Rise and run in, every format out: percent grade for site work, degrees for the math, ratio for the plans, and inches-per-foot for the level.
Slope inputs
ft
ft
Grade
—
Angle—
Ratio—
Inches per foot—
Fall per 100 ft—
Handy targets: drainage away from a foundation = 5% for the first 10 ft (6 in of fall). Sewer pipe = 1/4 in per foot (2%) for 3-in and smaller. ADA ramps = 1:12 max (8.33%).
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One slope, four languages
Percent grade is rise over run × 100 — the language of site work and roads. Degrees is the trig angle. Ratio (1:20) is how plans and ADA specs talk. Inches per foot is what your level and your pipe laser speak. They're all the same hill.
A yard that falls 2 ft over 100 ft is a 2% grade — 1.15°, a 1:50 ratio, about 1/4 inch per foot. That happens to be the standard minimum for getting water to move across finished grade, and exactly the classic slope for small drain pipe.
Numbers worth memorizing
Foundations want 5% falling away for the first 10 ft (that's 6 inches — more than most yards have). Driveways get uncomfortable past 10–12% and sketchy in ice past 15%. A 45° slope is a 100% grade, which surprises everyone the first time — percent and degrees are not the same scale.
Frequently asked questions
Is a 100% grade straight up?
No — 100% grade is a 45° angle, where rise equals run. Straight up would be an infinite percent. This is the most common slope misconception on any job site.
What slope do I need for drainage away from a house?
The standard is 5% for the first 10 feet from the foundation — a 6-inch drop. After that, 2% keeps surface water moving across lawns and swales.
What is the proper fall for a sewer or drain pipe?
1/4 inch per foot (about 2%) for pipes 3 inches and smaller; 1/8 inch per foot is commonly allowed at 4 inches and up. Too steep is a real problem too — liquids outrun solids and leave them behind.
🧰 From the same shop: HouseMath — project calculators for the house and yard (mulch, paint, flooring, fence, pavers) · AxleMath — trucker calculators (tandem slide, cost per mile, fuel, per diem).