Compound interest calculator
Project savings with a starting balance, monthly contributions and monthly compound interest.
Use the selected decimal separator and no thousands separators. Amounts: 0–1e12. Interest: 0–100%. Duration: 1–1200 whole months. Results round to two decimal places; calculations retain full floating-point precision.
Illustrative calculations, not personal financial advice. Constant user-entered rates; no taxes, fees, market data or guarantees.
Calculated on your device. Financial entries are not uploaded or saved.
Calculation schedule
Table values are rounded for display. CSV contains every row; displayed rounded rows may not sum exactly.
Formula and worked example
Monthly rate r = annual nominal rate ÷ 12 ÷ 100. Each month earns interest and adds the contribution. With 1000 initially, 100 at each month end, 12% annual nominal interest and 12 months, the balance is 2395.08; contributions total 2200.
Questions and assumptions
How are contributions and compounding modeled?
Compounding is monthly. A beginning-of-month contribution earns that month’s interest; an end-of-month contribution starts earning next month. The schedule shows each year end and any final partial year. The rate stays constant and is an assumption, not a forecast. APY/effective annual rates are not the same as the nominal annual input.