
When it comes to real measurements, SketchUp lets you work as loosely or as accurately as you like. Any time you use a tool SketchUp gives you accurate measurement information in the lower right of the screen. This text field is known as the "Value Control Box".
If you want to be precise when using a tool, just type in the dimensions, degrees, size or distance. Notice your exact values appear in the Value Control Box. Hit the Enter key when you are done typing your values.