Compound Interest Calculator
Enter your starting capital, what you'll add each month, the interest rate and the term, and you'll see how much you'll have at the end and how much of it is interest.
Enter a deposit or a monthly contribution, the interest rate and the number of years.
Gross result, before taxes. Add an inflation rate to also see what it is worth in today’s money.
Year-by-year breakdown
Enter a deposit or a monthly contribution, the interest rate and the number of years.
Results are estimates. Solvemis accepts no responsibility for the results obtained or for how they are used. Always double-check the result and talk to a qualified professional before making decisions.
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How it works
How to use it
- Enter the capital you start with and what you'll add each month. You can leave one of the two blank.
- Enter the annual interest rate (TIN) and the number of years.
- Choose how often interest compounds and whether you contribute at the start or the end of each month.
- To see what it will be worth in today's money, enter the average inflation you expect. The European Central Bank's target is 2 %.
- You'll see the final capital, how much you put in and how much is interest, with a growth chart. Open the table to see it year by year.
How it is calculated
It's simulated month by month. The annual rate is converted into its monthly equivalent according to the compounding frequency, (1 + TIN/n)^(n/12) − 1, and each month it's applied to the balance and the contribution is added. Without contributions, the result matches the classic compound interest formula exactly. If you enter inflation, each balance is divided by (1 + inflation)^years to express it in today's money.
Final capital = C × (1 + TIN/n)^(n × years)
Sources
- ¿Sabes qué es el tipo de interés compuesto? — Banco de España, Portal del Cliente Bancario
- Tipos de interés: cuestiones generales (TIN y TAE) — Banco de España, Portal del Cliente Bancario
- Compound Interest Calculator — U.S. Securities and Exchange Commission (Investor.gov)
- Objetivo de inflación del 2 % — European Central Bank
About this calculator
What compound interest is
Compound interest means that interest is added to the capital and in turn earns new interest. In the first year you earn interest on what you put in; in the second, on what you put in plus the first year’s interest; and so on. The effect is small at first and huge over time.
An example with this calculator’s figures: 10,000 € at 5 % for 10 years, compounded monthly, becomes 16,470 €. If you also add 100 € a month, you end up with 31,998 €: you put in 22,000 € and almost 10,000 € is interest.
The factor that matters most: time
The formula raises growth to a power that depends on the years. That’s why starting earlier matters more than contributing more. A quick reference is the rule of 72: divide 72 by the interest rate and you get roughly the years it takes for capital to double. At 5 %, about 14 years; at 8 %, about 9.
TIN, TAE and compounding frequency
TIN (nominal interest rate) is the annual percentage agreed. TAE (annual equivalent rate, like an APR) shows what it really earns in a year, taking into account how often interest compounds:
| Compounding | 5 % TIN equals |
|---|---|
| Annual | 5.00 % TAE |
| Quarterly | 5.09 % TAE |
| Monthly | 5.12 % TAE |
| Daily | 5.13 % TAE |
When you compare products, compare TAE with TAE: it’s the only figure that puts them all on an equal footing. In the calculator you enter the TIN and choose the frequency; the result already includes that effect.
Contributing at the start or the end of the month
If you contribute at the start of each month, that money earns interest for the whole month; if you do it at the end, it starts earning the following month. The difference is small but it adds up: with 100 € a month for 10 years at 5 %, contributing at the start gives about 65 € more.
Inflation: what your money will be able to buy
A euro in 20 years won’t buy the same as today. If you enter the average inflation you expect, the calculator adds a dashed line to the chart and another row to the result with the value in today’s money: each balance is divided by (1 + inflation) raised to the years elapsed.
The gap between the two curves is what inflation eats. And if interest doesn’t beat inflation, capital grows in euros but loses purchasing power: at 2 % interest with 2 % inflation, in 10 years you’ll have more euros than now and be able to buy practically the same.
For reference, the European Central Bank targets 2 % inflation over the medium term. In particular periods it can be very different: in 2022 it went above 8 % in the euro area.
What the result doesn’t include
- Taxes: in Spain, savings income is taxed in the savings base of income tax (IRPF), at rates starting at 19 %.
- Inflation, if you don’t enter it: without that figure, the result is in euros of each moment, not in today’s purchasing power.
- Fees: in funds and pension plans, management fees come straight off the return.
- Variable returns: the calculator assumes a fixed rate. In investments such as funds or shares, returns change every year and can be negative; the result is an estimate, not a forecast.
Frequently asked questions
What is the difference between simple and compound interest?
With simple interest, interest is always calculated on the starting capital. With compound interest it is added to the capital and in turn earns new interest. Over 10 years at 5 %, 10,000 € becomes 15,000 € with simple interest and 16,289 € with annual compounding.
Which is better, monthly or annual compounding?
At the same nominal rate (TIN), the more often interest compounds, the more you earn, because interest starts earning interest sooner. That's why Spanish banks also quote the TAE (APR), which converts any frequency to its annual equivalent: 5 % TIN compounded monthly equals 5.12 % TAE.
Does the result include taxes?
No, it's gross. In Spain, interest and investment gains are taxed in the savings base of income tax (IRPF), at rates starting at 19 %. Your real net capital will be somewhat lower, depending on the product and your tax situation.
What about inflation?
It's only deducted if you enter it. In that case you'll also see what the final capital is worth in today's money: 16,470 € in 10 years' time, with average inflation of 2 %, buys the same as 13,511 € today. For reference, the European Central Bank targets 2 % inflation over the medium term.
Updated 30 September 2026
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