Measure the longest wall of your room. This would be your length.
Measure the other wall of your room. This would be your width.
Measure the height of your room. This would be your height.
You would have to make a personal decision of how many coats you would want to apply to a wall.
* 2 coats are usually applied.
NB * If your wall has protrusions or hollows, neglect these and measure the wall as a smooth surface.
* The ceiling is not considered in this calculation and an allowance of 0.5 metres is given for doors and windows.