Drain Slope Calculator
Calculates required drain pipe slope per IPC code, total fall over run length, and verifies velocity meets self-scouring minimums using Manning's equation.
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How to Use This Calculator
1. Select pipe diameter. Choose the drain pipe size for the section you are designing. Most residential branch drains are 1.5" to 4". Building drains and sewers are typically 4" to 6".
2. Choose pipe material. Manning's roughness coefficient (n) varies by material. PVC and ABS are smooth (n=0.009), cast iron is rougher (n=0.013). Rougher pipes need more slope to maintain self-scouring velocity.
3. Enter DFU load. Total the Drainage Fixture Units on this pipe section. Use our Fixture Unit Calculator if needed. DFU load helps verify the pipe diameter is adequate.
4. Enter pipe run length. The horizontal distance of the drain run. This determines total fall — how many inches the pipe drops end to end.
How the Drain Slope Formula Works
Minimum slope is set by the IPC based on pipe diameter. Velocity is calculated using Manning's equation to verify self-scouring performance.
IPC Minimum Slope: 2.5" and smaller = 1/4" per foot, 3" and larger = 1/8" per foot
Total Fall = Slope (in/ft) x Run Length (ft)
Manning's Equation: V = (1.486 / n) x R^(2/3) x S^(1/2)
Where: n = roughness coefficient, R = hydraulic radius (d/4 for full pipe), S = slope in ft/ft
Self-scouring check: V must be ≥ 2 fps
Manning's equation calculates gravity-flow velocity in open channels and partially-filled pipes. The hydraulic radius (pipe cross-section area divided by wetted perimeter) and slope drive the velocity, with roughness acting as a drag factor.
When To Use This
Rough-in layout. Before cutting into the slab or running drain lines through floor joists, calculate the total fall required so you know if there is enough vertical space for the slope across the pipe run.
Sewer lateral design. Building drain to street sewer connections often have long runs. Calculate total fall to ensure the pipe reaches the main at the correct elevation while maintaining minimum slope.
Diagnosing recurring clogs. If a drain line clogs repeatedly, check if the installed slope meets code. Belly (sag) in the pipe or insufficient slope are common causes of chronic blockages.