Compound interest calculator
Watch money grow on itself. Start with a lump sum, add a regular deposit, pick a rate, and see the future value split into what you put in versus what the interest earned. The year-by-year chart makes the snowball effect impossible to miss.
Your plan
Growth over time
Year-by-year breakdown
| Year | Contributions | Interest | Balance |
|---|
How to use it
Set your starting point
Enter a lump sum you already have, or leave it at zero if you're building from scratch. Then add how much you plan to deposit and how often.
Choose a rate and horizon
Pick a realistic annual return and the number of years you'll stay invested. Longer horizons are where compounding really takes over.
See interest do the work
The result separates your own money from the interest it earns. Watch the green share of the chart grow as the years stack up.
Why compounding is worth understanding
Interest on interest
Simple interest pays only on your original deposit. Compound interest pays on your deposit and on every bit of interest already earned, so the balance grows faster and faster the longer it runs.
Time beats timing
Starting a few years earlier often outperforms contributing more later, because the earliest dollars compound the longest. This calculator makes that head start visible in the year-by-year table.
Numbers stay on your device
Your savings figures never touch a server. There's no login, no cookie wall and no tracking of what you type — the math happens entirely in your browser.
The compound interest formula
For a single lump sum, the future value is A = P(1 + r/n)^(nt), where P is the starting amount, r is the annual rate as a decimal, n is how many times a year interest is added, and t is the number of years. The (nt) exponent is the whole story: each compounding period multiplies the balance again, so the growth curves upward rather than rising in a straight line.
Adding regular contributions
Most people don't just deposit once — they add money every month or year. Each contribution then compounds for the time remaining until the end. This calculator sums the growth of your starting amount and the growth of every contribution, which is why the interest column in the table accelerates even when your deposit stays the same.
How often it compounds
More frequent compounding earns slightly more, because interest starts earning its own interest sooner. Moving from annual to monthly compounding at the same rate adds a small but real amount over long periods. Daily compounding adds a little more still, though the gap narrows quickly.
Choosing a realistic rate
The rate you enter should reflect the account or investment you have in mind: a savings account might return 1–4%, a diversified stock portfolio has historically averaged around 7% after inflation over long periods, though returns vary and are never guaranteed. Try a conservative and an optimistic rate to see the range of outcomes.
Frequently asked questions
What is compound interest?
Compound interest is interest calculated on both your original amount and on the interest already added. Because each period's interest earns interest of its own, savings grow faster over time than they would with simple interest.
How are contributions handled?
Each regular deposit is added on schedule and then compounds for the rest of the term. You can set contributions to monthly or yearly; the calculator grows every one of them to the end date and adds it to the total.
Does compounding frequency really matter?
It has a modest effect. At the same annual rate, monthly compounding earns a bit more than annual, and daily a bit more than monthly. The rate and the length of time you stay invested matter far more.
Is the return guaranteed?
No. This is a projection based on the fixed rate you enter. Real investments fluctuate, and the tool doesn't account for taxes, fees or inflation. Use it to compare scenarios, not as a promise of future results.
Can I model retirement or a savings goal?
Yes. Set your current savings as the starting amount, your planned monthly deposit as the contribution, and the years until your goal. The future balance tells you where that plan lands.
Do you store what I enter?
Never. Every figure stays in your browser. There is no account, no server call and nothing saved after you close the tab.