This calculator estimates the future value of a saving or investing plan by combining four inputs: the capital you start with, the amount you add each month, the annual return you assume and the number of years you keep the plan running. It helps you see the compounding effect without building a spreadsheet: returns are generated not only on your own money, but also on previous returns.
Compound interest calculator
Move the sliders and see how your money behaves when you contribute consistently. The calculator is at the top because you came here for an answer, not theory first.
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Simulator
Adjust capital, contribution, years and return. State stays consistent even if you drag the slider to a preset value.
Estimated result€190,958
Contributed€82,000
Interest generated€108,958
Horizon20 years
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Adjust the starting capital, monthly contribution, annual return and years. Starting capital can be zero. Monthly contribution is the recurring amount you add. Annual return is an assumption, not a promise. Years is the investment horizon. The result updates instantly as you move each control.
The calculator converts the annual return into a monthly equivalent and compounds month by month. Each month it adds your contribution to the balance and applies return to the accumulated total. The starting capital grows in parallel with the future value of the recurring contributions.
If you invest $300 per month for 20 years at 7% per year, you contribute $72,000 of your own money and end around $152,000. Roughly $80,000 comes from compounding. Extending the same habit to 30 years does not add just 50%; it more than doubles the effect.
Treat the final number as a scenario, not a guarantee. The useful part is sensitivity: compare what changes when you increase the monthly contribution, the horizon or the assumed return. Time and consistency usually matter more than chasing a higher return assumption.
Common mistakes are using an optimistic return as if it were guaranteed, forgetting that inflation reduces future purchasing power and ignoring fees. A one-point annual fee difference compounded over decades can absorb a meaningful share of the final amount.
Assumptions: the return is constant on average, real markets move unevenly, inflation is not deducted and taxes are not included. The result is educational and approximate.
Bank of Spain — Client Banking Portal · CNMV — Investor Education
Compound interest, straight to the point
What return do I use as a scenario?
Use your own assumption. 7% is often used to illustrate broad equity indices over long periods, but past returns do not guarantee future returns.
Does it include inflation?
No. Use the inflation calculator to reason in purchasing-power terms.
Does it include taxes?
No. It is a gross estimate. Tax treatment depends on the product and when gains are realized.
Does it work with small contributions?
Yes. Compounding rewards time and consistency more than the size of the first contribution.
Illustrative results for educational purposes · Educational information, not financial advice.