Percent of Fall Calculator

Percent of fall is one of the most important measurements in pipeline and drainage installation. It represents the slope of a pipe – specifically, the vertical drop over a given horizontal distance – expressed as a percentage.

Getting this number right is not optional. Gravity-assisted flow systems like sewer lines and stormwater drainage depend on correct slope to move fluid efficiently. Too little fall and you get stagnation and backflow. Too much and solids outrun the liquid, leaving buildup in the line. Most local building codes specify a minimum percent of fall for drainage systems, so it also has direct compliance implications on inspected work.

How to Calculate Percent of Fall

The formula is straightforward:

(Highest Invert − Lowest Invert) ÷ Length of Pipe × 100 = Percent of Fall

You need three measurements, all in feet:

• Highest invert — the vertical height at the pipe’s starting point
• Lowest invert — the vertical height at the pipe’s ending point
• Length of pipe — the total horizontal run

Decimal values are fine and common in field measurements. For example, a pipe with a highest invert of 102.5 feet, a lowest invert of 101.0 feet, and a length of 50 feet gives you:

(102.5 − 101.0) ÷ 50 × 100 = 3% fall

Use Our Percent of Fall Calculator

Skip the manual math. Our free percent of fall calculator handles the formula instantly – just enter your invert elevations and pipe length and get your result in seconds. You’ll also find additional construction calculators useful for field and estimating work.

Visit our construction calculator page for percent of fall and other useful tools!