Compound Interest Calculator
Enter a starting amount, a monthly contribution and an annual rate to project your balance year by year with yearly, quarterly or monthly compounding.
How the projection works
Each month the contribution is added to the balance, and at the end of every compounding period the balance grows by the periodic rate — the annual rate divided by the number of periods per year.
This is a simplified model: deposits and the rate are assumed constant, and fees, taxes and inflation are not included.
Yearly growth
| Year | Contributions so far | Balance at year end |
|---|
How to use
- Enter the starting principal, your monthly contribution and the annual interest rate as a percentage.
- Choose how many years to project and how often interest compounds — yearly, quarterly or monthly.
- Click Calculate to see the final balance, total contributions, interest earned and the year-by-year growth table.
Frequently asked questions
How does compounding frequency change the result?
The more often interest compounds, the more each period’s interest starts earning interest of its own, so the final balance grows slightly. At the same annual rate, monthly compounding beats quarterly, which beats yearly — and the gap widens with higher rates and longer periods. The difference is modest at typical savings rates, but the calculator lets you compare all three options directly.
What model does the yearly table use?
The calculation runs month by month: your contribution is added at the end of each month, and at the end of every compounding period the whole balance grows by the periodic rate — the annual rate divided by the number of periods per year. With monthly compounding this is an exact month-by-month simulation; with quarterly or yearly compounding, interest is applied only at those period ends.
Does it include inflation, fees or taxes?
No. The projection uses the nominal rate you enter and applies no fees, taxes or inflation. Real returns are lower once account fees, withholding tax on interest and the erosion of purchasing power are counted. A practical habit is to enter a rate a little below the advertised one, or to interpret the final balance in today’s money for long horizons.
What interest rate should I enter?
Enter the nominal annual rate your bank, fund or bond actually pays, not a marketing figure that assumes bonuses. For savings accounts that is usually a small percentage; for long-term stock index investing, people often use historical averages around 7 percent before inflation, but past returns do not guarantee future ones. Be conservative with horizons that really matter, such as retirement planning.
Are these results financial advice?
No. The tool is a deterministic calculator: it applies the rate and schedule you enter and nothing more. Real investing involves variable returns, missed contributions, fees, taxes and the risk of loss, none of which this simple model captures. Treat every figure as a mathematical estimate that supports your planning, not as financial advice, and consult a licensed adviser before investment decisions.