Video narration
Let's identify the parts of a circle. A circle is a round, closed shape whose boundary points are the same distance from its center. The radius, r, is the distance from the center to the boundary. The diameter, d, goes across the circle through the center, so it contains two radii: d equals 2r, and r equals d divided by 2. The distance around a circle is its perimeter, called circumference. Use C equals 2 pi r, or, if the diameter is given, C equals pi d. Pi is the constant ratio of circumference to diameter, approximately 3.14. Circumference is measured in units. The space inside the circle is its area. Use A equals pi r squared: first square the radius, then multiply by pi. Area is measured in square units. Remember: around means circumference, C equals 2 pi r; inside means area, A equals pi r squared. Convert diameter to radius when needed.