Two friends each put €100 a month into the same investment. Ana starts at 25 and stops at 35. Marko starts at 35 and keeps going until 65. Marko puts in three times as much money. Yet, under the same assumed return, Ana's money grows by more than his. The reason is , and it is the quiet engine behind long-term investing.
Before we start: what do you think?
You invest €1,000 at an assumed return of 6% a year and leave all the gains invested. Roughly how long until it doubles?
✗ Too short: after 6 years at 6% you have about €1,419, not €2,000.
✓ Right: 1.06 to the power of 12 is about 2.01, so the money roughly doubles.
✗ That is how long simple interest takes (17 × €60 = €1,020), but compounding is faster.
✗ Much too long: after 25 years at 6% the money has grown more than four times.
Simple growth vs compound growth
Imagine €1,000 that earns an assumed 5% a year. This is an assumption to show the maths, not a forecast. Actual returns change every year and can be negative.
With simple growth, you take the €50 out each year and spend it. After 10 years you have your €1,000 plus €500 that you received along the way. That is €1,500 in total.
With compound growth, you leave the €50 invested. Next year, 5% is calculated on €1,050, not on €1,000. So you earn €52.50. The year after, you earn 5% on €1,102.50. Each year's growth also grows. After 10 years you have about €1,628.89.
The difference looks small after 10 years. It does not stay small:
| Years | €1,000 at an assumed 5%, compounded |
|---|---|
| 10 | about €1,629 |
| 20 | about €2,653 |
| 30 | about €4,322 |
| 40 | about €7,040 |
Look at the steps. The first 10 years add about €629. The last 10 years add about €2,718. The same 5% does more work later, because it works on a bigger amount.
Quick check
Why does compound growth add more money in later years at the same rate?
A quick rule: the rule of 72
To estimate how long money takes to double, divide 72 by the yearly rate. At an assumed 6%, 72 ÷ 6 = 12 years. At 4%, it is about 18 years. At 9%, about 8 years. It is only an estimate, but it is close for normal rates.
The rule works in reverse too. Debt at 18% a year doubles in about 4 years if you pay nothing. Compounding does not care whether it works for you or against you.
Time: Ana and Marko
Worked example: Starting early vs putting in more
Both save €100 a month and get the same assumed return of 5% a year, compounded monthly. Nobody knows real future returns. This only shows the effect of time.
| Ana | Marko | |
|---|---|---|
| Saves €100 a month | from 25 to 35 (10 years) | from 35 to 65 (30 years) |
| Money put in | €12,000 | €36,000 |
| Value at 65 | about €69,376 | about €83,226 |
| Growth on top | about €57,376 | about €47,226 |
Marko ends with more, because he put in €24,000 more. But Ana's growth is about €10,000 larger than his, from a third of the money. Her euros simply had 30 more years to compound.
The lesson is not "start at 25 or never". Most people cannot. The lesson is that every year of waiting has a cost, and that cost is paid at the end, where you do not see it today.
Quick check
Ana put in €12,000 and Marko €36,000. Why did Ana's money grow by more?
Timing: waiting for the "right moment"
Many beginners want to wait until prices fall before they start. It sounds clever. In practice, nobody knows when that moment comes, and the waiting itself costs years of compounding.
Here is a simple comparison. Assume a market grows 5% a year on average. Petra invests €10,000 today. Ivan waits five years for a better moment, with his money in cash that earns nothing. Then he is lucky: he buys 10% below the price of that day, so his €10,000 buys as much as €11,111 would.
After 30 years from today:
- Petra has about €43,219.
- Ivan has about €37,626.
Ivan picked a perfect dip and still ended about €5,600 behind. His good timing could not make up for five lost years. In real life, timing is rarely that good, and returns are never this smooth. But the idea holds: time in the market tends to matter more than timing the market.
The tool starts at $100 a month, an assumed 4% and 20 years. You put in $24,000, and the growth on top is about $12,677. Move the years to 40: you put in $48,000, and the growth on top becomes about $70,196, more than the money you put in.
Quick check
Ivan waited five years and bought 10% cheaper. Why did he still end up behind Petra?
What compounding needs from you
Compounding is simple, but it asks for three things:
- Time. Start as early as you sensibly can, even with small amounts.
- Patience. Leave the gains invested. Every withdrawal resets part of the engine.
- Low costs. A yearly fee compounds against you in the same way. (See Fees: the silent cost.)
It also needs one thing it cannot promise: positive returns. In bad years, compounding works on a smaller balance. A long time horizon gives you the chance to recover, not a guarantee. (See Ups and downs.)
This is education, not advice. Check your own situation before you invest.
Key takeaways
- Compound growth means your growth also grows, because each year's percentage works on a larger amount.
- The rule of 72: divide 72 by the yearly rate to estimate how many years money takes to double.
- Starting early gives compounding more years. Each year of waiting has a cost you only see at the end.
- Waiting for the perfect moment usually costs more time than it saves in price.
- Every rate in this lesson is an assumption, not a forecast. Actual returns change and can be negative.
Try this
Open the tool above. Set your own realistic monthly amount and an assumed rate of 3%. Write down the result after 10, 20 and 30 years. Then ask yourself one question: "What would I need to change to start one year earlier than I planned?"