Plumbing Drain Pipe Slope Calculator

What is a drain pipe slope calculator?

Drain pipes that won’t drain lead to standing water. And standing water is a problem! That’s why plumbers factor in slope. The slope of a drain pipe is also known as the grade or fall. It’s the angle at which the pipe is installed to ensure the proper flow of wastewater and sewage. The slope is usually expressed as a percentage or ratio, like inches per foot of pipe. A drain pipe slope calculator helps plumbers get the ratio right so they can use gravity to move the contents of the pipe efficiently without allowing solids to settle and clog the pipe.

Why is the slope of a drain pipe important?

Drain pipes should slope slightly downhill to drain accordingly. Proper slope ensures that wastewater flows smoothly through the pipe, preventing backups and clogs. If the slope is too steep, water can flow too quickly, leaving solids behind. If it’s too shallow, water can move too slowly, leading to sediment buildup. The ideal slope is ¼ inch per foot of pipe, ranging no more than 3 inches. That means for every foot the pipe travels east and west, it should drop ¼ inch vertically.

Can you install a drain pipe without a slope?

Nope. Bad idea! Installing a drain pipe without a slope can lead to standing water and sewage in the pipe, causing backups, unspeakable odors, and health hazards.

How do you calculate the slope of a drain pipe?

Pipe Slope Formula
The following formula is used to calculate the pipe slope:

PS = PF/PL × 100

Where:
PS is the pipe slope (%).
PF is the pipe fall (ft).
PL is the pipe length (ft).

To calculate a drain pipe slope, first determine the pipe fall (ft) and the pipe length (ft). Once you have those measurement, divide the pipe fall by the pipe length, then multiply by 100.

Example
Let’s calculate the slope of a drain pipe that has a pipe fall of 25 feet and a pipe length of 97 feet.

PS = 25 / 97 × 100 = 25.77%

The pipe slope is 25.77%.