Simple Interest Calculator

Enter the principal: Type the original amount of money you want to calculate interest on.

Enter the annual interest rate: Provide the rate as a percentage, such as 5%.

Choose the time unit: Enter the duration in years, months, or days.

Select the day-count convention: If you use days, choose either a 365-day year or a 360-day year.

Choose a currency: Select the currency used to display your amounts.

Set decimal precision: Choose how many decimal places should appear in the results.

The calculator updates the results as you change the inputs. You can also use the reset control to clear the values or copy a summary of your calculation.

Calculate simple interest and the final amount using your principal, annual interest rate, and time period. Choose a duration in years, months, or days, select a currency, and see your results update as you change the inputs.

Simple interest is calculated only on the original amount of money, known as the principal. The principal stays constant throughout the calculation, so the same amount of interest is earned or charged for each equal period of time.

Simple interest is commonly used in some loans, short-term borrowing arrangements, and basic investment calculations.

Simple Interest Formula

The formula for calculating simple interest is:

I = P × r × t

Where:

I = Total simple interest

P = Original principal

r = Annual interest rate expressed as a decimal

t = Time period in years

For example, an annual interest rate of 5% is entered into the formula as 0.05.

To calculate the final amount:

A = P + I

Where A is the principal plus the total interest.

The main difference is how interest is calculated.

With simple interest, interest is always calculated on the original principal. Previously earned interest is not added to the principal for future calculations.

With compound interest, earned interest is added to the balance, and future interest can be calculated on both the original principal and accumulated interest.

For example, $10,000 invested at 5% per year for three years earns $1,500 in simple interest. With annual compounding at the same rate, the interest would be $1,576.25.

The calculator displays several values to help you understand the calculation:

Principal: The original amount used in the calculation.

Total simple interest: The interest accumulated over the selected period.

Final amount: The principal plus the total interest.

Average interest per year: The average interest earned or charged per year over the selected period.

Simple return: The total interest expressed as a percentage of the original principal.

Year-by-year schedule: An optional breakdown showing the interest and balance for each year.

The year-by-year schedule is useful for seeing how the balance changes over time. With simple interest, the interest added for each full year remains constant when the principal and annual rate do not change.

When you enter a duration in days, the calculator needs a day-count convention to convert the period into years.

365-day convention: Divides the number of days by 365. It uses a standard 365-day year for the calculation.

360-day convention: Divides the number of days by 360. This convention is used in some financial calculations and agreements.

For example, consider a principal of $10,000 at an annual rate of 5% for 180 days:

Using 365 days: $10,000 × 0.05 × (180 ÷ 365) = approximately $246.58 interest.

Using 360 days: $10,000 × 0.05 × (180 ÷ 360) = $250 interest.

The selected convention affects the result, so check the terms of the relevant loan or investment when applicable.

Example 1: $10,000 at 5% for 3 years

Principal: $10,000

Annual interest rate: 5%

Time: 3 years

Calculation:

$10,000 × 0.05 × 3 = $1,500

Total interest: $1,500

Final amount: $11,500

Average interest per year: $500

Simple return: 15%

Example 2: Changing the interest rate

Suppose you keep the principal at $10,000 and the duration at 3 years but increase the annual rate from 5% to 6%.

$10,000 × 0.06 × 3 = $1,800

The total interest increases from $1,500 to $1,800, and the final amount becomes $11,800.

Example 3: Changing the duration

Suppose the principal is $10,000 and the annual interest rate is 5%, but the duration is reduced from 3 years to 2 years.

$10,000 × 0.05 × 2 = $1,000

The total interest is $1,000, and the final amount is $11,000.

These examples show that, with a fixed principal and annual rate, simple interest increases in direct proportion to the time period.

Live calculations: Results update as you change the principal, rate, or duration.

Flexible time units: Calculate interest using years, months, or days.

Day-count options: Choose a 365-day or 360-day convention for day-based calculations.

Currency selection: Display amounts using your preferred currency.

Decimal precision: Choose the number of decimal places shown in the results.

Year-by-year schedule: View an optional breakdown of yearly interest and balances.

Copy summary: Copy the calculation results for reference or sharing.

Reset: Clear the inputs and start a new calculation.

Input validation: Receive clear feedback when entered values are invalid.