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Slope calculator
Turn rise and run, or two elevations and a distance, into percent grade, ratio, degrees and drop per foot, or work backward from a target slope to the total drop.
Measure run level, not along the ground. On gentle slopes the difference is tiny.
Use spot elevations from a survey or plan, or heights you shot with a level. A is the starting point.
Example: a patio running 12 ft out from the house at 2%.
Enter numbers greater than zero for the distances.
How it compares with common targets
In central PA? We regrade yards and build patio pitch into the base. See our grading and drainage work.
How it works
Percent grade = rise ÷ run × 100, with both in the same units. Ratio = 1 : (run ÷ rise). Two elevations: rise = elevation A − elevation B.
Angle = arctan(rise ÷ run). Drop per foot = rise ÷ run × 12 inches. Drop per meter = rise ÷ run × 1,000 mm.
Total drop = slope (as a decimal) × distance. 2% over 12 ft = 0.02 × 12 = 0.24 ft, or 2.88 in. (about 2 7/8 in.).
1 ft of rise over 50 ft: 1 ÷ 50 = 0.02 → 2% · ratio 1 : 50 · arctan(0.02) = 1.15° · 0.02 × 12 = 0.24 in. per foot
¼ in. per foot = 0.25 ÷ 12 = 2.08% · 1 in. per foot = 8.33% = 1 : 12
Common slope targets
| What | Slope | Per foot | Ratio | Source |
|---|---|---|---|---|
| Paver patio, minimum | 1.5% | 0.18 in. (about 3/16) | 1 : 67 | CMHA PAV-TEC-002 |
| Paver patio, maximum | 2% | 0.24 in. (about 1/4) | 1 : 50 | Unilock |
| Hard surfaces within 10 ft of a foundation | 2% min. | 0.24 in. | 1 : 50 | IRC R401.3 |
| Ground away from a foundation: 6 in. of fall in the first 10 ft | 5% | 0.6 in. | 1 : 20 | IRC R401.3 |
| New lawn graded away from a building | 1–4% | 0.12–0.48 in. | 1 : 100 to 1 : 25 | Penn State Extension |
| Walkway with a non-slip surface, maximum | 5% | 0.6 in. | 1 : 20 | Unilock |
| Wheelchair ramp, maximum running slope (30 in. max rise per run) | 8.33% | 1 in. | 1 : 12 | 2010 ADA Standards 405.2, 405.6 |
Your township or code office has the final word; Pennsylvania’s Uniform Construction Code is based on the IRC. Within the first 10 ft of a house, the code’s 6 in. of fall (5%) governs; Penn State’s 1–4% is general guidance for lawn grading. We don’t list a driveway maximum: limits for private driveways are set locally, so ask your municipality.
Related guides: how much slope a patio needs, sloped backyard ideas and why your yard holds water after it rains.
Questions
How do you calculate percent slope?
Divide the rise by the run, using the same units for both, and multiply by 100. One foot of rise over 50 feet of run is 1 ÷ 50 = 0.02, or 2%.
What is 1/4 inch per foot as a percent?
0.25 ÷ 12 = 0.0208, or about 2.08%. Going the other way, 2% is 0.24 inches per foot, and 1 inch per foot is 8.33%, a 1:12 ratio.
How much should the ground slope away from a house?
The International Residential Code, section R401.3, asks for at least 6 inches of fall within the first 10 feet of a foundation, which is 5%. Where that isn’t possible, drains or swales must carry water away, and hard surfaces within 10 feet must slope at least 2% away from the building.
How much slope does a paver patio need?
CMHA calls for a minimum of 1.5%, and Unilock caps patios at 2%, so about 3/16 to 1/4 inch per foot. A patio that runs 12 feet out from the house at 2% drops about 2 7/8 inches.
What is the maximum slope for a wheelchair ramp?
Under the 2010 ADA Standards, a ramp’s running slope can be no steeper than 1:12, or 8.33%, and each run can rise no more than 30 inches.
How do I convert percent grade to degrees?
Degrees = arctan(percent ÷ 100). A 2% slope is about 1.15 degrees, a 1:12 ramp is about 4.76 degrees, and a 100% slope is 45 degrees.
Sources: CMHA, Construction of Interlocking Concrete Pavements, PAV-TEC-002 (slopes minimum 1.5%); Unilock, pavers on a slope (patios max 2%; walkways 5% with non-slip surface); 2021 International Residential Code, R401.3 Drainage (6 in. fall within 10 ft; impervious surfaces 2% min.); Penn State Extension, Lawn Establishment (1–4% slope away from foundations); U.S. Access Board, Guide to the ADA Standards, Chapter 4 (ramp running slope 1:12 max; 30 in. max rise per run).
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