Free calculator
Compound interest calculator with monthly contributions
Work out how much your money could grow from a starting amount, a contribution every month, the number of years you choose and an annual return. The result updates as you type, with a year-by-year chart and table.
- Total contributed
- Accumulated growth
Year by year
Each bar is the value at the end of the year: the lighter part is what you contributed and the stronger part is accumulated growth. Tap or hover over a bar to see that year.
- Total contributed
- Accumulated growth
- Contributed that year
- Growth that year
See the year-by-year table
| Year | Total contributed | Accumulated growth | Total value | Contributed that year | Growth that year |
|---|
Educational simulation, not a forecast or financial advice. It excludes inflation, taxes and fees; investments can fall in value.
How it is calculated
The annual return is converted into its monthly equivalent: r = (1 + annual return)^(1/12) − 1. That way, a year without contributions grows by exactly the return you choose.
Each month that interest is applied to the balance and then the contribution is added, so what you put in one month starts growing the next. The starting amount grows from the first month.
The figures are nominal and gross: they do not deduct inflation, taxes or fees.
Compound interest questions
What is compound interest?
It is growth earned on growth: each year the return applies to the money you put in and also to the gains from previous years. That is why, with the same savings, the result depends so much on time.
What formula does this calculator use?
It converts the annual return into its monthly equivalent, r = (1 + annual return)^(1/12) − 1, and each month applies that interest to the balance and then adds the contribution. A year without contributions grows by exactly the chosen annual return.
Are contributions made at the start or the end of each month?
At the end of each month: a month's contribution starts earning interest the following month. The starting amount earns interest from the first month.
How long does it take for money to double?
A quick estimate is the rule of 72: divide 72 by the annual return. At 7%, about 10 years (72 / 7 ≈ 10.3). The exact figure is ln 2 / ln 1.07 ≈ 10.2 years, without contributions.
Does it account for inflation, taxes or fees?
No. The figures are nominal and gross. A simple way to see them in today's purchasing power is to subtract expected inflation from the return: for example, use 5% instead of 7% if you expect 2% inflation.
What return should I enter?
The calculator does not recommend one. The three options are historical USD references from S&P indices: U.S. Treasury bills, a mixed stock and bond portfolio and the S&P 500 with dividends. You can also type your own, from 0% to 999%. Past performance does not guarantee future results.
Why do some results show as “× 10¹⁸”?
With many years and high returns, the figure passes a quadrillion. From there it is shown in scientific notation: €1.2 × 10¹⁸ is 1.2 followed by 18 zeros. If the figure passes the largest number that can be calculated, the simulation stops at the last year with a real value.