Drainage Slope Calculator Formula

Understand the math behind the drainage slope calculator. Each variable explained with a worked example.

Use the Drainage Slope Calculator

Formulas Used

Total Drop

total_drop_in = run_length * slope_fraction

Total Drop

total_drop_ft = run_length * slope_fraction / 12

Slope Percentage

slope_percent = slope_fraction / 12 * 100

Variables

VariableDescriptionDefault
run_lengthPipe Run Length(ft)20
slope_fractionSlope(in/ft)0.25

How It Works

Drainage Slope

Total Drop = Run Length x Slope

The standard slope for most residential drain lines (3-inch and larger) is 1/4 inch per foot. Pipes smaller than 3 inches often require 1/4 in/ft as well, though codes may specify 1/8 in/ft for larger commercial mains.

Worked Example

A 20-foot drain run at the standard 1/4 in/ft slope.

run_length = 20slope_fraction = 0.25
  1. 01Drop = 20 x 0.25 = 5 inches
  2. 02In feet = 5 / 12 = 0.417 ft
  3. 03The outlet must be 5 inches lower than the inlet.

Ready to run the numbers?

Open Drainage Slope Calculator