Slope and Grade: How Much Fall for Drainage, Patios and Ramps
Part of the project planners: Planning a new driveway and Planning retaining walls and drainage.
Slope is how much a surface falls for the distance it runs. Water needs a fall of at least 1/4 inch per foot (about 2%) to run off a patio or away from a house, a drain pipe needs 1/8 to 1/4 inch per foot, and an accessible ramp must not be steeper than 1:12 (8.33%). The slope and grade calculator converts between rise, run, percent, degrees and ratio.
Common slopes
| Slope | Grade | Angle | Inches per foot |
|---|---|---|---|
| 1/8 in per ft | 1.04% | 0.60° | 0.125 |
| 1/4 in per ft | 2.08% | 1.19° | 0.250 |
| 1:20 | 5.00% | 2.86° | 0.600 |
| 1:12 (ramp) | 8.33% | 4.76° | 1.000 |
| 1:10 | 10.00% | 5.71° | 1.200 |
| 1:8 | 12.50% | 7.13° | 1.500 |
| 1:4 | 25.00% | 14.04° | 3.000 |
| 1:1 (45°) | 100.00% | 45.00° | 12.000 |
Fall for drainage
The drop over a given run, at a few common grades:
| Grade | Run | Fall | Per foot | Angle |
|---|---|---|---|---|
| 1% | 10 ft | 1.20 in | 0.120 in/ft | 0.57° |
| 2% | 10 ft | 2.40 in | 0.240 in/ft | 1.15° |
| 2% | 20 ft | 4.80 in | 0.240 in/ft | 1.15° |
| 5% | 10 ft | 6.00 in | 0.600 in/ft | 2.86° |
- Away from the house: 5% (about 6 inches in 10 feet) for the first 10 feet is the usual advice for soil, and 2% for hard surfaces.
- Patios and walks: 1/8 to 1/4 inch per foot, in the direction you want the water to go, and no more than about 2% across.
- Drain pipe: 1/8 to 1/4 inch per foot, the steeper figure for small pipes. Too little leaves solids behind; too much runs the water past them.
- Driveways: a steady slope of 1/4 inch per foot or more sheds water, and most are 5 to 10% along their length.
Ramp length for each rise
At the 1:12 (8.33%) limit for accessible ramps:
| Rise | Run needed |
|---|---|
| 6 in | 6.0 ft |
| 12 in | 12.0 ft |
| 18 in | 18.0 ft |
| 24 in | 24.0 ft |
| 30 in | 30.0 ft |
| 36 in | 36.0 ft |
Ramps also need landings at the top and bottom, and at least every 30 inches of rise, plus handrails. Check your local code.
How to measure slope
- Use a long straight board and a level, or a laser level, and measure the gap at one end when the board is level.
- Divide the drop by the distance in the same units.
- For a drain or patio, stretch a string with a line level between stakes, and measure down from it.
- Check the fall after you place the base, and again before the surface goes on.
See also the French drain calculator and the French drain materials list. Estimates only: check your local code.
Frequently asked questions
What slope do I need for drainage?
At least 1/4 inch per foot (about 2%) away from a house for the first 10 feet, and about 1/8 to 1/4 inch per foot for a patio or driveway. A drain pipe needs 1/8 to 1/4 inch per foot (1 to 2 inches in 8 feet), the steeper figure for small pipes.
How do I calculate slope?
Divide the rise by the run and multiply by 100. A rise of 6 inches over 12 feet is 4.17%, an angle of 2.39 degrees.
How do I convert percent grade to degrees?
Take the arctangent of the grade as a decimal. 4.17% is 2.39 degrees, and a 100% grade is 45 degrees.
What is the maximum slope for a wheelchair ramp?
1:12, which is 8.33% or about 4.8 degrees: one inch of rise for every foot of run. A 30 inch rise needs 30 feet of ramp, plus landings.
How much does a 2% slope fall over 20 feet?
4.8 inches (about 4¾ inches).
More guides
Tools & materials for this project
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Estimates only. Quantities depend on your materials, site and supplier. Confirm important numbers before you order, and follow local building codes for anything structural.