Free Slope & Grade Calculator - Rise, Run, Angle, Pitch (2026)

They all describe the same thing - how steep a surface is - but they use different formats. Grade is expressed as a percentage: a 6% grade means 6 feet of rise per 100 feet of horizontal run. Slope is the general term and can be expressed as a ratio like 1:12 (one foot of rise for every 12 feet of run). Pitch is used in roofing and framing, expressed as X:12, meaning the number of inches a roof rises per 12 inches of horizontal run. A 4:12 pitch roof rises 4 inches for every foot of run.

The simplest field method uses a level and a tape measure. Place a 4-foot level on the surface with one end at your high point. Level it, then measure the gap between the far end of the level and the ground below. That gap is your rise over a 4-foot run. Divide the rise by the run and multiply by 100 for grade percentage. For longer distances, a transit, laser level, or smartphone inclinometer app works well. I always double-check critical grades with a transit rather than relying on a phone.

The maximum ADA ramp slope is 1:12, which equals 8.33%. That means for every inch of vertical rise, you need at least 12 inches of horizontal run. ADA Standards Section 405.2 sets this limit. Ramps must also have level landings at the top and bottom, and every 30 inches of rise. The maximum single run before a landing is 30 feet. Cross slope (side to side) cannot exceed 1:48 or about 2.08%. These are federal minimums - your state or local codes may be stricter.

The IRC (International Residential Code) Section R401.3 requires the ground to slope away from the foundation at a minimum of 6 inches within the first 10 feet, which equals a 5% grade. Many contractors and building inspectors consider 2% the absolute minimum for surface drainage, but 5% or more within that 10-foot zone is the code requirement for most jurisdictions. For flatwork like patios and driveways, a minimum 1% grade (1/8" per foot) keeps water moving. I aim for 2% on concrete flatwork because 1% leaves almost no margin for settling.

To go from grade percentage to roof pitch: divide the grade by 100 and multiply by 12. A 33.33% grade = (33.33/100) x 12 = 4:12 pitch. To go from pitch to grade: divide the pitch number by 12 and multiply by 100. A 6:12 pitch = (6/12) x 100 = 50% grade. You can also convert pitch to angle: arctan(pitch/12). A 4:12 pitch = arctan(4/12) = 18.43 degrees.

Most riding mowers become unsafe above a 15% grade (about 8.5 degrees). Walk-behind mowers can handle up to about 25% (14 degrees), but it depends on conditions and the mower. OSHA and mower manufacturers generally recommend staying off slopes steeper than 15 degrees with any mower. For landscaping purposes, anything over 33% (3:1 ratio or about 18 degrees) is usually stabilized with retaining walls, terracing, or ground cover rather than grass. Erosion control matting is standard on slopes above 33% until vegetation establishes.

Slope length is the actual distance along the sloped surface, measured from the bottom to the top. It is always longer than the horizontal run. You need slope length when ordering materials that cover the surface - roofing, erosion fabric, concrete for ramps, sod on a hillside, or any material measured along the slope, not the flat ground below it. The formula is simple: slope length = square root of (rise² + run²). A ramp with 3 feet of rise and 36 feet of run has a slope length of 36.12 feet.

In roofing, low-slope is anything below 4:12 (18.4°, 33.3% grade). Low-slope roofs need special membranes, underlayment, and flashing details because water moves slowly and can pool. Standard asphalt shingles are typically rated for 4:12 and above. Steep-slope starts at about 8:12 (33.7°, 66.7% grade), where materials need extra fastening and safety harnesses become code-required for workers. Between 4:12 and 8:12 is standard residential pitch, where most shingle and metal roofing products perform best.