Compound Interest Calculator (with Monthly Contributions)
See how an initial deposit + steady monthly contributions grow over time with compound interest.
Result
- Initial deposit$10,000
- Total contributions (500 × 12 × 20)$120,000
- Total interest earned$170,850.72
- Final balance$300,850.72
Step-by-step
- Convert annual rate to decimal: 7% ÷ 100 = 0.07.
- Compounding periods per year (monthly): n = 12.
- Future value of initial deposit: $10,000 × (1 + 0.005833)^240 = $40,387.39.
- Future value of monthly contributions added on top: $260,463.33.
- Total final balance: $300,850.72 — of which $130,000 you put in, $170,850.72 is interest the money earned.
How to use this calculator
- Enter your initial deposit (existing savings going in).
- Set your monthly contribution (what you can realistically add each month).
- Pick an annual rate — 7% is a reasonable long-run S&P 500 estimate after inflation; high-yield savings sit around 4-5%.
- Pick the number of years and your compound frequency (monthly is the most common in practice).
About this calculator
Compound interest is interest earned on both your original deposit and on the interest already accrued — money making money on its own money. The longer you let it run, the more dramatic the curve. Adding monthly contributions on top accelerates this further: every dollar you add gets its own runway to compound, so a 30-year-old depositing $500/month for 35 years can end up with more than someone depositing $1,000/month for 15 years starting at 50, even though the lifetime total contributed is the same. The Rule of 72 estimates how long it takes money to double: divide 72 by your annual rate. At 7% annual return, your balance roughly doubles every 10 years. Compound frequency matters less than people think — the difference between annual and daily compounding at 7% over 20 years is under 1%.