Interest Calculator
A precision tool for modeling financial growth and borrowing costs.
Interest Calculator
Whether you're calculating interest on a savings account, a loan, or an investment, this calculator handles both simple and compound interest — and shows you how dramatically compounding accelerates growth over time.
Simple vs. Compound Interest
Simple interest is calculated only on the principal. Compound interest is calculated on the principal plus all previously earned interest — it grows exponentially. Einstein reportedly called compound interest "the eighth wonder of the world."
How to Use This Calculator
- Enter the principal amount (starting balance).
- Enter the annual interest rate.
- Enter the time period in years.
- Choose simple or compound interest.
- If compound, choose the compounding frequency (daily, monthly, quarterly, annually).
- Click Calculate to see total interest earned and final balance.
Formulas
Simple Interest: I = P × r × t
Compound Interest: A = P × (1 + r/n)^(n×t)
- P = Principal | r = Annual rate (decimal) | t = Time (years) | n = Compounding periods per year
Example: Simple vs. Compound
Principal: $10,000 | Rate: 7% | Time: 20 years
- Simple interest: $10,000 × 7% × 20 = $14,000 interest → $24,000 total
- Compound (monthly): $10,000 × (1 + 0.07/12)^(240) = $40,065 total
- Compounding earns $16,065 more on the same principal and rate
Common Mistakes to Avoid
- Assuming savings accounts compound annually — Most savings accounts compound daily or monthly, which is better for you as a saver.
- Forgetting compounding frequency matters — Daily compounding earns slightly more than monthly, which earns more than annually.
- Ignoring inflation — A 7% return isn't 7% in real purchasing power. Subtract inflation (≈3%) for real growth.
- Not starting early — Time is the most powerful variable in compound interest. Even small amounts started early can outperform large amounts started late.
Frequently Asked Questions
What is APY vs. APR?
APY (Annual Percentage Yield) accounts for compounding — it's the actual return earned in a year. APR (Annual Percentage Rate) is the stated rate before compounding. APY is always equal to or higher than APR.
How does compound interest work against me on debt?
The same power that grows savings also grows debt. Credit card interest compounding daily at 24% APR is devastating. Pay off high-rate debt as fast as possible. See our Credit Card Payoff Calculator.
How long to double my money?
Use the Rule of 72: divide 72 by the interest rate. At 7%, money doubles in ~10.3 years. At 10%, about 7.2 years.
What is the best compounding frequency for savings?
Daily compounding is best for the saver (earns the most interest). The difference between daily and monthly compounding is small but meaningful over long periods.
Conclusion
Interest is either working for you (savings, investments) or against you (debt). Understanding how it compounds over time is fundamental to financial success. Use this calculator to visualize the power of both scenarios.
Related: Compound Interest Calculator | Simple Interest Calculator | Savings Calculator | Investment Calculator
Financial Tip
The frequency of compounding matters. Compounding daily will always result in a higher balance than compounding annually, even if the interest rate is identical.
