How compound interest actually works
Compound interest is interest that earns interest. You put money in, it earns a return, and then that return starts earning its own return. The effect is small at first and then, given enough time, almost startling. It is the quiet engine behind most long-term savings and investing.
The calculator above shows it in action. Enter a starting amount, what you add each month, a rate, and a number of years, and it plots the whole journey — including the moment the interest starts outpacing what you put in yourself.
The formula, briefly
For a lump sum with no monthly additions, the classic formula is:
A = final amount | P = starting amount | r = annual rate (as a decimal)
n = times compounded per year | t = number of years
Once you add regular monthly contributions the maths gets busier, which is exactly why a calculator helps. The tool above handles the contributions for you and shows the year-by-year balance.
Why time matters more than the amount
The single biggest lever in compounding is time, not the size of your deposits. Money invested early has more years to multiply, and those extra years tend to matter more than a bigger contribution made later. Someone who starts modestly in their twenties often ends up ahead of someone who saves much more but begins in their forties.
This is the case for starting now with whatever you can rather than waiting for the "right" amount. Use the Goal tab to see how the required monthly figure drops sharply when you give yourself more years.
What changes the outcome
The rate
A higher rate compounds faster. The Compare tab shows how even two or three points of difference balloon over decades.
How often it compounds
Daily compounding edges out annual, though the gap is smaller than people expect. Time and rate matter far more.
Leaving it alone
Every withdrawal resets the snowball. Compounding rewards patience more than cleverness.
Regular contributions
Steady monthly deposits stack with the growth, and over long periods they make up a surprisingly small share of the final figure.
The same force in reverse
Compounding is not only a friend. Credit cards and some loans compound against you, which is why an unpaid balance can feel like it grows faster than you can pay it. If you are carrying high-interest debt, clearing it is often the best "investment" available, because you are guaranteed to save that rate. Our loan calculator and mortgage calculator show that side of the coin.
The Rule of 72 and the cost of waiting ten years
There is a mental shortcut investors have used for decades: divide 72 by your annual rate and you get roughly how many years it takes money to double. At 6%, that is 12 years, and the maths backs it up: €5,000 left alone at 6% a year grows to about €10,061 after 12 years, just past double. At 8%, doubling takes about 9 years; at 4%, about 18. The rule is not exact, but it is close enough to compare two rates in your head before touching a calculator.
The same maths shows why starting early beats saving more later. Put €200 a month into an account earning 7% a year, starting at 25, and by 65 you would have roughly €525,000. Start the same €200 a month at 35 instead, and by 65 you would have roughly €244,000, less than half, even though you only skipped ten of forty years. The missing decade is not just ten years of contributions; it is ten years of compounding on everything that came after.
These figures assume a steady 7% return, monthly contributions, and no taxes or fees. Real investments bounce around, charge fees, and get taxed, so treat the numbers as a demonstration of the force, not a promise. What the demonstration shows is the point most investors agree on: time in the market is the input you cannot buy back later, and it is worth more than a slightly higher rate.
Frequently asked questions
What is the difference between simple and compound interest?
Simple interest is paid only on your original amount, so it grows in a straight line. Compound interest is paid on your amount plus all the interest so far, so it accelerates over time.
How often should interest compound?
More frequent compounding gives a slightly higher result, but the difference between monthly and daily is small. Do not lose sleep over it — focus on the rate and the time horizon.
Is a higher rate always better?
For savings, yes. But higher returns usually come with more risk, so the rate you can realistically expect depends on where the money sits. This tool shows the maths; it is not investment advice.
Can I include monthly contributions?
Yes. Enter a monthly amount and the calculator adds it every month and compounds the growing balance, which is how most real savings plans work.
Is this calculator free?
Completely free, no signup, nothing stored. Use it as much as you like.