Slope Calculator: Calculate Grade, Angle, and Ratio for Any Slope
Use our free slope calculator to determine the grade percentage, rise:run ratio, and angle of any slope from just a rise and run measurement. Built for grading contractors, landscapers, and anyone checking drainage or ADA ramp compliance, it returns grade as a percentage, a simplified ratio, and the equivalent angle in degrees.
How to use this calculator.
- Enter the rise. Input the vertical elevation change between your two points, in feet.
- Enter the run. Input the horizontal distance between the same two points, in feet.
- Review your results. The calculator instantly shows grade percentage, the rise:run ratio, and the angle in degrees.
- Compare against code limits. Check your result against ADA, driveway, or drainage requirements before finalizing a design.
How do you calculate slope grade and angle?
Grade percentage is simply the rise divided by the run, expressed as a percentage. The angle is calculated as a linear approximation of the arctangent (rise ÷ run × 57.2958, the number of degrees per radian), which is accurate for the gentle slopes typical of grading and drainage work.
Grade (%) = (Rise ÷ Run) × 100
Ratio = Rise ÷ Run
Angle (°) = (Rise ÷ Run) × 57.2958
Where:
- Rise: The vertical change in elevation between two points, in feet.
- Run: The horizontal distance between the same two points, in feet.
- Ratio: Rise:run expressed as a simplified fraction, such as 1:20, which is often how slopes are called out on plans.
Example calculation.
Let's check the grade on a section of yard being sloped away from a foundation for drainage.
Scenario: The ground rises 5 ft in elevation over a 100 ft run.
- Calculate grade: (5 ft ÷ 100 ft) × 100 = 5.0%
- Calculate ratio: 5 ÷ 100 = 0.05, or 1:20 (rises 1 ft every 20 ft)
- Calculate angle: 0.05 × 57.2958 = 2.9°
Result: This section has a 5.0% grade, a 1:20 ratio, and a slope angle of approximately2.9° — well within the 1–2% minimum for drainage and comfortably below any driveway or ADA maximum.
Tips from the field.
- ADA ramps max out at 8.33% (1:12). A 30 ft ramp at this ratio can rise a maximum of 2.5 ft, and every 30 ft of run requires a 5 ft × 5 ft landing.
- Driveways top out around 12–15%. Keep the first 10–15 ft from the street under 5% so vehicles transition smoothly without scraping.
- Drainage needs a minimum 1–2% slope. Anything flatter and water pools instead of running off; use 1% for paved surfaces and 2% for unpaved ground.
- Road grades typically run 6–12%. The exact maximum depends on design speed and local standards, with steeper grades reserved for lower-speed roads.
- Turf slopes should stay under 25%. Beyond that, mowing becomes difficult and erosion risk climbs, especially on bare or newly seeded ground.
Disclaimer: This calculator provides estimates for planning purposes only. Verify slope requirements against local building codes, ADA standards, and site-specific drainage design before construction. PavementCalculator.com assumes no liability for design or compliance decisions based on these results.
Cost, price, and density figures on this page reflect approximate U.S. averages as of August 2026 and vary by region, supplier, and material availability.
Frequently asked questions.
How do I calculate slope percentage?
Divide the vertical rise by the horizontal run, then multiply by 100. For example, a slope that rises 5 feet over 100 feet = (5 ÷ 100) × 100 = 5% grade. A 5% slope means the ground rises 5 feet for every 100 feet of horizontal distance. Use our calculator above: just enter rise and run values.
What is the maximum slope for a driveway?
The maximum recommended driveway slope is 12-15%. For the first 10-15 feet from the street, keep the slope under 5% for a smooth transition. ADA-compliant ramps require a maximum slope of 8.33% (1:12 ratio). For drainage, a minimum slope of 1-2% is recommended to prevent water pooling.
How do I calculate slope for proper drainage?
For drainage, maintain a minimum 1-2% slope (1-2 feet of drop per 100 feet of run). For a 50-foot yard: 50 × 0.01 = 0.5 feet (6 inches) minimum drop at 1% slope. Steeper slopes (2-5%) provide better drainage but may cause erosion. Use 1% for paved surfaces and 2% for unpaved areas.
What is a 1:12 slope ratio?
A 1:12 ratio means 1 unit of rise for every 12 units of run: this equals an 8.33% grade. This is the maximum slope allowed by ADA for wheelchair ramps. A 30-foot long ramp at 1:12 can rise a maximum of 2.5 feet (30 ÷ 12 = 2.5). For every 30 feet of ramp length, a 5×5 foot landing is required.
How do I convert slope percentage to degrees?
Use the formula: angle = arctan(rise ÷ run) or arctan(grade ÷ 100). For a 5% slope: arctan(0.05) = 2.86°. For a 10% slope: arctan(0.10) = 5.71°. A 45° angle equals a 100% grade (1:1 ratio). Most construction slopes are under 15°, which equals about 27% grade.
What slope is needed for a French drain?
French drains need a minimum 1% slope (1 foot of drop per 100 feet of run) for proper water flow. A 2% slope is ideal: it provides good drainage without causing erosion. For a 50-foot French drain at 2% slope: total drop needed = 50 × 0.02 = 1 foot from start to end.