Drainage Slope Calculator
Enter the run and the fall. See the slope and if it drains properly.
100% in your browser - no upload, no sign-up.
Slope (grade)
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Fall per foot
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Ratio
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Angle
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✅ Good drainage - 2% or steeper meets the ¼ inch per foot rule for hard surfaces.
⚠️ Marginal - it drains, but it's below the ¼ inch per foot (2%) standard for patios and walks.
⛔ Too flat - under 1% will pond. Aim for at least ¼ inch per foot away from the house.
💡 These are planning numbers. Always slope away from the house - a common target is 6 inches of fall over the first 10 feet (about 5%) for the ground next to a foundation. Confirm local grading rules, and never drain toward a neighbor's lot.
⚠️ Measure the fall with a string - a 2% slope is too small to see.
How to use this drainage slope calculator
- Measure the horizontal run of the surface in feet.
- Measure how much it falls over that run - the drop - in inches.
- Read the percent grade, fall per foot, ratio and angle, and check the drainage status.
How drainage slope is calculated
Slope is the vertical fall divided by the horizontal run. Expressed as a percent grade it is fall ÷ run × 100, so 3 inches of fall over a 10-foot (120-inch) run is a 2.5% grade. The same slope as fall per foot is the drop divided by the run in feet - here 0.3 inches per foot. The ratio 1:n is the run divided by the fall in the same units, and the angle is arctan(fall ÷ run). For drainage the benchmark is the ¼ inch per foot rule - about 2% - the minimum fall to keep water moving off a patio, walk or driveway without ponding; under about 1% water tends to sit.
Frequently asked questions
- What slope do I need for drainage?
- For hard surfaces like patios, walkways and driveways the standard minimum is ¼ inch of fall per foot - about a 2% grade - so water sheds instead of ponding. Lawns and planting beds can drain at 2% and up. For the ground right next to a foundation, aim for about 6 inches of fall over the first 10 feet. Enter your run and fall above to check your grade.
- How do I convert slope to inches per foot or a percentage?
- A percent grade is the fall divided by the run times 100; inches per foot is the fall in inches divided by the run in feet. The two relate simply: 1% is about ⅛ inch per foot, and 2% is about ¼ inch per foot. The calculator shows both from a single run and fall so you can spec whichever your plan uses.
- Which way should the ground slope?
- Always slope away from the house and any structure so surface water drains toward the street, a swale or a drain - never toward the foundation or a neighbor's property. A minimum of about 2% (¼ inch per foot) on hard surfaces and roughly 5% for the first 10 feet of soil next to the house are common targets. Local grading codes can be stricter.
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