Area of a Circle

circumference or area of a Circle to find the other three. The calculations are done "live": images/circle-dia-circ.js How to Calculate the Area The area of a circle is : π (Pi) times the Radius squared: A = π r2 or， how 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 0.4m， Area of a Circle See How to below， so: 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: Calculating a square hole: 0.4 × 0.4 = 0.16 m2 Taking 80% of that (to estimate a circle): 80% × 0.16 m2 = 0.128 m2 And the volume of a 1 m deep hole is: 0.128 m3 A good estimate! And， when you know the Diameter: A = ( π /4) × D2 or， but first the calculator: The Calculator Enter the radius， when you know the Circumference: A = C2 / 4 π Example: What is the area of a circle with radius of 3 m ? Radius = r = 3 Area = π r2 = π × 32 = 3.14159... × (3 × 3) = 28.27 m2 (to 2 decimal places) 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 deep ， something interesting for you: See Circle Area by Lines Copyright 2025 Rod Pierce ， diameter， so the Area is: A = ( π /4) × D2 = (3.14159.../4) × 0.42 = 0.7854... × 0.16 = 0.126 m2 (to 3 decimals) And each hole is 1 m deep， to finish，。