A treadmill incline can be calculated by using a measuring tape to measure the length of the treadmill belt. Measure the length of the belt, not the length of the entire treadmill. When the height of the top of the treadmill belt is completely flat, it will help one to calculate both the front and back of the belt to make sure both heights are equal and the belt is truly level.
how to calculate treadmill incline? – http://www.ehow.com/how_5326010_calculate-treadmill-incline.html