Getting drain slope right is the difference between a line that self-scours and one that clogs. This calculator converts freely between slope, run length, and total fall so you can lay out and cut pipe to the correct gradient, and it checks the result against the IPC 704.1 minimum.
How it works
The three quantities are tied together by one relationship:
fall (in) = slope (in/ft) × run (ft)
slope (in/ft) = fall (in) ÷ run (ft)
run (ft) = fall (in) ÷ slope (in/ft)
percent grade = slope (in/ft) × 100 ÷ 12The standard slopes are 1/4 inch per foot for pipe up to 3 inches and 1/8 inch per foot permitted for 3 inch and larger, with 1/2 inch per foot used where extra fall is available. The tool flags any slope below the 1/8 inch per foot floor.
Standard slope quick-reference
| Slope | In/ft | % Grade | Use case |
|---|---|---|---|
| 1/8″/ft | 0.125 | 1.04% | 3″ and larger pipe minimum |
| 1/4″/ft | 0.25 | 2.08% | Standard for all drain pipe up to 3″ |
| 1/2″/ft | 0.5 | 4.17% | Short runs, extra fall available |
Worked examples
Finding fall: A bathroom drain runs 14 feet to the stack at the code minimum 1/4 inch per foot. Fall = 0.25 × 14 = 3.5 inches total drop. Mark the upstream invert 3.5 inches higher than the downstream entry.
Finding run from available fall: A 4-inch sewer line enters a cleanout with 9 inches of fall available before hitting an obstacle. At the 1/8 inch per foot minimum, the maximum run is 9 ÷ 0.125 = 72 feet. At the preferred 1/4 inch per foot it is 9 ÷ 0.25 = 36 feet.
Checking an existing slope: You measure a pipe that drops 6 inches over 28 feet. Slope = 6 ÷ 28 ≈ 0.214 inch per foot. That is below the 1/4 inch code minimum for a small-diameter line, so you would flag it for correction.
Common mistakes to avoid
Confusing run with pipe length. Run is the horizontal distance, not the distance along the pipe itself. For nearly flat lines the difference is negligible, but it matters in tight basement situations with noticeable vertical drop.
Going too steep on long runs. At 1/2 inch per foot over a 30-foot run you drop 15 inches — manageable. Over a 60-foot run that becomes 30 inches, which can make fitting connections and stack entries very difficult. Use the steeper slope only where the run is short.
Measuring at the wrong point. Slope is invert to invert (the bottom inside of the pipe), not outside-to-outside. Measuring to the outside of a large-diameter pipe and forgetting the pipe wall can make the slope look acceptable when the flow line is actually flat.