Simple Interest Calculator

Calculate interest using the formula SI = P × R × T / 100

Currency

Principal Amount

$1K$1M

Annual Interest Rate

0.5%50%

Time Period

years

1 yr50 yrs

Time Unit

Results

Adjust the inputs and click Calculate.

Frequently Asked Questions

Simple Interest (SI) is calculated only on the original principal amount for the entire duration of the loan or investment. It does not compound — interest is not added to the principal to earn further interest. The formula is: SI = (P × R × T) / 100, where P is the principal, R is the annual rate, and T is the time in years.

SI = (P × R × T) / 100
Total Amount (A) = P + SI
Where P = Principal, R = Annual interest rate (%), T = Time (years).
For months: T = months / 12. For days: T = days / 365.

Simple interest is calculated only on the principal for each period. Compound interest is calculated on the principal plus accumulated interest, causing exponential growth. Over short periods, both are similar. Over long periods, compound interest significantly outgrows simple interest. Most modern savings accounts and loans use compound interest.

Simple interest is commonly used in: short-term personal loans, car loans (US), some mortgages, treasury bills, certificates of deposit (CDs) for short terms, and informal lending. It is easier to calculate and understand than compound interest, making it popular for short-duration financial products.

Yes. Use the Time Unit selector to switch between Years, Months, and Days. The calculator automatically converts to the correct fraction of a year (months ÷ 12, days ÷ 365) before applying the formula. This is useful for short-term instruments like treasury bills, short loans, or daily interest calculations.

From SI = (P × R × T) / 100, rearranging:
P = (SI × 100) / (R × T)
R = (SI × 100) / (P × T)
T = (SI × 100) / (P × R)
Enter any three known values in this calculator and adjust until you match your target interest amount.

For simple interest, the effective annual rate (EAR) equals the nominal rate when the period is exactly one year. For sub-annual periods (months or days), the annualised rate is: EAR = (SI / P / T_years) × 100. This calculator displays the effective annual rate for the entered inputs.

For borrowers, simple interest is generally better because the total interest paid is lower than with compound interest over the same period. For lenders/investors, compound interest yields more over time. As a borrower, look for simple interest loans; as an investor, prefer compound interest instruments for maximum growth.

Simple interest is preferred for short-duration financial products where compounding frequency would make little practical difference. It is commonly used in consumer auto loans (particularly in the US), short-term personal loans, trade credit, and government treasury bills. Its transparency and predictability — you always know exactly how much interest accrues — make it ideal for terms under 2 years.

Common real-world uses include: US auto loans (interest accrues daily on outstanding balance using simple interest), short-term business loans, microfinance lending in developing economies, 90-day treasury bills, and pawn shop loans. Some education loans also use simple interest during the study period. Whenever straightforward, linear interest cost is needed, simple interest is the practical choice.

About This Simple Interest Calculator

This free simple interest calculator lets you instantly compute the interest earned or owed on any principal amount. Enter the principal, annual interest rate, and time period (in years, months, or days) to get the total interest, final amount, and a period-by-period breakdown table.

Simple interest is the most straightforward form of interest calculation. It is widely used in short-term loans, car financing, government bonds, and informal lending. Because interest does not compound, the total interest grows linearly — making it easy to plan and predict repayments.

When to use this calculator

Calculating interest on a short-term personal loan
Estimating returns on treasury bills or CDs
Comparing simple vs compound interest for the same rate
Computing daily or monthly interest for any amount

Standards & References

Wikipedia — Simple Interest

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Compound Interest

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