Circles: calculate area, circumference, radius, and diameter

Learning Objectives Define and identify the radius, diameter, circumference, and area of a circle. Explain the relationship between radius and diameter. Recall the approximate value of Pi (π) as 3.14. Calculate the diameter of a circle given its radius, and vice versa. Calculate the circumference of a circle using its radius or diameter. Calculate the area of a circle given its radius. Apply appropriate units to measurements of radius, diameter, circumference, and area. Have you ever wondered how much frosting you need to go around a circular cake 🎂, or how much space a round trampoline takes up in your backyard? 🤔 In this lesson, you'll learn all about circles! We'll explore their key parts and discover how to calculate important measurements like the distanc...

TermDefinitionExample CircleA perfectly round shape where all points on the edge are the same distance from the center.A coin, a clock face, or a hula hoop. CenterThe exact middle point of a circle.If you draw a circle with a compass, the point where the compass needle sits is the center. Radius (r)The distance from the center of a circle to any point on its edge.If a pizza has a radius of 8 inches, it means the distance from the center to the crust is 8 inches. Diameter (d)The distance across a circle, passing through its center. It's twice the length of the radius.If a circular table has a diameter of 4 feet, you can measure 4 feet straight across the middle of the table. Circumference (C)The total distance around the outside edge of a circle. It's like the perimeter of a squa...

Relationship between Radius and Diameter $d = 2r$ or $r = \frac{d}{2}$ To find the diameter, multiply the radius by 2. To find the radius, divide the diameter by 2. This rule helps you find one measurement if you know the other. Circumference of a Circle $C = \pi d$ or $C = 2\pi r$ To find the circumference, multiply Pi (π ≈ 3.14) by the diameter, OR multiply 2 by Pi (π ≈ 3.14) and then by the radius. Use this when you need to know the distance around the circle. Area of a Circle $A = \pi r^2$ To find the area, multiply Pi (π ≈ 3.14) by the radius squared ($r^2$ means $r \times r$). Use this when you need to know the amount of space inside the circle.