Compound Interest Calculator
See what your money does when the interest earns interest too. Enter an initial amount, a monthly contribution and an expected return, and watch the balance grow year by year. No login, no data leaves your browser.
Contributions are added at the end of each month. Interest is applied at each compounding period on the balance at that point — so with monthly compounding it is added every month, with annual compounding once a year. The return you enter is a guess, not a promise: real returns vary every year.
- Initial investment$0
- Monthly contributions$0
- Total you put in$0
- Interest earned$0
Balance over time
*Without monthly contributions, the same $10,000 initial investment at the same return would grow to the figure shown — the difference is what regular saving adds.
Year-by-year growth table
| Year | Contributions | Interest earned | Balance at year end |
|---|
Interest is shown as earned during that year. The last years earn far more interest than the first, because the balance they are calculated on is much larger — that is compounding doing the heavy lifting.
Worked example
Start with $10,000, add $200 a month, and earn 7% a year compounded monthly for 25 years. You put in $10,000 + $200 × 300 months = $70,000 in total. The balance grows to $219,268.52, so $149,268.52 of it is interest — more than double what you contributed. Without the monthly contributions, the same $10,000 would only reach $57,254.18. After 10 years the balance is $54,713.58; most of the growth happens in the last decade, when the balance is large enough that 7% a year is a big number in dollars.
How it works
With monthly compounding, the future value is A = P(1 + r/12)12t + PMT × (((1 + r/12)12t − 1) / (r/12)), where P is the initial investment, PMT is the monthly contribution, r is the annual return and t is the number of years. The first term is your starting money growing on its own; the second is every monthly contribution growing from the month it went in. Contributions are assumed to be added at the end of each month. With quarterly or annual compounding, the same idea applies with fewer, larger interest steps — the calculator simulates it month by month so the table and chart match the headline number exactly.
Common mistakes
Using a return that is too optimistic — 7% is roughly the long-run average for a broad stock market index after inflation is ignored, not a guarantee, and some years are negative. Forgetting inflation: $219,269 in 25 years buys less than $219,269 today. And stopping early: the last 10 years of a 25-year plan add more than the first 15 combined, so time in the market matters more than timing it.
Questions people actually ask
What is compound interest?
Interest earned on both your original money and the interest you have already earned. In year one you earn interest on $10,000; in year 25 you earn interest on more than $200,000, so the same percentage produces a much larger dollar amount.
What return should I assume?
For a broad stock index fund, many planners use 6–8% a year before inflation as a long-run planning figure, and 4–5% after inflation. A savings account might pay 1–4%. Use a lower number if you want a cautious plan, and remember no return is guaranteed.
Does monthly compounding beat annual compounding?
Barely. At 7% over 25 years, $10,000 plus $200 a month reaches $219,268.52 compounded monthly versus $215,812.37 compounded annually — the difference comes from interest being added (and then earning interest) sooner. The contribution habit matters far more than the compounding frequency.
Are taxes included?
No. Taxes on interest, dividends and capital gains depend on your country and account type — for example, a TFSA in Canada or an ISA in the UK can shelter the growth — so the figure here is before tax.
What if I stop contributing?
Set the monthly contribution to $0 and the calculator shows the initial investment growing alone. That is the “without monthly contributions” figure in the results: $57,254.18 for the default example.
Can I trust this number for a real investment?
For planning, yes — the maths is exact for a fixed return. For a real investment, no: returns vary year to year, fees reduce them, and inflation reduces what the final amount buys.
Estimates only — not financial advice
Results are estimates for a fixed annual return with the inputs you entered, assuming the return never changes and no fees or taxes apply. LootLoaf does not collect or store anything you type — every calculation happens in your browser. Formula and figures reviewed 10 November 2026.