Area of a Circle
See How to Calculate the Area belowbut first the calculator:
The Calculator
Enter the radiusdiametercircumference or area of a Circle to find the other three. The calculations are done "live":
How to Calculate the Area
The area of a circle is:
Example: What is the area of a circle with radius of 3 m ?
Radius = r = 3
How to Remember?
To help you remember think "Pie Are Squared"
(even though pies are usually round)
Comparing a Circle to a Square
It is interesting to compare the area of a circle to a square:
A circle has about 80% of the area of a similar-width square.
The actual value is (π/4) = 0.785398... = 78.5398...%
Why? Because the Square's Area is w2
and the Circle's Area is (π/4) × w2
Example: Compare a square to a circle of width 3 m
Square's Area = w2 = 32 = 9 m2
Estimate of Circle's Area = 80% of Square's Area = 80% of 9 = 7.2 m2
Circle's True Area = (π/4) × D2 = (π/4) × 32 = 7.07 m2 (to 2 decimals)
The estimate of 7.2 m2 is not far off 7.07 m2
A Real World Example
Example: Max is building a house. The first step is to drill holes and fill them with concrete.
The holes are 0.4 m wide and 1 m deephow much concrete should Max order for each hole?
The holes are circular (in cross section) because they are drilled out using an auger.
The diameter is 0.4mso the Area is:
And each hole is 1 m deepso:
Volume = 0.126 m2 × 1 m = 0.126 m3
So Max should order 0.126 cubic meters of concrete to fill each hole.
Note: Max could have estimated the area by:
A good estimate!
Andto finishsomething interesting for you: