Estimates only. Check measurements, product labels and local building codes before you buy or build.
How the Pipe Slope Calculator works
Drain lines are laid with a steady fall so waste keeps moving. This calculator works it both ways: give the run and a slope (1/4", 1/8" or any inches per foot) to get the total drop, or give the run and a measured drop to get the slope. It shows the slope in inches per foot, percent and degrees, rounds the drop to the nearest 1/16", and compares it with the IPC minimum for the drain size.
Drop = run in feet × slope in inches per foot, and slope = drop ÷ run in feet. Percent grade = slope ÷ 12 × 100, and the angle is arctan(slope ÷ 12). So 1/4" per foot is 2.08% (1.19°), 1/8" per foot 1.04% (0.60°) and 1/16" per foot 0.52% (0.30°).
Example: 40 ft of 2" drain at 1/4" per foot. Drop = 40 × 0.25 = 10". That is a 2.08% grade, and it meets the 1/4" per foot IPC minimum for a 2" drain.
The minimums are from IPC Table 704.1: 1/4" per foot for 2½" and smaller, 1/8" per foot for 3" to 6", 1/16" per foot for 8" and larger. The IRC (Table P3005.3) uses the same values for houses, and the UPC (708.1) calls for 1/4" per foot, allowing 1/8" for 4" and larger only where the AHJ approves. Your local code or amendments may differ, so treat the check as a reference, not an approval.
Under the IPC (Table 704.1): 1/4" per foot for pipe 2½" and smaller, 1/8" per foot for 3" to 6", and 1/16" per foot for 8" and larger. The UPC (708.1) calls for 1/4" per foot, with 1/8" allowed for 4" and larger pipe where the AHJ approves it. Check your local code.
How much fall per foot does a 3 or 4 inch drain need?
1/8" per foot under the IPC (3" to 6" pipe). Over 25' 6" of run that is 25.5 × 0.125 = 3 3/16" of fall. Under the UPC, 1/4" per foot is the rule and 1/8" needs the AHJ's approval.
What is 1/4 inch per foot in percent?
2.08%: 0.25 ÷ 12 × 100. In degrees it is arctan(0.25 ÷ 12) = 1.19°. 1/8" per foot is 1.04% (0.60°).
How do I find the slope of a pipe that is already in?
Measure the run and the difference in height between the two ends, then divide: slope = drop ÷ run in feet. A 30 ft run that falls 5" is 5 ÷ 30 = 0.167" per foot (1.39%), between 1/8" and 1/4": enough for a 3" to 6" drain under the IPC, not for 2½" and smaller. Pick Find the slope to do it in the calculator.