Drainage Pipe Calculator

Civil Hydrology & Manning's Flow Equation

Gravity drainage pipe hydraulics follow Manning's empirical equation:

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• Manning's Full-Flow Equation**:

Q = (1.486) / (n) · A · R_h^2/3 · S^1/2

Where:

• Q: Discharge capacity in Cubic Feet per Second (CFS)

• n: Roughness coefficient

• A = (π D^2) / (4): Cross-sectional pipe area (sq ft)

• R_h = (D) / (4): Hydraulic radius for full circular pipe (ft)

• S = Slope\%100: Hydraulic gradient / slope

• Flow in GPM: Q_GPM = Q_CFS × 448.831

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• Slope & Elevation Fall Formula**:

Total Fall (inches) = Length (ft) × (Slope\%100) × 12

• 0.5\% Slope = 1/16" per foot

• 1.0\% Slope = 1/8" per foot (Standard)

• 2.0\% Slope = 1/4" per foot (Code Standard)

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• Standard Pipe Capacity Benchmarks (@ 1% Slope, Smooth PVC)**:

• 3" Pipe: 51.2 GPM (0.114 CFS)

• 4" Pipe: 111.0 GPM (0.247 CFS)

• 6" Pipe: 350.7 GPM (0.781 CFS)

• 8" Pipe: 762.6 GPM (1.699 CFS)

• 12" Pipe: 2,324.2 GPM (5.178 CFS)

How to Use This Calculator

Enter your Drainage Trench Length (e.g. 50 in ft), Drainage Pipe Diameter — choose from 3" Pipe (Downspout Diverter / Small Garden Run), 4" Pipe (Standard Residential French Drain & Yard Runoff), 6" Pipe (High Volume Stormwater / Commercial / Long Runs), Pipe Slope / Gradient — choose from 0.5% (1/16" per ft - Minimum Flat Grade), 1.0% (1/8" per ft - Standard French Drain Minimum), 2.0% (1/4" per ft - Standard Gravity Flow Code), Pipe Material Type — choose from Smooth Interior PVC / SDR-35 / Dual-Wall HDPE (n = 0.010 - High Velocity), Single-Wall Corrugated Flexible Polyethylene (n = 0.015), and Civil & Drainage Benchmark — choose from Custom Drainage Layout, 50 ft Residential French Drain (4" PVC, 1% Slope) → 111 GPM Flow / 6" Total Fall, 100 ft Yard Drainage Swale (4" Corrugated, 1% Slope) → 74 GPM / 12" Fall. Measure twice before entering dimensions — small measurement errors scale up significantly in material estimates.

The Formula

D_ft = D_in / 12

Velocity (Manning's): V = (1.486 / n) (Rh)^(2/3) (S)^(1/2)

Geotextile Fabric: Trench perimeter + 6" overlap = (1 + 21.5 + 0.5) = 4.5 ft width Length

Worked Example

A civil contractor designing a 50-foot French drain selects a 4-inch smooth PVC pipe with a 1.0% grade (6.0" total drop), confirming that its 111.0 GPM capacity easily drains a 1,500 sq ft roof catchment during a heavy 2-inch/hr downpour.

Understanding Your Result

IPC/UPC residential water supply sizing: 3/4" main line typically serves homes up to 3,500 sq ft. Supply pressure between 40–80 psi is standard; pressure reducing valve (PRV) required when street pressure exceeds 80 psi. Drain slope: 1/4" per foot fall for 3" and smaller pipes; 1/8" per foot for 4" drain lines. Fixture unit calculations (DFU) determine drain line sizing — consult the applicable plumbing code for local adoption.