Drain Slope Calculator

Calculate the required drop over a drain run based on pipe length and slope standard.

Calculator

Pipe Length (ft)

Slope Standard

Custom

Total Drop

0.00 in

0.000 ft

Enter values above and click calculate.

How It Works

This calculator determines how much vertical drop is required over a horizontal drain run to maintain proper drainage flow and prevent blockages. Use it when roughing in drain lines or verifying existing slope before inspection.

Drop (in) = Length (ft) x Slope (in/ft)

Where:
Pipe Length (ft) = total horizontal run of the drain pipe in feet
Slope Standard (in/ft) = the fall per foot selected from the dropdown - use custom to enter your own value
Drop (in) = total vertical drop required over the full run in inches

Typical slope standards:
¼" per foot — standard minimum for 3" pipe and under
⅛" per foot — permitted minimum for pipes larger than 3"
½" per foot — steep slope for short runs only, avoid on long runs

Common example: A 20ft drain run at ¼" per foot requires a total drop of 5.00 inches (0.417 ft).