Use a level against the underside of the rafter youre measuring to get the most accurate value. Rise run pitch where pitch is expressed in percent. To do this you will need to write down the measurements that you have taken from the rafters.
Slope tan degrees.
Outside of the US a degree angle is typically used. If you know the roofs angle in degrees you can find the roof pitch by converting the angle in degrees to a slope then finding the rise by multiplying the slope by 12. First find the slope by finding the tangent of the degrees eg. Rise run pitch where pitch is expressed in percent.