Calculate pipe capacity using the Manning Equation for full-pipe gravity flow.
PVC / Plastic (0.009)
Smooth Steel (0.011)
Cast Iron / Ductile Iron (0.012)
Concrete – Smooth (0.013)
Vitrified Clay (0.015)
Corrugated Metal (0.024)
Flow Velocity:0 ft/s
Flow Rate (CFS):0 ft³/s
Flow Rate (GPM):0 Gallons/Min
Flow Rate (MGD):0 Million Gallons/Day
How to Calculate Sewage Flow Rate
Designing an efficient sewer system requires precise calculations to ensure that the pipe diameter and slope can handle the anticipated hydraulic load without overflows or excessive sedimentation. This calculator uses the Manning Equation, which is the industry standard for calculating gravity-driven flow in open channels and pipes.
The Manning Equation Formula
For US Customary units, the formula for velocity is:
V = (1.486 / n) × R^(2/3) × S^(1/2)
V: Velocity (ft/s)
n: Manning's Roughness Coefficient (unitless)
R: Hydraulic Radius (ft), which for a full circular pipe is Diameter / 4
S: Slope of the pipe (ft/ft)
Common Manning's n Values
Material
Typical "n" Value
PVC / HDPE
0.009 – 0.011
Ductile Iron
0.011 – 0.013
Concrete (New/Smooth)
0.012 – 0.014
Brickwork
0.015 – 0.017
Example Calculation
Suppose you have an 8-inch PVC pipe with a 2% slope:
This flow rate converts to roughly 1,100 Gallons Per Minute (GPM). If your actual peak sewage inflow exceeds this, you would need a larger pipe or a steeper slope.
function calculateSewageFlow() {
var dInches = parseFloat(document.getElementById("pipeDiameter").value);
var slopePercent = parseFloat(document.getElementById("pipeSlope").value);
var n = parseFloat(document.getElementById("manningN").value);
var resultDiv = document.getElementById("sewageResult");
if (isNaN(dInches) || isNaN(slopePercent) || dInches <= 0 || slopePercent <= 0) {
alert("Please enter valid positive numbers for diameter and slope.");
return;
}
// Conversion to feet
var dFeet = dInches / 12;
var slopeDecimal = slopePercent / 100;
// Area of full pipe (PI * r^2)
var radius = dFeet / 2;
var area = Math.PI * Math.pow(radius, 2);
// Hydraulic Radius (R = Area / Wetted Perimeter)
// For full pipe: (PI * r^2) / (2 * PI * r) = r/2 = D/4
var hydraulicRadius = dFeet / 4;
// Manning's Equation for Velocity (ft/s)
// V = (1.486/n) * R^(2/3) * S^(1/2)
var velocity = (1.486 / n) * Math.pow(hydraulicRadius, (2/3)) * Math.sqrt(slopeDecimal);
// Flow Rate Q = V * A (Cubic Feet per Second)
var flowCFS = velocity * area;
// Unit Conversions
// 1 CFS = 448.831 GPM
var flowGPM = flowCFS * 448.831;
// 1 MGD = 1.547 CFS
var flowMGD = flowCFS / 1.547;
// Display Results
document.getElementById("velocityResult").innerText = velocity.toFixed(2);
document.getElementById("cfsResult").innerText = flowCFS.toFixed(3);
document.getElementById("gpmResult").innerText = Math.round(flowGPM).toLocaleString();
document.getElementById("mgdResult").innerText = flowMGD.toFixed(3);
resultDiv.style.display = "block";
resultDiv.scrollIntoView({ behavior: 'smooth', block: 'nearest' });
}