Compound interest explained: what €200 a month really becomes
The closest thing personal finance has to a cheat code, explained with numbers instead of formulas. Plus the fee that quietly takes nearly a fifth of it.
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The short version
- Compound interest means your returns start earning returns of their own. Slowly at first, then fast.
- €200 a month at 7% a year grows to about €104,000 in 20 years, from €48,000 paid in.
- Time matters more than amount. Starting ten years earlier can roughly double the result.
- Fees compound too, against you. A 1% yearly fee can cost tens of thousands over a lifetime.
Compound interest is the closest thing personal finance has to a cheat code. It is also badly explained almost everywhere, usually with a formula nobody remembers. So here it is with numbers instead.
What compound interest actually is
Say you invest €1,000 and it grows 7% in a year. You now have €1,070. Next year, you earn 7% on €1,070, not on the original €1,000. That extra €70 is now earning money too.
In year one the difference is tiny. Over decades it becomes most of the money. That is the whole idea: growth on top of growth.
The rule of 72: divide 72 by your yearly return to see roughly how many years it takes to double your money. At 7%, money doubles about every 10 years. At 4%, about every 18.
What €200 a month turns into
Here is what happens if you invest €200 every month, with growth added monthly. Two scenarios: 4% a year, which is closer to a cautious mix, and 7% a year, a figure often used to illustrate long-term returns from a diversified stock fund before inflation.
| After | You paid in | At 4% a year | At 7% a year |
|---|---|---|---|
| 10 years | €24,000 | €29,450 | €34,617 |
| 20 years | €48,000 | €73,355 | €104,185 |
| 30 years | €72,000 | €138,810 | €243,994 |
| 40 years | €96,000 | €236,392 | €524,963 |
Look at the 7% column. After 10 years, growth has added about €10,600. Between year 30 and year 40, growth adds over €250,000 while you only put in another €24,000. That is the curve doing the work.
An honest caveat: real returns are not a smooth 7%. Markets fall, sometimes by a third or more, and can take years to recover. These numbers show how compounding works, not what any investment will do. Investing always carries the risk of getting back less than you put in.
Try it: Compound growthWhat regular saving grows into, and what fees quietly take.Why starting early beats saving more
Two friends each invest €200 a month at 7% until age 65.
The early starter put in just €24,000 more, but ends up with more than twice as much. The friend who started at 35 would need to invest about €430 a month to catch up. Those first ten years are the most valuable years you will ever invest, because that money has the longest to compound.
If you are 35 or 45 reading this, the lesson is not that you missed out. It is that today is the earliest you can start from now on.
The other side: fees compound too
Compounding does not care whose money it is working for. A yearly fee takes a percentage of your whole pot, every year, so it compounds against you.
Take €200 a month for 30 years at 7%. That ends at about €244,000. Now take 1% a year in fees, so you effectively earn 6%. It ends at about €201,000. That 1% fee cost roughly €43,000, more than half of everything you paid in.
This is why low-cost index funds are so popular. Many charge 0.1% to 0.3% a year, while some actively managed funds and advisers charge 1% to 2% or more. A small-looking number has a very large effect.
How to put compounding to work
- Start now, even small. €50 a month started today beats €200 a month started "when things settle down".
- Automate it. A monthly transfer on payday means you never have to decide again.
- Keep fees low. Check the total yearly cost of any fund or platform before you invest.
- Do not interrupt it. Selling in a panic during a crash, or dipping in for non-emergencies, breaks the chain. Have an emergency fund first so you never have to.
- Reinvest the returns. Choose accumulating funds or switch dividend reinvestment on, so growth keeps growing.
The best time to start was ten years ago. The second best time is your next payday.
Keep reading: accumulating vs distributing ETFs, the easiest way to keep compounding on autopilot, and how much you should have saved by 30, 40 and 50.
The bottom line
Compound interest rewards two things above everything else: time and patience. You do not need a big salary or perfect timing. You need to start, keep fees low, and leave it alone long enough for the curve to bend upwards.