Slope / Grade Calculator
Enter rise and run in feet to calculate grade percentage, slope ratio, angle in degrees, and rise per foot of run. Use for grading, drainage, roads, ramps, and landscape slopes.
Estimate only — not professional advice. Always verify results independently before purchasing materials or beginning work. Terms of Use
What is Slope / Grade?
A slope calculator converts rise and run measurements into the standard formats used in construction, civil engineering, and landscape design. Slope (also called grade or gradient) describes the steepness of a surface and is expressed in four common ways: as a percentage, as a ratio, as an angle in degrees, and as inches of rise per foot of horizontal run. Each format serves a specific purpose in different trades and code requirements. Grade percentage is the most common slope expression in civil engineering and site grading. It equals the rise divided by the run, multiplied by 100. A 6-foot rise over a 100-foot run equals a 6 percent grade. Grade percentage is used for road design, parking lot drainage, septic system leach fields, and ADA ramp compliance. Positive grades go uphill; negative grades go downhill relative to the direction of travel. Slope ratio expresses the relationship as 1:X, meaning one unit of vertical rise for every X units of horizontal run. A 6-foot rise over 100 feet of run is a 1:16.67 slope. Slope ratios are standard in earthwork, embankment design, and cut-and-fill calculations. The Corps of Engineers and OSHA use slope ratios for trench safety — for example, a 1:1.5 slope (or 1.5 horizontal to 1 vertical) is the maximum for Type B soil in trenching. The angle in degrees is the geometric angle between the sloped surface and a horizontal plane, calculated as the arctangent of rise over run. A 6 percent grade corresponds to 3.43 degrees. Angles are useful for structural engineering, roof design, and any application requiring trigonometric calculations. Wheelchair ramps must not exceed 4.76 degrees (1:12 slope or 8.33 percent grade) per ADA standards. Rise per foot of run, expressed in inches, is commonly used by plumbers for drain pipe slope. The standard drainage slope for most residential waste pipes is 1/4 inch per foot (2.08 percent grade). Storm drains and gutters typically require 1/8 to 1/4 inch per foot. This format translates directly to what a plumber measures with a level and tape on a pipe run. Understanding slope is critical for preventing water damage, ensuring accessibility, meeting building codes, and designing safe earthwork. Too little slope causes ponding and drainage failure; too much slope causes erosion and accessibility problems.
How to Calculate
- Measure the vertical rise (height change) in feet
- Measure the horizontal run (distance) in feet
- The calculator displays grade percentage, slope ratio, angle, and rise per foot
- For drainage, compare the rise per foot value against code requirements (typically 1/4 inch per foot for drain pipes)
- For ADA ramps, verify the grade does not exceed 8.33% (1:12 slope)
- For earthwork, use the slope ratio for cut and fill design
Formula
Grade Percentage = (Rise / Run) x 100 Slope Ratio = 1 : (Run / Rise) Angle (degrees) = arctan(Rise / Run) x (180 / pi) Rise per Foot = (Rise / Run) x 12 inches Where arctan is the inverse tangent function and pi = 3.14159. The run is guarded with a minimum of 0.1 ft to prevent division by zero.
Example Calculation
A slope with 6 feet of rise over 100 feet of run: Grade Percentage = (6 / 100) x 100 = 6.00% Slope Ratio = 1 : (100 / 6) = 1:16.67 Angle = arctan(6 / 100) x (180 / pi) = arctan(0.06) x 57.2958 = 3.43 degrees Rise per Foot = (6 / 100) x 12 = 0.72 in/ft
Frequently Asked Questions
What is the minimum slope for drainage away from a building?
The International Residential Code requires a minimum 5 percent slope (6 inches of fall in the first 10 feet) away from the foundation. Beyond 10 feet, the grade should slope at least 2 percent (1/4 inch per foot) to direct surface water away from the structure. Poor grading is the most common cause of basement water problems.
What slope is required for drain pipes?
The standard slope for residential drain pipes (3-inch and 4-inch) is 1/4 inch per foot (approximately 2.08 percent grade). Pipes 2 inches and smaller require 1/4 inch per foot minimum. Some codes allow 1/8 inch per foot for pipes 4 inches and larger. Too much slope (over 1/2 inch per foot) can cause liquids to outrun solids, leading to clogs.
What is the maximum slope for an ADA-compliant ramp?
ADA requires a maximum slope of 1:12 (8.33 percent grade or 4.76 degrees) for any ramp along an accessible route. The maximum rise for any single ramp run is 30 inches, after which a level landing is required. Cross-slope must not exceed 1:48 (2.08 percent). These requirements apply to all public buildings and most commercial construction.
How do I measure slope in the field?
Use a long level (4 feet or longer) placed horizontally. Measure the gap between the low end of the level and the ground surface. Divide the gap by the level length to get the slope. For example, a 1-inch gap over a 4-foot level is 1/48 = 2.08 percent (1/4 inch per foot). For longer distances, use a transit, laser level, or string line with a line level.
What is the difference between slope and grade?
In common construction usage, slope and grade are interchangeable and both describe steepness as rise over run. Technically, grade usually refers to the percentage (6% grade), while slope can refer to the ratio (1:16.67) or the angle. In road engineering, grade is always a percentage. In earthwork, slope is usually a ratio. Context determines the format.
What slope is too steep to mow safely?
Most riding mowers are rated for slopes up to 15 degrees (approximately 27 percent grade or 1:3.7 ratio). Push mowers can handle slightly steeper grades but become dangerous above 20 degrees. Slopes steeper than 3:1 (33 percent, 18.4 degrees) should use ground cover, retaining walls, or terracing instead of maintained turf.