Compound Interest Calculator
Free compound interest calculator: see how your savings grow with monthly contributions, any compounding frequency, and a year-by-year table.
- Free
- No sign-up
- Runs in your browser
- Updates as you type
What you're putting in today
Added every month. Set to 0 for none.
Your expected yearly rate, before compounding
How long the money stays invested
How often interest is added to the balance
Future value
- Starting amount$0
- Total contributions$0
- Interest earned$0
The gap between the two lines is compound interest. Hover or tap the chart for any year's figures.
An estimate from your inputs. Real accounts round to the cent and post interest on their own schedules.
| Year | Paid in | Interest that year | Balance |
|---|
How this calculator works
Compound interest is interest earned on your starting amount and on the interest already added to it. The calculator applies the standard formula, then layers your contributions on top period by period.
- P
- your starting amount
- r
- the annual rate as a decimal
- n
- how many times per year interest compounds
- t
- the number of years
Contributions are added each month and start compounding from the period they arrive.
Results update as you type. There is no Calculate button because compounding questions are rarely one-shot: the useful part is nudging the rate or the years and watching what changes. The year table below the calculator is the same computation written out the long way, so you can check any year's math yourself.
Every projection here is an estimate built on the rate you enter. Real accounts round differently, post interest on their own schedules, and investment returns vary year to year. The S&P 500 preset uses 10% because that is the index's widely cited long-run average annual return (about 10% since 1957, per Fidelity, with dividends reinvested), not because any future return is promised. After inflation, that long-run figure is closer to 6% to 7% (per SmartAsset and SoFi). More on who builds and checks these tools is on the about page.
This calculator was built by Cedrick Reese. The formula and its worked results are checked against the U.S. Securities and Exchange Commission's compound interest calculator at investor.gov, and the year table is generated by the same code that produces the headline figure, so the two can never disagree. Rate context cited on this page comes from the sources linked beside each claim and is reviewed on a maintenance cycle. Last reviewed: October 30, 2026.
Frequently asked questions
How much will $10,000 grow to in 20 years?
It depends entirely on the rate and compounding schedule. At 5% compounded monthly with no additions, $10,000 becomes about $27,126 in 20 years. At 7% it is about $40,387. Enter your own numbers above and the calculator shows the exact path year by year.
Can I become a millionaire by investing $1,000 a month?
The math says yes, with enough time and a high enough return. At a 7% annual return compounded monthly, $1,000 a month reaches $1 million in about 28 years the way this calculator counts it, with each contribution added at the start of its month. At 10%, about 22 years. These are estimates built on a steady assumed return, which real investments do not deliver in a straight line. See our monthly savings grid for the same math at other amounts, rates, and time horizons.
What is the 8-4-3 rule of compound interest?
An illustration of how compounding accelerates: at roughly a 12% annual return, growth in years 9 through 12 roughly matches the first 8 years, and years 13 through 15 roughly match that again. See the full breakdown, checked against real numbers, in our dedicated 8-4-3 rule article.
Does this work for a savings account?
Yes. Enter your account's rate (banks quote APY, which already includes compounding) and pick the compounding frequency your bank uses. Many savings accounts compound daily and post the interest monthly. For a daily-specific version, use our daily compound interest calculator. For how the two numbers differ, see APY vs interest rate, and for what taxes do to the result, see taxes on savings interest.
All calculators on this site
- Compound Interest CalculatorThe main tool: growth with contributions at any compounding frequency.
- Daily Compound Interest CalculatorLocked to daily compounding, with a daily-vs-monthly comparison.
- Compound Interest Calculator With WithdrawalsGrowth while taking regular money out, with depletion detection.
- Simple Interest CalculatorThe I = Prt version, with a live comparison against compounding.
- Compound Interest Formula, ExplainedThe math behind every tool here, with worked examples.