The Measurement Tool can be used to measure the distance between two points. You can choose to measure along the grid or directly from point to point.
Start measuring by pressing the ruler icon in the Action Bar, then click and drag to define two points to measure between.
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Or you can right click on the background where you want to start measuring, select Measure Distance, then click and drag to where you would like to stop measuring.
Before you start draging you can change the measuring options in the action bar.
Choose between snap to grid and free measuring. Choose how diagonals are handled, either by letting their distance equal 5 (the same as horizontal and vertical), alternate between 5 and 10, 7 or disable diagonal movement. Choose if the measurement should be visible to other people in the scene.
Make waypoints while measuring by pressing space as you drag, this will create a waypoint at the current end of the measuring tool, you can continue dragging from there and make more waypoints.