Look over the equation for calculating horizontal distance, which is slope = rise/run x 100. Plug your slope percentage and rise into the equation. For example, if you have a slope percentage of 6 and a rise of 25 feet, the equation would look like 6 = 25/run x 100.
Multiply each side of the equation by the run, which is still yet unknown. Continuing with the example of a slope percentage of 6 and a rise of 25, the equation will look like this: run x 6 = (25/run x 100) x run. Multiplying 25/run x 100 by the run on the right side of the equation will cancel out the run entirely on that side. You can then multiply your rise (25) by 100. After multiplication, the equation will look like: 6 x run = 2,500.
Divide each side of the equation by the slope percent. Continuing with the example of a slope percentage of 6 and a rise of 25, the equation should look like this: (run x 6) / 6 = 2,500 / 6. Dividing run x 6 by 6 will cancel out the slope percent entirely on the left side of the equation. After completing the division, the equation becomes run = 416.6. The horizontal between the two points is then 416.6 feet.