Quick compound interest calculator
A compact monthly-savings tool: enter a starting balance, month-end contribution, hypothetical annual rate and number of years. Compare the projected balance with what you put in; the rate is an assumption, not a forecast.
How it works
- P is your starting balance (principal).
- PMT is your monthly contribution.
- r is the annual return, expressed as a decimal (7% = 0.07).
- n is the number of compounding periods per year (we use 12 for monthly).
- t is the number of years.
- The first term compounds the starting balance; the second compounds the monthly contributions.
- Contributions are modeled at month-end. Taxes, fees, inflation and variable market returns are excluded.
- Use a nominal annual rate divided by 12, not an APY without conversion; annual yield already includes compounding.
Worked examples
- Hypothetical: $200 at each month-end, a zero starting balance, and a constant 7% nominal annual rate for 40 years gives about $524,963. Contributions total $96,000.
- Hypothetical: $10,000 with no added deposits at a constant 5% nominal annual rate for 20 years gives about $27,126 with monthly compounding.
- These are invented constant-rate scenarios, not actual investment results. Use the detailed guide for a year-by-year schedule and manual calculation.
Frequently asked questions
What rate should I use?
Use several hypothetical rates, including a lower-return case. This tool does not predict stock returns. For a deposit account, check whether the quoted rate is nominal interest or APY, which already includes compounding.
What is the Rule of 72?
A quick shortcut: divide 72 by the annual return to estimate how many years it takes for money to double. At 8%, roughly 9 years.
How is this different from the detailed compound interest guide?
This page is a compact tool showing future value, contributions and growth. The detailed guide explains the formula step by step and includes a year-by-year schedule. Both use month-end contributions and monthly compounding.