Use a measuring tape to measure the length of the treadmill belt. Measure just the length of the belt, not the length of the entire treadmill.
Use a straightedge to measure from the floor to the top of the treadmill belt when the treadmill is completely flat. Measure both the front and back of the belt to make sure both heights are equal and the belt is truly level. This is a zero percent incline.
Set the treadmill to your chosen incline and remeasure the height of the front of the belt.
Subtract to find the difference between the new height of the treadmill and the original height. For example, if the original height was 3 inches and the new height at an incline is 6 inches, the difference is 3.
Divide the difference in height by the length of the treadmill belt. If your treadmill is 45 inches long and you determine the incline is 3 inches higher than it was at a zero percent incline, you have an incline grade of 3/45 or 6.7 percent.
Repeat for your other incline settings. Write down all the incline grades so you do not forget.