See how your savings grow over time with the power of compounding. Enter your principal, rate, and duration to get started.
This is an estimate for informational purposes only. Not financial, medical, or professional advice.
Think of a snowball rolling down a hill. It starts small, but every metre it travels adds more snow, and a bigger ball picks up more snow with each rotation. Compound interest works the same way: the interest your money earns is reinvested, so future interest is calculated on a larger base. The longer the slope, the bigger the ball.
Unlike simple interest (where a €10,000 deposit at 5% earns exactly €500 every year), compound interest accelerates. In year one you earn €500. In year two, you earn interest on €10,500. By year twenty, each year's interest is worth more than the entire first year combined.
“Compound interest is the eighth wonder of the world. Those who understand it, earn it; those who don't, pay it.”
Divide by the rate
Without a calculator, you can estimate how long any investment takes to double using one simple rule: divide 72 by the annual interest rate. The result is the approximate number of years to double.
Three quick examples
The rule works best for rates between 2% and 15%. For higher rates the true doubling time is slightly longer, but for quick comparisons between options, it is accurate enough to matter.
“Time is your friend; impulse is your enemy. Take advantage of compound interest and don't be captivated by the siren song of the market.”
A
Final amount
The total value at the end of the period
P
Principal
Your starting balance
r
Annual rate
As a decimal: 5% becomes 0.05
n
Frequency
Times per year interest compounds
t
Time
Duration in years
In the formula, t (time) sits in the exponent. Doubling the duration does not merely double the result; it squares the growth factor. Time outweighs rate in almost every realistic scenario. When recurring contributions are added, the calculator advances the balance period by period and adds each deposit at the correct interval, handling any combination of compounding and contribution frequencies accurately.
€5,000 initial deposit · 4.5% annual rate · €100/month contribution · 20 years · monthly compounding
Final balance
€41,300
after 20 years
Total contributed
€29,000
€5k + 240 × €100
Interest earned
€12,300
pure growth, added for free
At the end of 20 years, roughly 30% of the final balance comes from interest alone, money that was never deposited. The monthly €100 contribution adds up to €24,000 over two decades; compound growth turns that discipline into an additional €12,300 on top.
4.5% is a realistic rate for a diversified bond-equity ETF portfolio available to EU retail investors in 2024. It is not guaranteed (past performance does not imply future returns), but it serves as a reasonable reference point for conservative long-term planning.
This example works forward: a starting balance and a contribution, growing toward whatever final amount they reach. Working backward from a specific target instead, the savings goal calculator solves for the monthly contribution, rate, or time needed to reach it.
Compounding frequency determines how often interest is added to your balance and starts earning interest itself. Monthly compounding (12 times per year) beats annual compounding (once per year) because each month's interest starts working sooner.
On a €10,000 deposit at 5% for 10 years, the difference between annual and monthly compounding is about €130. Real, but not the main story. The difference between investing for 10 years versus 20 years at the same rate is closer to €6,500.
The gap between annual and daily compounding is even smaller, roughly €10 on that same deposit. Frequency has diminishing returns; time has compounding ones.
€10,000 at 5% for 10 years
Three ideas that sound right but mislead most people:
“My wealth has come from a combination of living in America, some lucky genes, and compound interest.”
“I need a large amount to start.”
False, or at least the wrong priority. Time in the market has far more impact than the starting balance. €1,000 invested at age 25 at 6% grows to about €10,286 by age 65. The same €1,000 invested at 35 grows to only €5,743. Those ten missing years cost more than the original principal.
Starting small and starting early beats starting large and starting late.
“A higher rate is always better.”
Partially true, but fees and taxes erode it. A 7% gross return with 1.5% annual management fees and 30% withholding tax on gains leaves closer to 3.7% real growth. A 5% product with 0.2% fees and the same tax treatment ends up producing a higher net balance over 20 years. The rate printed on the label is rarely the rate that matters.
Net rate after costs is what compounds, not the headline figure.
“It works the same in reverse.”
It does, but the rates are asymmetric. Savings products across the EU typically offer 2–5%. Consumer credit and revolving credit lines typically charge 10–20%. Debt compounds just as aggressively as savings, and usually at a higher rate. A €3,000 credit card balance at 18% that you make only minimum payments on can take over a decade to clear, paying more than double in total.
Compound interest is equally powerful as a force against you.
No. The result is a nominal figure, growth calculated purely from the rate, contributions, and time you enter, with no adjustment for rising prices. A nominal balance of €41,300 in 20 years will not buy as much as €41,300 buys today, since prices generally rise over that same period. To compare purchasing power across different time horizons, subtract an assumed inflation rate from the annual rate before entering it; the result is then a rough real (inflation-adjusted) figure instead of a nominal one, useful for judging what the balance could actually buy rather than its face value.
No. The formula compounds whatever rate you enter without deducting management fees, platform charges, or tax on gains. Those costs vary widely by country, account type, and product, a low-fee index fund and an actively managed fund with the same headline return can end up with very different real balances after twenty years once fees and tax are subtracted. Enter a net-of-fees, after-tax rate if you want the result to reflect what you would actually keep, rather than the fund's advertised gross return.
The calculator assumes a fixed contribution at a fixed interval for the entire duration. If your actual contributions grow or shrink over time (a raise, a career break, an increased deposit later on), the real balance will differ from this single estimate, since more money contributed earlier has more time to compound than the same amount contributed later. Re-run the calculator with an average contribution figure, or break the timeline into separate segments (each with its own steady contribution) and combine the results, to approximate a changing contribution schedule.
Not by much. Moving from annual to monthly, or monthly to daily, compounding has a real but modest effect, typically a small fraction of the final balance on realistic deposits, since each step simply adds interest sooner rather than changing the underlying rate. Choosing a longer time horizon or a meaningfully higher rate has a far larger effect than optimizing compounding frequency alone, so it is rarely worth switching products, or paying a higher fee, purely to chase a more frequent compounding schedule.