Free Interest Calculator
Compare simple vs. compound interest so you can see how contributions and rates change your balance over time.
No sign-up required. 100% free to use.
Try the Interest Calculator
Enter your starting balance, rate, time, and optional contributions to see total value and interest earned with simple and compound settings.
Interest Calculator (Simple / Compound)
Inputs
Results
Final Amount
$16,470.09
Total Interest Earned
$6,470.09
Total Contributed
$10,000.00
Year-by-Year Breakdown
In Compound Interest mode, interest is calculated on both the principal and accumulated interest at the selected frequency. Monthly contributions are added at the end of each month and begin earning interest immediately.
| Year | Starting Balance | Contributions | Interest | Ending Balance |
|---|---|---|---|---|
| 1 | $10,000.00 | $0.00 | $511.62 | $10,511.62 |
| 2 | $10,511.62 | $0.00 | $537.79 | $11,049.41 |
| 3 | $11,049.41 | $0.00 | $565.31 | $11,614.72 |
| 4 | $11,614.72 | $0.00 | $594.23 | $12,208.95 |
| 5 | $12,208.95 | $0.00 | $624.63 | $12,833.59 |
| 6 | $12,833.59 | $0.00 | $656.59 | $13,490.18 |
| 7 | $13,490.18 | $0.00 | $690.18 | $14,180.36 |
| 8 | $14,180.36 | $0.00 | $725.49 | $14,905.85 |
| 9 | $14,905.85 | $0.00 | $762.61 | $15,668.47 |
| 10 | $15,668.47 | $0.00 | $801.63 | $16,470.09 |
How this works
- Compound Interest Formula: The selected frequency is converted to a monthly rate: rmonthly = (1 + r/n)n/12 − 1, where n is the compounding frequency per year.
- Each month: balance = balance × (1 + rmonthly) + monthly contribution
- This means your interest earns interest, leading to exponential growth over time.
- "Total Contributed" includes your initial principal plus all monthly deposits over the time period.
Learn more: Simple vs Compound Interest—Which One Grows Your Money Faster?
How to use the interest tool
- Principal: Your starting balance or loan amount.
- Rate (APR): Annual rate expressed as a percentage.
- Time horizon: Enter years or months to see how long you earn or accrue interest.
- Contributions: Add recurring deposits to model savings growth; set to zero for a one-time deposit.
- Simple vs. compound: Toggle to see how compounding frequency changes the ending balance.
Small changes to rate, time, or contribution amount can dramatically change your ending balance. Run a few scenarios to choose the plan that fits your goal.
Popular ways people use it
- Compare a high-yield savings account vs. a CD before locking funds.
- See how extra monthly deposits accelerate an emergency fund.
- Check how interest-only vs. amortizing loans differ over time.
- Model long-term compounding for college, retirement, or big purchases.
Interest Calculator FAQ
When should I use simple interest?
Use simple interest for short-term loans or scenarios that do not compound (many auto loans or some personal loans).
Why is compound interest larger?
Compound interest grows on both your principal and previously earned interest, so balances snowball faster as time and rate increase.
Do I need an account to use it?
No. The interest calculator is free and runs instantly in your browser.
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