Compound Interest Calculator
Forecast the exponential growth of your investments and savings over time with initial capital, monthly deposits, and compound interest.
Investment Parameters
1.6x Growthof total capital deposited.
Compound Interest with Regular Deposits Formula
A = P(1 + r/n)^(nt) + PMT ร [ ((1 + r/n)^(nt) โ 1) / (r/n) ]- A: Final future balance of the investment portfolio.
- P (Principal): Starting initial deposit amount.
- r: Annual nominal interest or investment return rate (as a decimal, e.g. 8% = 0.08).
- n: Number of times interest is compounded per year (12 for monthly compounding).
- t: Number of years the money remains invested.
- PMT: Monthly recurring contribution deposited at each compounding period.
Step-by-Step Worked Example
Calculation Steps:
- Total Principal Deposited = $10,000 + ($500 ร 120) = $70,000
- Future Value of Initial Deposit = $10,000 ร (1 + 0.08/12)^120 = $22,196.40
- Future Value of Monthly Deposits = $500 ร [ ((1 + 0.08/12)^120 โ 1) / (0.08/12) ] = $91,473.02
- Total Portfolio Balance = $22,196.40 + $91,473.02 = $113,669.42
- Total Compound Interest Earned = $113,669.42 โ $70,000 = $43,669.42
How to Use This Calculator
- 1Enter your initial starting investment or current savings balance.
- 2Input the monthly recurring amount you plan to deposit.
- 3Set your anticipated average annual return rate (% per year).
- 4Adjust the investment horizon slider to see your portfolio value compound over time.
Assumptions & Limitations
- Assumes regular contributions are deposited at the start or end of each monthly period.
- Assumes a constant annualized rate of return, whereas market returns fluctuate year to year.
- Does not account for income taxes or inflation unless an inflation-adjusted return rate is entered.
Frequently Asked Questions
What is compound interest?
Compound interest is interest earned on top of previously accumulated interest. Over long periods, it creates exponential wealth growth because your returns generate their own returns.
What is a realistic annual return rate to use?
Historically, the US S&P 500 index has returned approximately 10% annually before inflation (about 7% to 8% inflation-adjusted). For conservative bond portfolios, 4% to 5% is standard.
Why does time matter more than contribution size in compounding?
Because compound interest is exponential (t is in the exponent), money invested early has more doubling cycles. An investor who starts 10 years earlier can contribute less money yet end up with significantly more wealth.
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