Measure the entire length of the ramp, from one end to the other.
Measure the distance from the high end of the ramp to the ground. If the low end of the ramp is not on the ground, measure from the highest point of the ramp to the lowest. This is called the rise.
Divide the length of the ramp by the height. This is the angle, or slope, of the ramp and is the second number in your ratio. The first number is always one. If the ramp measures 12 feet long and the rise is 2 feet, you would divide 12 by 2 to get 6, and your ratio would be 1 to 6. This means that for every foot you go up, you will need 6 feet of ramp to get you there.