Compound Interest Calculator
Estimate how a starting balance and monthly contributions could grow when interest is compounded monthly. Enter the initial amount, annual rate, time horizon, and regular monthly contribution to see future balance, total money contributed, and estimated interest. The result is a mathematical projection, not a promise of investment performance. It excludes taxes, fees, inflation, changing rates, withdrawals, and irregular deposits, and it runs privately in this browser.
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Add to Chrome — freeWhat it does
- Monthly compounding
- Monthly deposits
- Contributions versus interest
- Local formula
How to use Compound Interest Calculator
- 1
Set the starting balance
Enter the amount already saved or invested before the projection begins.
- 2
Add rate and time
Use an annual percentage rate and a number of years; the page compounds the rate monthly.
- 3
Add recurring deposits
Enter a monthly contribution to separate money you put in from interest produced by the formula.
How it works
Reading the projection
The result separates money deposited from the interest attributed to the assumed rate. Try a zero-contribution case, a lower rate, and a shorter horizon before drawing a conclusion. Real accounts may compound daily, change rates, charge fees, withhold tax, or lose value. Preserve the starting balance, contribution, rate, frequency, and term beside any number copied into a plan or spreadsheet so a future comparison can see the assumptions rather than only the attractive final balance.
The starting balance uses monthly growth (1 + annualRate / 12) ^ months. Monthly
contributions use the corresponding annuity factor, so the tool can report total money
contributed separately from the difference attributed to the constant rate. A zero-rate
case is handled directly instead of dividing by zero. Values are formatted as currency
for readability while the calculation retains the available numeric precision.
Use scenarios, not certainty
Run the calculation with conservative and optimistic rates, then compare the output with fees and inflation. A forecast is most useful when it makes assumptions visible: monthly deposits may stop, rates may change, and a balance may lose value. For regulated financial decisions, use the institution's disclosure and professional advice.
Assumptions worth recording
Scenario checklist
Save the starting balance, monthly contribution, annual rate, compounding frequency, and time horizon with the displayed result. Run more than one rate so the output shows a range instead of one attractive point estimate. Then compare the projection with fees, tax, inflation, withdrawal plans, and the account's actual terms before using it in a financial decision.
The projection assumes deposits happen every month and that the rate compounds monthly. Some accounts compound daily, credit interest on a different schedule, or pay no fixed return at all. The display also ignores inflation, which means a future dollar may buy less than a current dollar. Write the rate, deposit timing, and fee assumptions beside any scenario you save so a future comparison remains meaningful.
The contribution field is not a withdrawal or a one-time deposit. It assumes the same amount is added each month for the whole period, which is useful for a clean illustration but unlikely to match every real savings habit. Change the contribution to zero to see the starting balance grow on its own, then compare that scenario with regular saving.
Examples
Saving with monthly deposits
The exact display depends on the unrounded formula result and assumes deposits at the monthly period boundary.
Zero-interest comparison
This case makes the difference between contributed money and interest easy to audit.
Frequently asked questions
Does this compound interest calculator include monthly deposits?
Yes. The input is treated as a regular monthly contribution and is added through the same monthly compounding schedule. Real accounts may deposit at the beginning or end of a period, change rates, charge fees, or apply tax, so compare the simple projection with the account's actual terms before making a decision.
Is the interest rate guaranteed for the whole projection?
No. The calculator holds the annual rate constant so the formula can show a clear scenario. Savings and investment rates can change, and investment returns are not guaranteed. Use several rate scenarios and account for fees, inflation, and tax when a long-term plan depends on the result.
Why is compound interest larger over a longer period?
Earlier interest remains in the balance and can itself earn later interest. The effect grows with time, rate, and contribution schedule. This tool shows that relationship mathematically, but a larger projected number does not remove market, credit, inflation, liquidity, or tax risk from the underlying choice.