Compound interest is the reason why saving a little for a long time can beat saving a lot for a short time. The idea is simple: the interest you earn is added to your capital and, from then on, it also earns interest.
How it works
With simple interest, you always earn the same amount each year, calculated on your initial capital. With compound interest, each year the interest is calculated on the capital plus the interest already accumulated. The formula for a single deposit is:
Final amount = initial amount × (1 + annual return)^years
For example, with 10,000 euros at 5% a year for 10 years:
| Type of interest | Final amount | Interest earned |
|---|---|---|
| Simple | €15,000.00 | €5,000.00 |
| Compound | €16,288.95 | €6,288.95 |
The 1,288.95 euros of difference is interest that has itself earned more interest.
The power of time
The effect takes off with the years. This is what you get by contributing 100 euros a month at a 5% annual return, starting from zero:
| Years | Total contributed | Final amount | Interest earned |
|---|---|---|---|
| 10 | €12,000 | €15,436 | €3,436 |
| 20 | €24,000 | €40,580 | €16,580 |
| 30 | €36,000 | €81,538 | €45,538 |
Between year 20 and year 30 your contributions grow by only 12,000 euros, but the balance doubles. Time is what matters most.
Fees compound too
One point less of annual return looks small, but it accumulates. With the same contribution of 100 euros a month for 30 years:
| Annual return | Final amount |
|---|---|
| 4% | €68,527 |
| 5% | €81,538 |
| 6% | €97,451 |
Moving from 5% to 4% costs you more than 13,000 euros. That is why it pays to look at the costs of the products you invest in, as we explain in our guide to index funds.
A quick rule: the rule of 72
To estimate how many years it takes for money to double, divide 72 by the annual return. At 6%, it doubles in about 12 years (the exact calculation gives 11.9). It is an approximation that works best with moderate returns.
Try it with your own numbers
Enter your starting amount, how much you contribute each month, the return you expect and the number of years:
How it calculates: it converts the annual return into an equivalent monthly rate and adds the contributions at the end of each month. It does not deduct taxes, fees or inflation, so the result is an estimate.
Compound interest works against you with debt
The same mechanism works in reverse when you borrow. Interest on an unpaid credit card balance is added to what you owe, and then it generates more interest. That is one more reason to clear expensive debt before you start investing for the long term.
Before you get excited
- The return is not guaranteed. Use cautious scenarios and try several figures.
- Inflation reduces purchasing power. 80,000 euros in 30 years will not buy what it buys today.
- Taxes reduce your gain. In Spain, gains are taxed in the savings base of the income tax (IRPF).
- Consistency. Compound interest requires you to keep contributing even when the market falls.
Frequently asked questions
How often is interest compounded?
It depends on the product: it can be monthly, quarterly or yearly. The more often interest is added, the slightly higher the final amount. Our calculator converts the annual return into a monthly rate.
What return should I assume?
There is no certain figure. Use prudent scenarios, for example 3%, 5% and 7%, and look at the range of results rather than a single number.
If you do not have a cushion yet, start with your emergency fund and with organising your monthly budget.
This article is for information and education only. It is not personalised financial, tax or legal advice: for an important decision, consult a qualified professional. Last reviewed: October 2026.