How to use this tool

1. Fill in the inputs.
2. Read the result.
3. Copy or download as needed.

About the Interest Calculator

This Interest Calculator compares simple interest and compound interest side by side for the same principal amount, interest rate, and time period, so you can see exactly how much of a difference compounding makes. You can also set how often the interest compounds each year, from annually to monthly, to match a specific savings account, investment, or loan structure.

Understanding the difference between these two methods matters whether you are saving money (where compounding works in your favor) or borrowing money (where compounding works against you), since even a small difference in compounding frequency can noticeably change the outcome over a long enough period.

Simple interest is calculated with the formula I = P × r × t, where P is the principal, r is the annual interest rate as a decimal, and t is the time in years — the interest earned or owed stays constant every period because it is always calculated on the original principal only.

Compound interest instead uses the formula A = P × (1 + r/n)^(n×t), where n is the number of times interest compounds per year, and the interest for each period is calculated on the current balance (principal plus any interest already added), not just the original principal. This is why compound interest grows faster the longer the money is left untouched and the more frequently it compounds — each period's interest starts earning interest of its own.

Common use cases

• Comparing how a savings account or certificate of deposit will grow under simple versus compound interest.
• Understanding the true long-term cost of compounding debt, such as credit card balances.
• Estimating investment growth over time for financial planning purposes.
• Teaching or learning the practical difference between simple and compound interest.
• Testing how changing the compounding frequency (annual, quarterly, monthly) affects the final amount.

Tips and best practices

• The more frequently interest compounds (monthly versus annually, for example), the higher the final amount will be for the same nominal interest rate.
• Compound interest becomes dramatically more powerful over long time periods — the gap between simple and compound interest widens the longer the money is invested.
• When comparing loan or savings offers, always check the compounding frequency in addition to the headline interest rate, since two loans with the same rate but different compounding can have different real costs.
• For debt, compound interest means unpaid interest can itself start accruing interest, which is why paying down high-interest, compounding debt quickly is usually a financial priority.

Frequently asked questions