Geometry Calculators

Area of a Circle Calculator

The area instantly calculates when you input the size. Try it out!
Size of circle =

Lesson on Area of a Circle

Lesson Contents

How to Find the Area of a Circle by Hand

We can easily hand calculate the area of a circle by using the area formula. The formula for area of a circle is given as:
A = πr2
Where A is the area and r is the radius.

If we do not know the radius but know some other dimension of the circle, we can easily convert to radius before applying the area formula. Here are the conversions for diameter and circumference:
r = d2
r = C
Where r is the radius, d is the diameter, and C is the circumference.

How the Calculator Works

The calculator on this page is written in the programming language JavaScript. The JavaScript function area runs in your internet browser when an input change is detected. This allows the area to be calculated nearly instantly.

The first step in the calculator’s routine is to check the input unit. If the unit is diameter or circumference, it converts the size input to radius using the same conversion formulas given above. If the unit is radius, the size input is left alone.

Then, the area of the circle is calculated using the same area formula given above. The raw area output is rounded to the fourth decimal place and displayed on this page as the area of your circle.

Result :

Worksheet 1


Cheat sheet


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