Calculate full-flow pipe velocity, CFS capacity, and GPM using Manning's equation. Enter diameter, slope, and pipe material to get velocity in fps with self-cleaning check for sewer design. Includes Manning's n reference table.
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Shop Express Tools →Manning's equation is the workhorse of open-channel and full-flow pipe hydraulics. It relates velocity to channel geometry, roughness, and slope. For a circular pipe flowing full, the hydraulic radius R = D/4, where D is the diameter. This simplification makes the equation practical for tabular lookup and field calculation.
ASCE/WEF design standards require a minimum of 2.0 fps at full flow to prevent solids deposition in sanitary sewers. For an 8-inch PVC pipe (n = 0.009), the minimum slope for 2.0 fps is approximately 0.28%. For a 12-inch concrete pipe (n = 0.013), minimum slope for 2.0 fps is about 0.22%. The minimum slope decreases as pipe diameter increases because larger pipes have a more favorable hydraulic radius. For very flat sites, check with the project engineer for alternative designs such as force mains.
This calculator shows full-flow conditions — the pipe flowing 100% full. Gravity sewers are designed to flow at 50–80% full at peak design flow, leaving headroom for surges. Interestingly, maximum velocity in a circular pipe occurs at about 93% full — not at 100% full — due to the reduced wetted perimeter at partial flow. For design flow analysis at partial depths, use hydraulic engineering tables (Hydraulic Design of Highway Culverts, HDS-5, or similar references).
Manning's equation calculates mean flow velocity in open channels and full-flow pipes: V = (1.49/n) × R^(2/3) × S^(1/2), where V is velocity in fps, n is Manning's roughness coefficient, R is hydraulic radius in feet (D/4 for a full circular pipe), and S is the slope as a decimal. The 1.49 factor is for US customary units; use 1.0 for SI units. The equation assumes uniform, steady flow in a straight pipe.
Standard Manning's n values: Concrete pipe (RCP) — 0.013; PVC pipe (SDR-35, smooth bore) — 0.009; HDPE smooth-interior — 0.009; Corrugated metal pipe (CMP) — 0.024; Cast iron — 0.013; Brick sewer — 0.015; Vitrified clay — 0.013. Use the lowest n value for new, clean pipe. Older pipes with deposits, corrosion, or joint offsets may have higher effective n values. Manufacturer data takes precedence over generic values.
The minimum self-cleaning velocity for gravity sewers is generally 2.0 fps at full flow or at design flow conditions. At this velocity, solids remain in suspension rather than settling and accumulating on the pipe invert. Some standards specify 2.5 fps for force mains and laterals with grease potential. Maximum velocity for sewer pipes is typically 10–12 fps to prevent erosion — velocities above 15 fps in concrete pipe can cause significant invert wear.
Manning's equation for circular pipes gives full-flow (completely full pipe) velocity and capacity. In practice, gravity sewers are designed to flow at 50–80% full at design flow, not 100% full. A pipe at 80% full capacity has approximately 108% of the full-flow velocity (velocity actually peaks around 93–94% full due to the geometry). For partial flow depths, use hydraulic flow tables or a hydraulics textbook. Full-flow values are useful for checking minimum slope requirements and maximum capacity.
Slope % = (Upstream Invert − Downstream Invert) / Pipe Length × 100. Example: upstream invert 105.20 ft, downstream invert 104.70 ft, pipe length 100 ft → Slope = (105.20 − 104.70) / 100 × 100 = 0.50%. Use the Pipe Grade Calculator or Invert Elevation Calculator on Sitemark to compute inverts from station data.
Inside diameter of pipe
Pipe grade in percent (e.g. 0.5 = 0.5%)
RCP, CMP concrete pipe
Manning's n Values for Common Pipe Materials
| Material | Manning's n | Notes |
|---|---|---|
| Concrete | 0.013 | RCP, CMP concrete pipe |
| PVC | 0.009 | SDR-35, schedule 40/80 |
| HDPE (smooth) | 0.009 | Smooth-interior HDPE pipe |
| Corrugated Metal | 0.024 | CMP, galvanized or aluminized |
| Cast Iron | 0.013 | Similar to concrete |
| Brick sewer | 0.015 | Older brick sanitary sewers |