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Simple Interest Calculator

Work out simple interest with the I = PRT formula and full working.

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I = P × R × T

Fill in any three and leave the one you want blank.

—

Interest—
Total (P + I)—
Time in years—
Interest per year—
Interest per day—
Simple interest is charged on the original principal only. Anything that compounds will grow faster — compare the two on the interest calculator.

Quick Tool Overview

The Simple Interest Calculator applies I = P × R × T and shows the substitution rather than just the answer. You can solve for any of the four values: leave the principal, rate, time or interest blank and the formula is rearranged to find it.

What it does
Solves simple interest for any of its four variables
You give it
Any three of principal, rate, time and interest
You get back
The missing value with the rearranged formula and working
Best for
Coursework, short-term loans and checking a lender's figures
Calculators Free, unlimited use No sign-up Runs in your browser

About the Simple Interest Calculator

Simple interest is the straightforward case: interest is calculated on the original principal for the whole period and never earns interest of its own. The formula is I equals P times R times T, and this calculator shows that substitution rather than just the answer.

You can solve for any of the four values. Leave the principal, rate, time or interest blank and the formula is rearranged to find it, which covers the reverse questions that come up in coursework and in checking a lender: what rate am I actually paying, or how long would it take to earn a given amount. Time can be entered in years, months or days, since short-term simple interest is often quoted daily.

Key features of the Simple Interest Calculator

Solve in any direction

Leave the unknown blank and the formula is rearranged for it, which covers the reverse questions that actually come up.

Full working shown

The substitution into I = PRT is printed, so the answer can be copied into a homework answer.

Years, months or days

Short-term simple interest is often quoted daily, and the calculator takes the period in whichever unit you have.

Answers the practical questions

What rate am I actually paying, or how long would it take to earn a given amount.

No compounding confusion

Interest is calculated on the original principal for the whole period and never earns interest of its own — exactly as the definition says.

How to use the Simple Interest Calculator

  1. 1

    Enter what you know

    Fill in any three of principal, rate, time and interest.

  2. 2

    Leave the unknown blank

    The calculator rearranges the formula to solve for whichever field is empty.

  3. 3

    Read the working

    The substituted formula is shown with the result so you can check or reproduce it.

Frequently asked questions

Interest equals principal times rate times time, written I = PRT. The rate is a decimal, so 5 percent is entered as 5 and converted to 0.05 internally, and time is in years.

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