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Interest formula tool

Free Simple Interest Calculator

A simple interest calculator finds interest from the original principal, annual rate, and time period. Use it for non-compounding personal loans, informal lending plans, and quick formula checks.

Formula

I = P x R x T

Modes

Interest, principal, rate, time

Cost

Free, no signup

Formula breakdown

$10,000.00 x 8% x 2 years = $1,600.00

Interest amount

$1,600

Simple interest owed or earned before compounding.

Total repayment

$11,600

Principal plus calculated simple interest.

Monthly interest

$67

Average interest cost spread across the period.

Interest amount

$1,600.00

The selected value solved from the formula.

How the simple interest formula works

Simple interest does not add interest back into the principal. That makes it useful for quick loan estimates, promissory-note checks, and comparing basic non-compounding terms before you create a full repayment plan.

Step 1

Choose what to solve

Pick interest, principal, annual rate, or time depending on which value is missing from the loan scenario.

Step 2

Enter known loan values

Add the principal, interest amount, annual rate, and time period that are available.

Step 3

Select the time unit

Use years, months, or days. The calculator converts the period to years for the formula.

Step 4

Review the formula

Check the formula line to see exactly how the answer was calculated.

FAQ

Simple interest questions

What is a simple interest calculator?

A simple interest calculator estimates interest using only the original principal, the annual rate, and the time period.

What is the simple interest formula?

The simple interest formula is I = P x R x T, where P is principal, R is annual interest rate as a decimal, and T is time in years.

Can simple interest be calculated monthly?

Yes. Convert months to years by dividing by 12, then multiply principal by annual rate and the year fraction.

When should I use simple interest instead of APR?

Use simple interest for straightforward non-compounding loans. Use APR when fees, compounding, or lender comparison rules matter.

Can this calculator solve for principal or rate?

Yes. Choose the value you want to solve for and enter the other known values to calculate principal, annual rate, or time.