Herramienta
Compound Interest Calculator
Project how your money could grow with recurring contributions and compare the result against simple interest.
Enter your assumptions to estimate potential growth based on annual return, recurring contributions, and total years. The comparison against simple interest helps quantify the compounding effect.
This simulation uses a constant annual return and excludes inflation, fees, and taxes. It is intended for educational planning purposes.
Inputs
Configure investment assumptions and contribution frequency to build the projection and compare compound interest vs simple interest.
Calculation assumption: contributions are applied at the end of each period and return is constant.
Results
Final value (simple interest)125.825 €
Compound advantage44.794 €
Year 20
Compound interest170.619 €
Simple interest125.825 €
Why compound interest scales better over time
This calculator shows why consistency matters. When interest is reinvested, growth applies not only to the initial principal but also to previously earned interest, creating a snowball effect over time.
With simple interest, interest does not generate additional interest. Over long horizons, this usually creates a visible gap compared to compound growth.
How to use the compound interest calculator
Enter an initial investment, a recurring contribution, the expected annual return, and the number of years. The calculator projects the final value with compound interest and compares it against simple interest, so you can clearly see how much compounding adds.
Try increasing the annual return or the time horizon: you will see the compound interest curve gradually pull away from the simple interest line. That is the effect of earning interest on interest — the longer the period and the higher the return, the wider the gap between the two lines on the chart.
You can also experiment by raising the periodic contribution. While the relative compounding effect stays the same, the absolute final value grows considerably. The key is combining consistent contributions, a long time horizon, and a reasonable annual return.
What is compound interest and why it matters
Compound interest is the process by which investment returns are reinvested to generate returns of their own. Unlike simple interest, which always calculates on the initial capital, compound interest acts on a growing capital base. The result is exponential growth that accelerates the longer the time horizon and the more frequent the compounding.
The practical impact is enormous: €10,000 invested at 7% annual interest with monthly compounding over 30 years grows to more than €81,000 — with no additional contributions. Add monthly contributions of €200 and the result exceeds €320,000. The calculator lets you visualise exactly this effect using your own numbers.
Index funds and the power of time
Index funds are the most common vehicle for harnessing compound interest over the long term. By automatically reinvesting dividends and rebalancing the portfolio, they allow the time effect to act on capital without active intervention. A fund tracking the MSCI World has historically delivered annual returns in the 7–9% range before inflation — translating into very significant capital multiplications over 20 or 30-year horizons.
The key is not to time the market, but to make consistent contributions through up and down cycles. Dollar-cost averaging reduces the risk of buying at peaks and turns corrections into opportunities to accumulate more units at lower prices.
Compound interest, pension plans, and time horizon
Pension plans in Spain combine the compound interest effect with a direct tax advantage: contributions reduce the IRPF taxable base in the year they are made. This accelerates the net growth of savings, since untaxed capital is reinvested from day one. The 20–30-year horizon typical of a pension plan is exactly where the compound effect is most powerful.
However, withdrawals are taxed as employment income, so planning the fiscal timing of the drawdown is critical to the final outcome. The calculator can be used to compare different contribution and time horizon scenarios before committing capital to a product with liquidity restrictions.
Frequently asked questions about compound interest
What is the difference between compound and simple interest?
With simple interest, returns are always calculated on the initial capital. With compound interest, the returns of each period are added to the capital and generate additional returns in subsequent periods. In the short term the difference is small; over the long term the gap becomes very significant.
How often should interest compound for the best result?
The more frequently interest compounds (daily, monthly, quarterly, annually), the higher the final result — though the practical difference between monthly and daily compounding is very small. In investment funds, compounding is continuous since the net asset value reflects all fund returns on a daily basis.
Are ETFs or index funds better for harnessing compound interest?
Both harness compound interest in a similar way, as they replicate accumulation indices that reinvest dividends. In Spain, index funds have the tax advantage of transfers without triggering capital gains tax, which allows capital to be moved between funds without interrupting the compound effect.
How much do I need to invest to reach a specific goal?
It depends on your initial capital, expected annual return, time horizon, and regular contributions. Use the calculator, adjusting these parameters until you find the combination that reaches your target. As a reference, the rule of 72 states that dividing 72 by the annual interest rate gives you the approximate number of years needed to double the capital.