.
In this article I try to explain it the way I'd explain it to myself a few years ago: starting from the basics, and working up to the actual numbers at the end.
Try it before reading — the numbers will help the rest make sense.
Quick investment calculator
Estimated final value
145.180 €
Estimated gains
87.180 €
The difference between simple and compound interest
The best way to understand it is with a concrete example — and seeing how the two diverge over time.
Imagine you have €1,000 saved and someone offers you 7% per year. With simple interest, you earn exactly €70 every year: always 7% of the original €1,000. After 10 years you have €1,700. Straightforward.
Compound interest works differently. At the end of year one you have €1,070. But in year two you don't earn 7% on the original €1,000 — you earn 7% on the €1,070 you now have. And so on each year: the interest from the previous year joins the principal, and the following year it earns more interest.
After 10 years, with those same €1,000 at 7%, you have €1,967 instead of €1,700. The extra €267 was generated by interest on interest. At 20 or 30 years, that gap becomes enormous.
Why time changes everything
What surprised me most when I started looking into this was how badly we intuitively grasp the effect of time. It doesn't add — it multiplies.
Take €10,000 at 7% per year and see what changes depending on the term:
| Term | Result | Gain |
|---|---|---|
| 10 years | €19,672 | +€9,672 |
| 20 years | €38,697 | +€28,697 |
| 30 years | €76,123 | +€66,123 |
What struck me about this table: the last 10 years (from year 20 to 30) add more than the first 20 combined. The final decade outpaces everything before it. That's hard to grasp intuitively, but it's exactly what compound interest does.
There's a quick rule for estimating how long it takes an investment to double: divide 72 by the interest rate. At 7%, that's about 10 years. At 4%, it's 18 years. It's an approximation, but useful for a quick sense-check.
What happens when you add money along the way
Most of us don't start with a large lump sum. The realistic way to invest is to contribute month by month — and compound interest works on every euro you add.
Say you start with €10,000 and add €200 per month for 20 years at an average annual return of 7%. In total you'll have put in €58,000: the initial €10,000 plus €48,000 in monthly contributions.
The final value comes close to €145,000. That means roughly €87,000 you didn't put in yourself — generated by compound interest working on everything you accumulated. More than half the final result comes from the compounding effect, not from your direct contributions.
You can see this in the calculator above by adjusting the numbers — try changing the term and watch how the "gains" block grows out of proportion.
The formula behind the numbers (for those who want to understand it)
If the examples above already make sense, this is the mathematical explanation for what we've seen. You don't need it — but if you ever want to verify a projection yourself, here it is.
The basic compound interest formula is:
FV = PV × (1 + r)n
- FV
- future value — what you'll have at the end of the period
- PV
- present value — what you invest at the start
- r
- annual interest rate as a decimal (7% = 0.07)
- n
- number of years
Using the numbers from before: €10,000 at 7% for 20 years is 10,000 × (1.07)20 = 10,000 × 3.87 ≈ €38,700.
What's interesting is that in this formula, the exponent n (time) is what produces the biggest effect. Doubling the initial capital matters less than doubling the number of years. That's why the table above shows such unequal gains across decades.
How this affects real decisions
Understanding compound interest changes how you frame certain financial questions that don't have an obvious answer.
It's the question that naturally comes up when a mortgage is your only remaining debt: is it better to make extra repayments or invest that money instead? There's no single answer because compound interest is at work on both sides — saving mortgage interest on one side, growing the invested capital on the other.
The two key variables are your mortgage rate and the expected return on your investment. If the second consistently exceeds the first — accounting for risk and time horizon — investing may make more mathematical sense. When it doesn't, early repayment is the safer option.
You can run that specific comparison with real numbers in the mortgage vs investment comparison tool.
One last thing worth noting: the figures in this article don't account for inflation. The nominal return (the one on paper) isn't the same as the real return once you subtract what prices rise by. It's an important distinction — one we'll cover in a separate article.
Run the numbers with your own scenario: