Interest Calculator
Interest Calculator
Interest is the cost of borrowing money or the return on invested capital. It is one of the most fundamental concepts in finance, affecting everything from savings accounts and certificates of deposit to mortgages, car loans, and credit cards. Understanding how interest works in its various forms is essential for making informed financial decisions.
The Interest Calculator supports three different methods: simple interest, compound interest, and continuous compounding. Simple interest is calculated only on the original principal. Compound interest is calculated on both the principal and accumulated interest, leading to exponential growth. Continuous compounding represents the theoretical limit of compounding at infinitely small intervals.
Compound interest is often called the eighth wonder of the world. When interest compounds, you earn interest on your interest, creating a snowball effect that significantly boosts returns over long periods. The more frequently interest is compounded, the higher the total return at the same nominal rate.
Simple interest is used for short-term loans and certain bonds. Compound interest is standard for savings accounts, CDs, and retirement accounts. Continuous compounding is used in advanced financial mathematics and options pricing.
Select the calculation type: simple, compound, or continuous. Enter the principal amount, which is the initial amount being borrowed or invested. Enter the annual interest rate as a percentage.
Enter the time period in years. For periods shorter than one year, enter fractions such as 0.5 for six months. For compound interest, select the compounding frequency from the available options: annually, semi-annually, quarterly, monthly, weekly, or daily.
Press Calculate to view the interest earned and final amount. For example, $10,000 at 5 percent compounded monthly for 10 years yields approximately $16,470. The same investment with simple interest yields only $15,000.
Simple interest is calculated on the original principal only:
Where A is the final amount, P is the principal, r is the annual rate, and t is the time in years.
Compound interest with m compounding periods per year:
For $10,000 at 5 percent compounded monthly for 10 years: A = $10,000 x (1 + 0.05/12)^(120) = $16,470.
Continuous compounding (theoretical limit of infinite compounding frequency):
For the same investment with continuous compounding: A = $10,000 x e^(0.5) = $16,487.
$10,000 at 5 percent over different time periods:
| Years | Simple | Annually | Monthly | Daily | Continuous |
|---|---|---|---|---|---|
| 1 | $10,500 | $10,500 | $10,512 | $10,513 | $10,513 |
| 5 | $12,500 | $12,763 | $12,833 | $12,840 | $12,840 |
| 10 | $15,000 | $16,289 | $16,470 | $16,487 | $16,487 |
| 20 | $20,000 | $26,533 | $27,126 | $27,180 | $27,183 |
| 30 | $25,000 | $43,219 | $44,677 | $44,793 | $44,817 |
Start saving early to maximize compound interest. A person investing $5,000 per year starting at age 25 will have significantly more at retirement than someone starting at 35, even if the late starter invests more per year.
When comparing financial products, always check the compounding frequency. A savings account compounding daily earns slightly more than one compounding monthly at the same nominal rate. The APY standardizes comparisons.
For loans, higher compounding frequency means more interest paid. Credit cards typically compound daily. Pay off balances before interest accrues whenever possible.
This calculator assumes a constant interest rate throughout the entire period. It does not account for additional contributions or withdrawals after the initial principal. For scenarios involving regular contributions, use a future value calculator.
The calculator does not consider taxes, fees, or inflation. Taxable accounts may require paying taxes on interest each year, reducing the effective compounding benefit. Simple interest calculations do not reflect most real-world financial products.
- What is the difference between simple and compound interest?
- Simple interest is calculated only on principal: I = P x r x t. Compound interest is on principal plus accumulated interest: A = P(1 + r/n)^(nt).
- How do I use this calculator to compare savings vs a loan?
- For savings, use compound interest to show growth. For short-term loans, use simple interest. For credit cards, use compound. Toggle the mode to match your scenario.
- What does compounding frequency mean?
- How often interest is calculated and added — annually (1), semiannually (2), quarterly (4), monthly (12), or daily (365). Higher frequencies yield slightly more total interest.
- Can this calculator show monthly payments for a loan?
- Yes. Enter principal, rate, and term. The calculator displays total interest and an estimated monthly payment.
- Is the result accurate for real-world products?
- The math is accurate, but real products may include fees, variable rates, or penalties not modeled here. Confirm with your institution.
- U.S. Securities and Exchange Commission. "Compound Interest Calculator." investor.gov.
- Federal Reserve. "Consumer Handbook on Adjustable-Rate Mortgages." federalreserve.gov.
- Investopedia. "The Power of Compound Interest." investopedia.com.
- NerdWallet. "Compound Interest Calculator." nerdwallet.com.
- Bankrate. "Simple vs Compound Interest." bankrate.com.
Last updated: May 12, 2026