Compound Interest Calculator
See what time does to money. Enter a starting amount, a monthly contribution and an expected return — watch contributions and compounding separate over the years. Free, private, no signup.
Your plan
Long-run stock market average is ~7-10% before inflation. Be conservative.
Your future balance
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You put in-
Interest earned-
Interest share of final balance-
Growth year by year
■ Your contributions ■ Compound interest
Year-by-year breakdown
| Year | Contributions | Interest | Balance |
|---|
Why compounding is the closest thing to magic in finance
Compound interest means you earn returns on your returns. In year one, a 7% return on $10,000 is $700. By year twenty, that same 7% applies to a balance swollen by two decades of contributions and growth — the interest alone can exceed what you put in.
- Time beats amount. Starting 10 years earlier usually beats doubling your monthly contribution later. The chart above shows interest overtaking contributions in the later years — that is compounding doing the heavy lifting.
- Rate matters, but do not chase it. The difference between 7% and 10% over 30 years is enormous — but so is the risk. Use a conservative rate for planning and treat anything above as a bonus.
- Consistency beats timing. This calculator assumes steady monthly investing. Missing months costs more than picking the perfect day to invest.
Adjusting for inflation
These are nominal figures — they do not subtract inflation. At 3% inflation, money halves in purchasing power roughly every 24 years. For a "today's dollars" view, subtract ~3% from your expected return (e.g. use 4-5% instead of 7-8%).
FAQs
What is a realistic return to assume?
Broad stock market index funds have returned ~7-10% per year over long periods before inflation. For planning, 6-7% nominal (3-4% after inflation) is the prudent range most advisors use.
Are contributions assumed at the start or end of the month?
End of month — slightly conservative. The difference vs start-of-month is small over long horizons.
Does this include taxes and fees?
No. Fund fees (even 1% a year compounds against you) and taxes on gains both reduce the real outcome. Think of this as the gross figure.
Why does interest eventually dwarf my contributions?
Because each year's growth applies to an ever-larger base. Early on, contributions dominate; later, returns on accumulated returns dominate. That is the whole argument for starting early.
Is my data sent anywhere?
No. Everything is computed in your browser; nothing leaves your device.
Disclaimer: Estimates for illustration, computed in your browser — no data leaves your device. Markets do not return a smooth fixed rate; actual results will vary and can be lower. This is not investment advice. Verify with a qualified advisor before investing.