Drain Slope Calculator
Calculate the required vertical drop for horizontal drain pipes to achieve proper self-cleaning velocity. Results include total drop in inches and feet, slope percentage, and IPC/UPC code compliance status.
Results
How the Calculation Works
- :
Drop (in) = Pipe Length (ft) × Slope (in/ft) - :
Drop (ft) = Drop (in) ÷ 12 - :
Slope % = (Slope in/ft ÷ 12) × 100
Quick Reference: Pipe Slope by Diameter
| Pipe Diameter | Min Slope (IPC) | Standard Slope | Max Slope |
|---|---|---|---|
| 1-1/2" | 1/4" per ft | 1/4" per ft | 1/2" per ft |
| 2" | 1/4" per ft | 1/4" per ft | 1/2" per ft |
| 3" | 1/8" per ft | 1/4" per ft | 1/2" per ft |
| 4" | 1/8" per ft | 1/4" per ft | 1/2" per ft |
Pro Tips
- [object Object]
- [object Object]
- [object Object]
- [object Object]
Frequently Asked Questions
What slope does the plumbing code require for drain pipes?
The International Plumbing Code (IPC) and Uniform Plumbing Code (UPC) require 1/4" per foot (about 2%) for drain pipes 3 inches in diameter or smaller. Pipes 3 inches and larger may use a minimum of 1/8" per foot. Always check your local authority having jurisdiction (AHJ) for any local amendments.
Can a drain pipe be too steep?
Yes. The maximum allowed slope is 1/2" per foot for horizontal drain pipes. Steeper slopes cause hydraulic separation — the liquid portion races ahead while solids are left behind, building up and eventually causing blockages. Very steep runs should use an approved fitting to slow flow.
What happens if a drain has no slope or negative slope?
A flat drain will pool water and allow solids to settle, leading to chronic clogs and sewer gas buildup. A negatively sloped (uphill) drain will not drain at all. Both situations violate plumbing code and require correction before inspection.
Does drain slope apply to vent pipes?
Vent pipes that run horizontally still need a slight slope (1/8" per foot minimum) back toward the drain to prevent condensation from pooling in the vent. Vertical vent pipes do not require a calculated slope since gravity handles drainage naturally.
Recommended Products
Empire 48" Digital Level
Accurate digital readout for setting drain slope.
View on Amazon →Oatey 4" PVC Drain with Strainer
Standard floor drain for showers and utility areas.
View on Amazon →4" ABS DWV Pipe — 10 ft
Schedule 40 ABS drain pipe for residential drain runs.
View on Amazon →Klein Tools Line Level Set (3-pack)
String line levels for establishing slope over long runs.
View on Amazon →