Compound Interest Calculator
See how your money grows with compound interest over time, with a year-by-year balance breakdown.
About the Compound Interest Calculator
Compound interest is often called the most powerful force in personal finance because interest earns interest — every period, growth is calculated on the new, larger balance rather than the original amount. This calculator projects how a lump sum grows at a given rate and compounding frequency, showing the final balance and a year-by-year breakdown of growth.
How It Works
The calculator applies the compound growth formula for your chosen compounding frequency, stepping the balance forward month by month at the equivalent periodic rate. Every year it records the running balance and the interest earned to that point, so you can see how growth accelerates as the balance itself gets larger.
A = P × (1 + r/n)^(n×t), where P is principal, r is the annual rate, n is compounds per year, and t is time in years.
Worked example
Scenario: $10,000 invested at 7% a year, compounded monthly, for 10 years, with no further deposits.
- Rate per period: 7% ÷ 12 = 0.5833%; periods: 10 × 12 = 120.
- Final balance = 10,000 × (1.005833)^120 = $20,096.61.
- Interest earned: $20,096.61 − $10,000 = $10,096.61.
- For comparison: yearly compounding gives $19,671.51 and daily compounding $20,136.18.
Result: $20,096.61 after 10 years — the money roughly doubles, with about $425 more than yearly compounding.
How $10,000 grows at different rates
Compounding rewards time more than anything else. The table shows $10,000 left untouched with monthly compounding, alongside two ways to estimate how long it takes to double: the Rule of 72 (72 ÷ rate) and the exact doubling time for monthly compounding.
| Annual rate | After 10 years | After 20 years | After 30 years | Rule of 72 (years) | Exact doubling (years) |
|---|---|---|---|---|---|
| 3% | $13,493.54 | $18,207.55 | $24,568.42 | 24 | 23.1 |
| 5% | $16,470.09 | $27,126.40 | $44,677.44 | 14.4 | 13.9 |
| 7% | $20,096.61 | $40,387.39 | $81,164.97 | 10.3 | 9.9 |
| 10% | $27,070.41 | $73,280.74 | $198,373.99 | 7.2 | 7 |
The Rule of 72 is a mental shortcut, and for rates between 3% and 10% it lands within about a year of the exact answer, and it is closest around 7–10%. Notice how the 30-year column runs away from the others: at 7%, the balance roughly doubles every ten years, so the last decade adds more than the first two combined. The schedule under the result now shows every year of your term, and you can download it as a CSV file.
What changes compound growth most?
Starting balance, contribution amount, time, return rate, and compounding frequency all affect the projection. Time can have an especially large effect because each period can earn returns on earlier returns. Treat the rate as an assumption, not a promise of future investment performance.
Tips and common mistakes
- The Rule of 72 gives a quick check: 72 ÷ 7 ≈ 10.3 years to double.
- Compounding frequency matters far less than the rate and the time invested.
- For regular deposits on top of the starting amount, use the Savings or Investment Calculator.
Assumptions & Limitations
- Assumes the rate stays constant for the entire period with no withdrawals.
- No additional contributions are included in this basic version — see the Investment Calculator for recurring deposits.
- Does not account for taxes on interest earned or inflation eroding purchasing power.
- Real-world rates fluctuate; this model uses a single fixed rate throughout.
Sources and review
Method: Standard compound-interest formula A = P(1 + r/n)^(nt), stepped monthly.
Sources:
- U.S. Securities and Exchange Commission, Investor.gov guidance on compound interest.
Last reviewed: October 2026 by Faheem Ali, founder of CalculatorAnything. The method and examples are provided for transparency; results remain estimates and may contain errors.
Results are estimates for planning and comparison, not financial advice or a lender's quote. Rates, fees, tax rules and regulations vary by country and change over time; confirm figures with your lender or a licensed adviser. Spotted an error? Tell us — see our editorial policy for how corrections are handled.