How this calculator works
Slope is how much a surface rises for the distance it runs. Grade is the rise divided by the run, as a percent. The angle is the arctangent of the grade, and the ratio is the run for each unit of rise. If you know the grade, the rise is the grade times the run, and the run is the rise divided by the grade.
Worked example: a rise of 6 inches over a run of 12 feet is a 4.17% grade, an angle of 2.39°, a 1:24.0 slope and 0.500 inch per foot. A 2% grade over 20 feet falls 4.80 inches. A ramp at the 8.33% (1:12) limit that rises 30 inches needs a run of 30.0 feet.
Common slopes
Grade, angle, ratio and fall per foot.
| 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 in per 8 ft (pipe) | 1.04% | 0.60° | 0.125 |
| 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 |
Tips before you buy
- Water needs a fall: 2% (1/4 inch per foot) away from the house for the first 10 feet is a common rule, and about 1% (1/8 inch per foot) is the least for a patio to drain. A drain pipe needs 1/8 to 1/4 inch per foot, the steeper figure for small pipes.
- Accessible ramps are limited to 1:12 (8.33%) by the ADA and most codes, with landings. Check your local rules.
- To measure a slope, use a long straight board and a level, or a laser level, and divide the drop by the distance.
- A string line and a line level are accurate enough to set the fall of a drain or a patio.
- Roof pitch is the same idea in a different form: a 6/12 roof rises 6 inches in 12, which is a 50% grade or 26.6 degrees.
Tools & materials for this project
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Frequently asked questions
How do you calculate the slope of a surface?
Divide the rise by the run and multiply by 100. A rise of 6 inches over 12 feet is 0.5 ft over 12 ft = 4.17%.
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 inch per foot (1%) for a patio. A drain pipe needs 1/8 to 1/4 inch per foot, the steeper figure for small pipes.
How do I convert percent grade to degrees?
Take the arctangent of the grade as a decimal: 4.17% is 2.39 degrees. A 100% grade is 45 degrees.
What is the maximum slope for a wheelchair ramp?
1:12, which is 8.33% or 4.8 degrees: one inch of rise for every foot of run. A 30 inch rise needs 30.0 feet of ramp.
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Estimates only. Results assume standard materials and simple shapes. Supplier products vary, and the final call on quantity, structure and safety is yours. Check your local code and your supplier's guidance before ordering.