Slope formula
Slope percent = fall in feet / run length in feet x 100. Target fall = run length x target slope x 12.
RainYard Tools
Grade check
Drainage Slope Calculator
Check the percent slope and fall needed for a downspout extension, swale, or small drain line.
Actual slope2.08%Based on measured fall.
Target fall2.9 inFall needed for selected target.
Difference+0.1 inMeets target.
Example
A 12 ft run with 3 inches of fall has about 2.08% slope. A 2% target over 12 ft needs about 2.9 inches of fall.
Watch the outlet
Slope only helps if water has a place to go. Do not send concentrated flow toward a house, sidewalk hazard, or neighboring property.
Questions
Common checks.
Is more slope always better?
No. Too much slope can erode soil or cause splash at the outlet.
Can I use this for pipe?
Yes for a quick grade check, but pipe capacity and local code are separate questions.