How to install Knife Circle?
Go to Edit > Preferences then to the Add-ons tab. Click Install.
Choose the downloaded zip and click Install Add-on.
After the installation is complete it will show up in the add-on list. Enable the add-on by ticking the checkbox.
How to use Knife Circle?
Select one or more mesh objects.
Activate the Knife Circle tool from the toolbar. It works in both Object Mode and Edit Mode. In Edit Mode it lives under the Knife tool group - open the Knife tool group and pick Knife Circle.
Click to place the shape center, then move the mouse to adjust the radius. The shape is drawn on a plane determined by the current Orientation - view-aligned by default, or a fixed global axis / surface normal.
Confirm with Left Click or Cancel with Right Click/ESC. The shape is cut into all selected meshes, and the helper mesh is removed automatically.
How to cut on a view plane or a fixed axis?
Press O to cycle the projection plane: View (screen-aligned), Normal (the surface the center was placed on) and the global X, Y and Z axes.
The fixed planes are cut parallel to their normal, so the result is a true circle regardless of the viewport angle - useful for clean cylindrical cuts. View orientation follows the current viewport projection instead.
The current orientation is shown in the header text while the tool is running.
How to create stars and rotate the shape?
Press P to toggle Star mode, then hold I and move the mouse to set the inner radius. Star shapes start at 5 segments.
Hold R and move the mouse to rotate the shape around its center before confirming. Hold Shift for fine rotation or Ctrl to snap to Blender's angle increments.
How to switch between circle, square and triangle?
Scroll the mouse wheel to increase or decrease the number of segments (3-256). The header shows the current value.
Press C for a circle (32 segments), S for a square (4 segments) and T for a triangle (3 segments).
Press F to toggle between Fixed mode (uniform radius from the center) and Freeform mode (independent X and Y radii for elliptical shapes).
Press V first if you want to type an exact segment count instead of a radius.
How to type an exact radius?
Type a number with the keyboard (top row or numpad, . for decimals) to enter an exact radius. The value appears in the header.
Press Backspace to delete the last character and Enter to confirm the typed radius.
Hold Shift while moving the mouse for precise (slower) radius adjustment, or Ctrl to snap the radius in increments.
How to cut through the entire mesh?
Press X to toggle Cut Through. When enabled, the cut sweeps through all faces of the mesh instead of only the visible side.
With a fixed orientation, the near-only mode stops the sweep at the first surface, while through mode cuts a clean prism across the whole mesh.
The circle outline color indicates the state: it uses the theme's Z axis color normally and switches to the X axis color when Cut Through is active.