Compound Interest Calculator
Compound interest shows how money can grow when interest is added to the balance and future interest is calculated on that larger amount. This calculator is useful for savings estimates, investment comparisons, loan scenarios, and learning how the timing and rate of compounding affect long-term growth.
Quick Answer
The standard compound interest formula is A = P(1 + r/n)nt. Here, A is the ending balance, P is the starting principal, r is the annual interest rate written as a decimal, n is the number of compounding periods per year, and t is the number of years.
What the Inputs Mean
Principal (P): The amount you start with. Annual rate (r): The yearly interest rate, such as 6% entered as 0.06. Compounds per year (n): Common choices are 1 for annual, 4 for quarterly, 12 for monthly, and 365 for daily compounding. Time (t): The number of years the money remains invested or borrowed.
How to Use the Calculator
- Enter the starting principal.
- Enter the annual interest rate and select the appropriate compounding frequency.
- Enter the investment or loan period in years.
- Review the ending balance and compare it with the original principal to see the interest earned or added.
Worked Examples
Example 1: Suppose $5,000 earns 6% annually for 5 years with annual compounding. Using A = 5000(1 + 0.06/1)5, the ending balance is about $6,691.13.
Example 2: With $5,000 at 6% for 5 years compounded monthly, A = 5000(1 + 0.06/12)60, giving about $6,744.25. The difference illustrates why more frequent compounding can slightly increase growth when the stated rate is the same.
Example 3: A $10,000 balance at 4% compounded annually for 10 years grows to about $14,802.44. The increase is larger than simply adding 4% of the original $10,000 every year because interest is being earned on previous interest as well.
Compound Growth Table
| Starting Amount | Rate | Compounding | Time | Approx. Ending Balance |
|---|---|---|---|---|
| $1,000 | 5% | Annual | 5 years | $1,276.28 |
| $1,000 | 5% | Monthly | 5 years | $1,283.36 |
| $5,000 | 6% | Annual | 10 years | $8,954.24 |
| $10,000 | 4% | Annual | 10 years | $14,802.44 |
Common Mistakes
- Entering 6 instead of 0.06 when the calculator expects a decimal rate.
- Using monthly periods with an annual rate without selecting monthly compounding correctly.
- Assuming a calculated return is guaranteed rather than an estimate based on the entered rate.
- Ignoring deposits, withdrawals, taxes, fees, or rate changes that may apply in a real account.
Why Compounding Frequency Matters
For the same nominal annual rate, increasing the number of compounding periods can increase the ending balance because interest is credited more frequently. The effect is usually modest over short periods but becomes more noticeable over long periods and at higher rates.