Free compound interest calculator for comparing initial investment, regular contributions, return assumptions, goal amounts, and CSV-ready growth tables.
- Calculations run in the browser and show principal, interest earned, total balance, and progress toward a target amount.
- The tool is educational and does not predict market returns, taxes, brokerage costs, inflation, or personal investment suitability.
- Use the detailed guides to compare monthly contribution, goal amount, and return-rate scenarios with the same assumptions used by the calculator.
What this compound interest calculator does
This calculator projects how a balance grows when interest is reinvested instead of withdrawn. You enter a starting amount, a recurring contribution, an expected annual return, and a time horizon, and the tool builds a period-by-period schedule showing principal, interest earned, and the running total. Every figure is computed in your browser, so nothing you type is sent to a server or stored in an account.
The output separates the money you put in from the money the investment generates. That distinction is the whole point of compounding: in the early years almost all of the balance is your own contributions, but as the interest base grows, the share produced by returns gradually overtakes the share you deposited. Watching that crossover happen on the chart is the clearest way to understand why time in the market matters more than precise timing.
Why compounding rewards patience
Simple interest pays a return only on your original deposit. Compound interest pays a return on your deposit plus all the interest already credited, so each period starts from a slightly larger base. The effect looks modest over a year or two and dramatic over decades. A single $10,000 deposit growing at 7% a year becomes about $19,700 after 10 years, roughly $38,700 after 20 years, and about $76,100 after 30 years. The balance does not merely add a fixed amount each decade; the gains themselves keep getting larger.
Regular contributions accelerate this further because every new deposit also begins compounding. Someone who invests $300 a month at a 7% annual return contributes $108,000 of their own money over 30 years but ends with roughly $352,000. The extra quarter-million dollars is interest earning interest. Use the calculator to test how a higher monthly amount or a few additional years changes that gap.
How to read your results
The summary cards show the final balance, the total you contributed, and the total interest earned. The chart plots the balance over time, with the lower band representing contributed principal and the upper area representing accumulated growth. The breakdown table lists each period so you can see exactly when interest begins to outpace deposits, and the CSV export lets you keep that schedule for your own records.
Treat the result as a planning scenario, not a forecast. Real markets do not deliver a smooth fixed percentage every year; returns arrive unevenly, and taxes, fees, and inflation all reduce what you actually keep. The value of the tool is in comparing options side by side under the same assumptions, not in predicting a specific future number.
Frequently asked questions
Is this compound interest calculator free?
Yes. The calculator is completely free and runs entirely in your browser. There is no sign-up, no paywall, and no limit on how many scenarios you can run.
Does the calculator store the numbers I enter?
No. All calculations happen locally on your device. Your inputs are used only to draw the chart and table on screen and are never transmitted to or saved on a server.
What return rate should I assume?
There is no single correct figure. Many long-term investors model a diversified portfolio somewhere between 5% and 8% before inflation, then stress-test with lower numbers. Because real returns vary year to year, it is wise to compare a conservative case against an optimistic one rather than relying on a single rate.
Does the projection account for taxes and inflation?
No. The figures are nominal and pre-tax. To approximate purchasing power, subtract an estimated inflation rate from your return assumption, and remember that taxes and fees will further reduce real-world results.
How often is interest compounded here?
You choose the period. Daily, monthly, and yearly compounding all produce slightly different results because more frequent compounding credits interest sooner. The differences are small at typical rates but grow over long horizons.