Compound Interest Calculator
Project how a lump sum and regular contributions grow over time — and see how much of the final figure is interest rather than your own money.
Runs entirely in your browser. Nothing you type here is uploaded, stored or logged.
Why time matters more than the amount
Compound growth is exponential, which means the last decade contributes more than the first two combined. Someone who saves 200 a month from 25 to 35 and then stops usually ends up ahead of someone who saves the same amount from 35 to 65 — despite paying in a third as much. The ten extra years of compounding do the work.
This is the single most useful thing to know about saving in your twenties, and the single most frustrating thing to learn in your fifties. Adjust the years field and watch what happens.
The rule of 72
Divide 72 by your annual return to estimate how many years it takes to double your money. At 6% it's twelve years; at 9%, eight. It's a rough approximation, but it's accurate enough between 4% and 12% to do in your head, and it makes the cost of a 1% platform fee immediately obvious.
Nominal versus real returns
A projection that ends at 750,000 in forty years is quoting a number in future money, which will buy considerably less than the same number does today. At 2.5% inflation, prices roughly double every 28 years. Tick the inflation adjustment to see the purchasing power rather than the headline.
What this projection assumes — and where reality diverges
- A constant return. Real markets deliver +20% and −30% years. The average may hold over decades, but the path is not a smooth curve, and the order of good and bad years matters if you're withdrawing.
- No fees. A 0.5% annual platform charge on a 7% return is over 7% of your growth, every year, compounded.
- No tax. Depending on the account and country, gains may be taxed on the way out.
- Contributions never rise. In practice most people increase them with income, which improves the outcome.
This is an arithmetic projection, not financial advice or a prediction. Past returns do not guarantee future ones, and investments can lose value.
Frequently asked questions
What is compound interest, in one sentence?
Interest that earns interest — each period's return is added to the balance, so the next period earns on a larger amount. Simple interest pays only on your original deposit; compound interest pays on everything that has accumulated.
How much difference does compounding frequency make?
Less than most people expect. On 10,000 at 7% for 10 years, annual compounding gives about 19,672 and monthly gives about 20,097 — a 2% difference. The rate and the time horizon matter far more than whether interest lands monthly or yearly.
Should I account for inflation?
Yes, if you want the answer in today's money. Tick “adjust for inflation” and the projection is shown in real terms. A pot of 500,000 in 30 years buys roughly what 275,000 buys today at 2% inflation — the nominal number flatters itself.
Is 7% a realistic return?
It is roughly the long-run average of a broad global stock index after inflation-adjusted history, but it is an average across decades that included severe crashes. Savings accounts pay far less; individual investments can lose money. Use a range rather than a single hopeful figure.
Does this account for tax or fees?
No. Investment platform fees of 0.5% a year and tax on gains both bite meaningfully over decades. Subtract your expected fee percentage from the return rate for a more honest projection.