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Simple Interest Calculator

Calculate simple interest and the final amount.

Finance Calculators Free, no sign-up Runs in your browser

Only needed when solving for something else.

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Interest
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Final amount
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If it compounded
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Compounding would add

About the Simple Interest Calculator

Simple Interest Calculator works out interest charged on the original amount only, using the formula I = P x R x T / 100, where P is the principal, R is the annual rate as a percentage and T is the time in years.

Simple interest is the easier of the two kinds, and the rarer. It does not build on itself: a sum earning simple interest earns the same amount every year, for ever. That makes it predictable and easy to check by hand, which is exactly why it still appears in textbooks, in short-term lending, in some bonds and in many informal arrangements.

The important comparison is against compound interest, where each period's interest is added to the balance and earns interest itself. Over one year the two are identical. Over five they differ noticeably. Over twenty-five the compound figure can be several times the simple one. This page shows both, because the gap is the single most useful thing to understand about interest and it is invisible if you only look at one.

The arithmetic works in any currency and over any period. Time can be entered in years, months or days, and the conversion is exact rather than rounded to a convenient number of days.

This is arithmetic, not financial advice. It tells you what the numbers do; it cannot tell you whether a decision is right for your circumstances. For anything that matters, check the figures against the actual offer and talk to someone qualified.

Everything is worked out inside your browser. Nothing you enter is uploaded, logged or stored, which matters for the financial calculators in particular: the figures you put into them are nobody else's business.

Looking for something this does not cover? FreeToolsZone.org is more free browser-based tools, organised by job, and it is worth a look if simple interest calculator is only part of what you are working on.

How to use the Simple Interest Calculator

  1. Enter the principalThe original amount lent, borrowed or invested.
  2. Enter the annual rateAs a percentage, for example 7.5.
  3. Enter the timeIn years, months or days.
  4. Read the interest and the totalThe compound equivalent is shown alongside for comparison.

Features

  • Interest and final amount from the standard formula
  • The formula shown with your own numbers substituted in
  • Time in years, months or days
  • Compound interest shown alongside, so the gap is visible
  • Solve for any one of principal, rate or time
  • Works in any currency
  • Entirely in your browser

Who uses this tool

The simple interest calculator is used by writers, developers, students, marketers and anyone else who needs the job done once without installing software. Common cases include:

  • Checking the interest on a short-term loan
  • Homework and exam practice where the formula must be shown
  • Working out what a fixed-rate bond pays
  • Seeing how much compounding would have added
  • Settling an informal loan between friends or family

Frequently asked questions

When is interest actually simple?

Short-term lending, some government bonds, many car finance agreements and most informal arrangements. Savings accounts and nearly all long-term borrowing use compound interest instead.

How do I solve for the rate instead?

Switch what you are solving for at the top. Give any three of principal, rate, time and interest, and the fourth is calculated.

Is a month a twelfth of a year here?

Yes, and a day is a 365th. Some lenders use 360 days a year, which gives a slightly different figure; if that is what your agreement says, enter the time in years as a decimal instead.

Why is the compound figure higher?

Because compound interest earns interest on interest. The longer the period, the wider the gap, which is why the comparison is shown rather than described.

If the simple interest calculator is not quite what you need, these other free tools solve closely related problems, and there are 38 more in finance calculators.

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EMI, total interest and the full repayment schedule for a loan of any size.

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Percentage Calculator

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CAGR Calculator

The smoothed yearly growth rate between a starting and an ending value.

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Checked and rebuilt on 2026-10-14. The method this calculator uses is described in the section above, and corrections go through the contact page.