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How the calculator works
Updated 4 November 2026 · Educational content
The calculator compares spending an amount today with setting that amount aside in a hypothetical investment. All figures are in euros. No live prices, investment products or market forecasts are used.
A one-off amount
The formula is future value = P × (1 + r)^y, where P is the initial amount, r is the effective annual return as a decimal, and y is the number of years. At 5%, €100 over ten years becomes €162.89 before rounding.
Recurring contributions
Daily uses 365 contributions per year, weekly uses 52, and monthly uses 12. Contributions are assumed to arrive at the end of each period. Leap years and exact calendar dates are ignored. We convert the effective annual rate to a periodic rate using i = (1 + r)^(1/m) − 1, where m is the number of contributions per year.
The balance is P × ((1 + i)^(m × y) − 1) / i. When the assumed return is zero, the balance is simply P × m × y. The displayed contribution total is the amount multiplied by the number of payments; modelled growth is the final balance minus that total.
What the model assumes
- A constant effective annual rate throughout the selected timeline.
- All returns stay invested; contributions remain fixed in euro terms.
- No missed payments or withdrawals.
- No taxes, platform fees, fund charges or trading costs.
- No adjustment for inflation or exchange rates.
Input limits and rounding
Choose a whole number from 1 to 100 years and a return from −50% to 30%. These are input limits, not statements about plausible or maximum market outcomes. Prices must be positive and no more than €1 billion. Calculations retain decimal precision internally; results are displayed to the nearest euro.
Why the default is 10%
The default 10% approximates the S&P 500’s long-term historical annualised total return with dividends reinvested. It is before inflation, fees and taxes, and is not a forecast. The historical figure is in US dollars; this euro illustration does not model exchange-rate changes.
Schwab Asset Management describes annualised S&P 500 returns of about 10% since 1926 and explains that total return assumes dividend reinvestment. Earlier history includes predecessor market series, since the modern 500-stock index launched in 1957. The exact compound annual growth rate depends on the start and end dates. We use a rounded historical benchmark, rather than a live historical backtest. You can adjust the rate to compare other assumptions.
Sources and further reading
Our worked examples use the formulas above. For broader education, see Investor.gov's introduction to investing, its fees guide, and the FCA's investing preparation guide. These organisations do not endorse this site.
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