Slope and grade calculator
Percent grade, ratio, degrees, and fall per foot. Work it either way: measure what you have, or figure the fall you need over a run.
Ratio is written run:rise, so 3:1 means 3 feet across for every 1 foot up. Shoot your grades on site before you cut or set pipe.
- Sewer / drain pipe, 4 inch: 2.08%. The common minimum, 1/4 inch per foot. Flatter than this and solids stall.
- Sewer / drain pipe, 6 inch: 1.04%. 1/8 inch per foot. Bigger pipe carries at a flatter grade.
- Grade away from a foundation: 5%. 6 inches of fall in the first 10 feet is what most codes ask for.
- Lawn and turf areas: 2%. 1 to 2 percent sheds water without scalping when it gets mowed.
- Parking lot and paved areas: 2%. Usually 1 to 5 percent. Flat spots are where the puddles and the ice show up.
- Driveway, comfortable: 8%. Up to about 8 percent rides fine. Past 12 to 15 percent you start scraping bumpers.
- ADA ramp, maximum: 8.33%. 1:12 is the hard ceiling for an accessible ramp.
- Mowable bank, maximum: 33.3%. 3:1. Steeper than this and nobody is putting a mower on it.
These are the common rules of thumb, not code. Local code and the engineer on the job win every time.
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Grade is only half of it. What the pipe sits on and how the ground was put back decide whether it holds.
Common questions
Percent grade is the fall divided by the distance, times 100. A 2 percent grade drops 2 feet over 100 feet, which works out to about 1/4 inch of fall per foot.
The common minimum is 1/4 inch per foot for 4 inch pipe, which is about 2 percent, and 1/8 inch per foot for 6 inch pipe, about 1 percent. Too flat and solids stall in the pipe. Too steep and the water outruns the solids and leaves them behind. Your local code governs, so confirm before you set pipe.
Most codes want 6 inches of fall in the first 10 feet away from the building, which is 5 percent. Getting this wrong is the single most common cause of a wet basement.
Three feet of horizontal run for every 1 foot of vertical rise. That is about 33 percent, or roughly 18 degrees. 3:1 is the usual practical limit for a bank you still intend to mow.
Divide the inches of fall by 12, then multiply by 100. So 1/4 inch per foot is 0.25 divided by 12, times 100, which is about 2.08 percent.
In the field they get used for the same thing. Grade is usually said as a percent and slope is usually said as a ratio, but both describe how much the ground rises or falls over a distance.
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