Slope Calculator

What Is Slope in Construction?

Slope measures the steepness of a surface, expressed as the ratio of vertical change (rise) to horizontal change (run). In construction, slope applies to roofs, roads, drainage pipes, landscapes, wheelchair ramps, and any surface that must direct water or provide accessibility. The calculator above converts between rise and run, percentage grade, angle in degrees, and ratio notation, giving you the value in whichever format your project specification requires.

How to Calculate Slope?

Divide the rise by the run. A surface that rises 3 feet over a horizontal distance of 12 feet has a slope of 3/12 = 0.25, or 25%. In degrees, this is arctan(3/12) = 14.04 degrees. In ratio notation, it is 1:4 (one unit of rise per four units of run). For roof pitch, the convention is to express slope as rise per 12 inches of run: 3 inches of rise per 12 inches of run = 3/12 pitch. The calculator accepts any one of these inputs and provides all other equivalent values instantly.

What Are Common Construction Slopes?

ADA wheelchair ramp: maximum 1:12 slope (8.33%, 4.76 degrees). Residential driveway: 2-15% grade (1.15-8.53 degrees). Drainage swale: 1-2% grade (0.57-1.15 degrees). Sewer pipe: 1-2% (1/8 to 1/4 inch per foot). Roof pitch: varies from 1/4:12 (nearly flat) to 12:12 (45 degrees). Yard grading away from foundation: minimum 5% for the first 10 feet. Patio surface: 1-2% slope away from the building for water drainage. Each application has a code-specified minimum and sometimes maximum slope that must be followed for safety, function, and compliance.

How to Convert Between Slope Units?

Percentage to ratio: divide 100 by the percentage. A 5% slope = 1:20 ratio. Percentage to degrees: arctan(percentage/100). A 5% slope = arctan(0.05) = 2.86 degrees. Degrees to percentage: tan(degrees) x 100. A 10-degree slope = tan(10) x 100 = 17.6%. Rise per 12 inches of run to percentage: divide rise by 12 and multiply by 100. A 6/12 pitch = (6/12) x 100 = 50%. These conversions matter because different trades use different conventions. A civil engineer specifies grade in percentage. A plumber uses inches per foot. A roofer uses rise per 12 run. A surveyor uses degrees.

What Is the ADA Maximum Slope for Ramps?

The Americans with Disabilities Act requires a maximum slope of 1:12 (one inch of rise per 12 inches of run, or 8.33%) for ramps accessible to wheelchair users. A ramp serving a 30-inch elevation change must be at least 30 feet long. Ramps exceeding 30 feet in length require an intermediate landing (at least 60 inches long) for rest. Cross slope (side-to-side) must not exceed 1:48 (2.08%). Handrails are required on both sides of ramps with rise greater than 6 inches. These requirements ensure safe, independent navigation for people using wheelchairs, walkers, and mobility aids.

How to Measure Slope in the Field?

A spirit level and tape measure provide a basic slope measurement. Place the level horizontally, lift one end until the bubble centers, and measure the vertical gap at the lifted end. Divide the vertical gap by the level length. A 4-foot level with a 2-inch gap: 2 / 48 = 4.17% grade. A transit or laser level provides precise elevation readings at multiple points along a slope. A digital inclinometer placed on a surface reads the angle directly in degrees. Smartphone apps using the built-in accelerometer can approximate slope measurements but are less accurate than dedicated instruments for construction-grade work.

Slope for Yard Grading and Drainage

Proper yard grading directs surface water away from the building foundation. The minimum recommended slope is 6 inches of fall in the first 10 feet from the foundation wall (5% grade). Flatter areas beyond the initial 10 feet should maintain at least 1-2% grade toward storm drains, swales, or property drainage points. Low spots that collect standing water (birdbaths) indicate negative grading that must be corrected by adding fill dirt. Grading is one of the most effective and least expensive ways to prevent basement water problems, foundation damage, and mosquito breeding around a home.

Slope Distance vs Horizontal Distance

The slope distance (measured along the inclined surface) is always longer than the horizontal distance. The relationship is: slope distance = horizontal distance / cos(angle). At a 10% grade (5.71 degrees), the slope distance is only 0.5% longer than the horizontal distance. At 50% grade (26.57 degrees), it is 11.8% longer. At 100% grade (45 degrees), it is 41.4% longer. This difference matters for material estimation: roofing material covers the slope distance, not the horizontal footprint. A steep driveway uses more pavement per horizontal foot than a flat driveway. The calculator provides the slope distance for your entered rise and run values.

Frequently asked questions