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Interest Calculator

Compare simple and compound interest on the same figures side by side.

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YearSimple balanceCompound balanceDifference

Compound balance

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Simple interest—
Simple balance—
Compound interest—
Effective annual rate—
Doubling time (rule of 72)—

Quick Tool Overview

The Interest Calculator shows simple and compound interest on the same figures side by side. Simple interest is charged on the original amount only; compound interest is charged on the balance including interest already added, so the two diverge — slowly at first, then decisively.

What it does
Compares simple and compound interest on identical inputs
You give it
A principal, a rate, a period and a compounding frequency
You get back
Both interest totals and the gap between them
Best for
Understanding a savings offer, a loan quote or coursework
Calculators Free, unlimited use No sign-up Runs in your browser

About the Interest Calculator

Enter a principal, a rate and a period to see the interest under both methods at once. Simple interest is charged on the original amount only; compound interest is charged on the balance including interest already added, so the two diverge over time.

Seeing them side by side makes the gap concrete. Over a year or two the difference is minor. Over ten or twenty years it becomes the dominant term, which is why compounding is the more important idea in both saving and borrowing. The frequency control shows the second effect: at the same nominal rate, monthly compounding beats annual, and daily beats monthly, though the gains shrink quickly as frequency rises.

Key features of the Interest Calculator

Both methods at once

The two are calculated on the same figures, which is the only way to see how far apart they really run.

The gap grows with time

Over a year or two the difference is minor; over ten or twenty years it becomes the dominant term.

Compounding frequency control

At the same nominal rate monthly beats annual and daily beats monthly, though the gains shrink quickly as frequency rises.

Works for saving and borrowing

The same arithmetic applies whether the interest is being paid to you or by you.

Clear totals

Interest and final balance are reported for each method rather than a single blended figure.

How to use the Interest Calculator

  1. 1

    Enter the principal

    The starting amount, whether it is a deposit you are making or a balance you owe.

  2. 2

    Set rate and period

    Enter the annual rate and how long the money is invested or borrowed for.

  3. 3

    Choose the frequency

    Pick how often interest is compounded — annually, quarterly, monthly or daily.

  4. 4

    Compare the two methods

    The simple and compound totals are shown together, with the gap between them highlighted.

Frequently asked questions

Simple interest is always calculated on the original principal. Compound interest is calculated on the principal plus any interest already earned, so the balance grows faster each period.

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