Drainage Slope Calculator
Calculate drainage slope as a percent grade and in degrees from rise and run, to check it against minimum drainage guidelines.
How this calculator works
Drainage slope describes how much a surface drops over a given distance, which determines whether water runs off or sits and pools. This calculator takes a rise (the vertical drop) and a run (the horizontal distance) and converts them into both a percent grade and an angle in degrees.
Formula: Slope percent = (rise in feet ÷ run in feet) × 100. Angle = arctangent(rise ÷ run).
Worked example
A 6-inch rise over a 25 ft run:
- Rise: 6 in = 0.5 ft
- Run: 25 ft
- Slope: 0.5 ÷ 25 × 100 = 2.0%
- Angle: arctangent(0.5 ÷ 25) ≈ 1.15°
Notes
A common minimum for reliable surface drainage — French drains, or yard grading that carries water away from a foundation — is about 1-2% slope. Anything flatter than that risks standing water, since there isn’t enough drop to reliably move water along. Anything much steeper than roughly 5-6% can erode without some form of stabilization, since faster-moving water is more likely to carry soil with it.
Grading immediately next to a house foundation typically wants at least 2% slope for the first several feet, so water is directed away from the foundation rather than pooling against it.
How to use
Enter the rise (vertical drop) and run (horizontal distance) between two points on the surface you’re grading, in the same units. The calculator returns the slope as both a percent grade and an angle in degrees, which you can compare against typical minimum and maximum drainage slope guidelines.
A planning estimate. Local drainage codes and soil type affect the actual minimum slope needed — confirm with a civil engineer or your local building department for anything code-governed.
Frequently asked questions
How do I calculate drainage slope as a percentage?
Divide the rise (the change in height, in feet) by the run (the horizontal distance, in feet), then multiply by 100. A 6-inch rise over a 25 ft run is a rise of 0.5 ft, giving 0.5 ÷ 25 × 100 = 2.0% slope.
What is a good minimum slope for yard drainage?
A common minimum for reliable surface drainage, such as French drains or grading a yard away from a foundation, is about 1-2% slope. Anything flatter than that risks standing water because gravity isn't pulling it away fast enough.
Can a slope be too steep for drainage?
Yes. Anything much steeper than about 5-6% can start eroding without some form of stabilization, like grass, mulch, or a hardscaped channel, since the water moves fast enough to carry soil with it. The right slope balances moving water away without washing the ground away too.
How much slope do I need right next to my house foundation?
Grading away from a house foundation typically wants at least a 2% slope for the first several feet, so water moving off the roof and ground near the foundation is directed away rather than pooling against it.
Estimates only. Verify quantities with your supplier before purchasing.