Compound Interest Calculator
Forecast wealth accumulation, investment growth, and savings yields with customizable monthly contributions, interest rates, and compounding frequencies.
Total balance after 20 years
62% of final portfolio
Initial + monthly contributions
Capital return multiple
| Year | Starting ($) | Annual Deposits ($) | Interest Earned ($) | Total Interest ($) | Ending Balance ($) |
|---|---|---|---|---|---|
| Year 1 | $10,000 | $6,000 | +$1,096 | $1,096 | $17,096 |
| Year 2 | $17,096 | $6,000 | +$1,685 | $2,782 | $24,782 |
| Year 3 | $24,782 | $6,000 | +$2,323 | $5,105 | $33,105 |
| Year 4 | $33,105 | $6,000 | +$3,014 | $8,119 | $42,119 |
| Year 5 | $42,119 | $6,000 | +$3,762 | $11,882 | $51,882 |
| Year 6 | $51,882 | $6,000 | +$4,573 | $16,454 | $62,454 |
| Year 7 | $62,454 | $6,000 | +$5,450 | $21,905 | $73,905 |
| Year 8 | $73,905 | $6,000 | +$6,401 | $28,305 | $86,305 |
| Year 9 | $86,305 | $6,000 | +$7,430 | $35,735 | $99,735 |
| Year 10 | $99,735 | $6,000 | +$8,544 | $44,279 | $114,279 |
| Year 11 | $114,279 | $6,000 | +$9,752 | $54,031 | $130,031 |
| Year 12 | $130,031 | $6,000 | +$11,059 | $65,090 | $147,090 |
| Year 13 | $147,090 | $6,000 | +$12,475 | $77,565 | $165,565 |
| Year 14 | $165,565 | $6,000 | +$14,008 | $91,573 | $185,573 |
| Year 15 | $185,573 | $6,000 | +$15,669 | $107,242 | $207,242 |
| Year 16 | $207,242 | $6,000 | +$17,467 | $124,709 | $230,709 |
| Year 17 | $230,709 | $6,000 | +$19,415 | $144,124 | $256,124 |
| Year 18 | $256,124 | $6,000 | +$21,525 | $165,649 | $283,649 |
| Year 19 | $283,649 | $6,000 | +$23,809 | $189,458 | $313,458 |
| Year 20 | $313,458 | $6,000 | +$26,283 | $215,742 | $345,742 |
How Compound Interest Drives Long-Term Wealth
Compound interest has famously been called the eighth wonder of the world. Because interest is reinvested into your balance each compounding cycle, your money accelerates over time. In early years, capital growth feels linear; after 10 to 20 years, compound interest begins to outpace your personal contributions, generating substantial passive growth.
Understanding the Variables
- Initial Principal ($): The starting capital you invest today in a brokerage account, mutual fund, or high-yield savings account.
- Monthly Additions ($): Systematic savings contributions made every month, which harnesses dollar-cost averaging.
- Annual Interest / APY (%): The expected annual rate of return, representing stock market gains, bond yields, or savings dividends.
- Compounding Interval: Frequency at which returns are credited to your balance (daily, monthly, quarterly, or annually).
Practical Investment Scenarios
- ✓Retirement & 401(k) / Roth IRA Projections
Forecast how recurring monthly contributions to index funds, ETFs, or IRA accounts will compound over 20, 30, or 40 years.
- ✓High-Yield Savings Account (HYSA) Estimations
Determine how much interest an emergency fund or certificate of deposit (CD) will yield over 1 to 5 years.
- ✓Early Financial Independence (FIRE)
Model aggressive savings rates and simulate how reinvesting dividends accelerates passive investment income.
- ✓College Savings & 529 Plans
Calculate the monthly deposit required from birth to age 18 to fully fund a child's college tuition.
Why Use This Compound Growth Calculator
Flexible Compounding Intervals
Model compounding interest annually, quarterly, monthly, or daily to mirror bank accounts and brokerages.
Monthly Contribution Modeling
Add regular dollar deposits to simulate real-world payroll dollar-cost averaging (DCA).
Detailed Annual Breakdown Table
Inspect every year's starting balance, annual contributions, interest earned, and cumulative growth.
One-Click Summary & CSV Export
Copy your calculation summary or download an itemized spreadsheet compatible with Excel and Google Sheets.
100% Private Client-Side Calculation
Your financial figures remain on your computer; no data is ever uploaded or stored on any server.
Frequently Asked Questions
What is compound interest and how does it work?
Compound interest is interest calculated on both the initial principal and the accumulated interest from previous periods. Unlike simple interest, which only generates earnings on the starting amount, compounding creates exponential growth as your interest earns interest.
What is the compound interest formula with monthly contributions?
For a lump sum without contributions, the formula is A = P(1 + r/n)^(nt), where P is principal, r is the annual nominal interest rate, n is compounding periods per year, and t is time in years. When making regular monthly contributions (PMT), the total future value combines the compound principal plus the future value of an ordinary annuity: FV = P*(1 + r/n)^(nt) + PMT * [((1 + r/n)^(nt) - 1) / (r/n)].
What is the Rule of 72?
The Rule of 72 is a fast shortcut to estimate how many years it takes for your investment to double at a given fixed rate of return. Divide 72 by the annual interest rate (e.g., at 8% interest, 72 / 8 = 9 years to double your initial capital).
How does compounding frequency impact returns?
The more frequently interest compounds (daily vs monthly vs annually), the higher your effective annual yield (APY). While the difference between daily and monthly compounding is subtle, daily compounding yields slightly more interest over long multi-decade horizons.
What average annual return should I expect for index funds?
Historically, the S&P 500 has delivered an average nominal return of approximately 10% per year over long historical stretches (or roughly 7% to 8% when adjusted for inflation). Past performance does not guarantee future results.
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