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Compound Interest Calculator
Explore an initial investment and monthly contributions under a constant return assumption.
Your inputs
Starting values are examples. Edit them to explore your own scenario.
Use one currency for all amounts. Changing this label does not convert numbers or apply any country’s tax rules. Amounts may include up to two decimal places.
Starting balance before investment returns.
Added at the end of each month.
An editable constant assumption, not a forecast or guaranteed return.
Whole years, with monthly compounding.
Illustrated future balance: USD 50,969.84
Your current scenario
Recalculated from the inputs above. Rounded display amounts may not reconcile exactly except where a rounding split is explicitly shown.
- Illustrated future balance
- USD 50,969.84
- Total amount contributed
- USD 34,000.00
- Gain or loss under the assumption
- USD 16,969.84
- A negative gain means the constant return assumption reduces the contributed balance. All amounts are before taxes and fees.
How this calculation works
Future value = P × (1 + r)^n + C × ((1 + r)^n − 1) ÷ r. Here r = annual assumption ÷ 1,200; n = years × 12. At 0%: P + C × n.
Assumptions and limits
- End-of-month contributions and a constant nominal return compounded monthly.
- Taxes, fees and inflation are excluded. Negative returns are allowed; actual investment paths fluctuate.
Worked example
Example: 1,000 initially plus 100 each month for one year at 0% becomes 2,200, all from contributions.
Related educational tool: Investor.gov’s compound interest calculator. Check contribution timing and compounding frequency when comparing results.
Questions about this tool
Is the assumed return a prediction?
No. Change it to compare scenarios, including negative returns. The model does not estimate market probabilities or investment suitability.
Does changing the currency convert my amounts?
No. Currency selects the unit and rounding used to display the numbers you enter. Enter every amount in that same currency. No exchange rate, local tax rule or benefit entitlement is inferred.
Check the next part of your plan
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- Financial Independence Target Calculator — Relate an investment target to spending and an explicit withdrawal-rate assumption.
- Loan Payment Calculator — Compare a fixed debt payment with the money available to invest, without assuming investment returns will cover it.