Compound Interest Calculator
See how your investments grow over time with compound interest and regular monthly contributions. Perfect for savings planning and investment decisions.
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Enter investment details to see compound growth.
Formula Explanation
The calculator computes your investment growth using compound interest formulas and calculates your overall rate of return:
- FV: Future value (your total investment amount at the end)
- P: Initial principal (starting investment)
- r: Monthly rate of return (expected annual return / 12)
- n: Total number of months
- PMT: Monthly contribution amount (deposited at beginning of each month)
- ROR: Actual Rate of Return (your total gains as a percentage of what you contributed)
Interest is compounded monthly. This means your earnings are calculated and added to your balance every month, allowing you to earn returns on your returns. The more frequently interest compounds, the faster your money grows.
What Is Compound Interest?
Compound interest is interest calculated not just on the original principal, but also on all previously accumulated interest. This creates a snowball effect — your earnings generate their own earnings over time. It is the most powerful force in long-term investing and saving: a $10,000 investment at 8% simple interest grows to $34,000 in 30 years, while the same amount at 8% compound interest (monthly) grows to over $109,000.
The flip side is equally important: compound interest works against you with debt. Credit card balances, personal loans, and student loans all use compound interest — meaning unpaid balances grow faster than most people realize.
How Compounding Frequency Affects Growth
The more frequently interest compounds, the faster your money grows. Here's how a $10,000 investment at 8% annual interest compares after 20 years under different compounding schedules:
| Compounding | Balance After 20 Years |
|---|---|
| Annually | $46,610 |
| Quarterly | $47,571 |
| Monthly | $49,268 |
| Daily | $49,530 |
Frequently Asked Questions
What is the Rule of 72?
The Rule of 72 is a quick way to estimate how long it takes an investment to double. Divide 72 by the annual interest rate. At 8%, your money doubles in approximately 72 ÷ 8 = 9 years. At 6%, it takes about 12 years. It's a useful mental shortcut for comparing investment options.
Why does starting early matter so much?
Time is the most powerful variable in compound interest. A 25-year-old who invests $5,000/year at 8% until age 35 (then stops) will accumulate more money by age 65 than a 35-year-old who invests $5,000/year continuously for 30 years. The early investor contributed $50,000; the late starter contributed $150,000 but ends up with less. Start as early as possible — even small amounts invested early outperform large amounts invested late.
What interest rate should I use for savings projections?
For a high-yield savings account or money market fund, use the current APY (currently 4–5% for competitive online banks). For a diversified stock and bond portfolio, historical averages suggest 6–8% annually after inflation adjustments. Be conservative in your projections — markets are volatile and past performance does not guarantee future results.