On the toolbar, select the Circle tool from the drop-down menu next to the Rectangle tool. Or press the C key. The cursor changes to a pencil with a circle, and the Measurements box indicates the default number of sides: 24. To change the number of sides, you can type a value now or wait until after you’re done drawing the circle.
Move the cursor out from the center point to define the circle’s radius. As you move the cursor, the radius value is displayed dynamically in the Measurements box. Press Esc at any point to start over.
Click to finish the circle. SketchUp creates a circle-shaped face.
(Optional) Until you select a new tool or draw a new circle, you can use the Measurements box to change the circle’s radius or the number of sides as follows:
To change the radius: Type a number and a unit (if desired), such as 6”, 8’, 34cm, or 7m. Then press Enter or Return.
To change the number of sides: Type a number and s, such as 48s or 12s. Then press Enter or Return.
source
