PVC / Plastic (C=150)
Copper / Brass (C=140)
New Cast Iron (C=130)
Steel – Welded (C=120)
Steel – Riveted (C=110)
Old Cast Iron (C=100)
Corroded Steel (C=80)
Estimated Flow Rate0.00 GPM
Fluid Velocity0 ft/sec
Head Loss0 ft
Pipe Cross-Section Area0 sq in
Calculations based on the Hazen-Williams equation for water at ambient temperature.
function calculatePipeFlow() {
// Get Input Values
var diameterInches = parseFloat(document.getElementById('pipeDiameter').value);
var lengthFeet = parseFloat(document.getElementById('pipeLength').value);
var pressurePsi = parseFloat(document.getElementById('pressureDrop').value);
var cFactor = parseFloat(document.getElementById('pipeMaterial').value);
// Validation
if (isNaN(diameterInches) || diameterInches <= 0 ||
isNaN(lengthFeet) || lengthFeet <= 0 ||
isNaN(pressurePsi) || pressurePsi <= 0) {
alert("Please enter valid positive numbers for diameter, length, and pressure.");
return;
}
// 1. Convert Pressure Drop (PSI) to Head Loss (feet of water)
// 1 PSI = 2.307 feet of head
var headLossFeet = pressurePsi * 2.307;
// 2. Hazen-Williams Equation solved for Flow Rate (Q in GPM)
// Formula: Q = 0.442 * C * D^2.63 * (Hf / L)^0.54
// Where D is diameter in FEET… wait, standard US customary form is often:
// hf = 0.002083 * L * (100/C)^1.85 * (Q^1.85 / d^4.8655) (where d is inches, Q is GPM)
// Let's use the rearranged standard industry formula for Q (GPM):
// Q = ( ( hf * C^1.852 * d^4.87 ) / ( 10.44 * L ) ) ^ (1 / 1.852)
// hf = Head loss in feet
// C = Roughness coefficient
// d = diameter in inches
// L = Length in feet
var numerator = headLossFeet * Math.pow(cFactor, 1.852) * Math.pow(diameterInches, 4.87);
var denominator = 10.44 * lengthFeet;
// Base calculation before exponent
var base = numerator / denominator;
// Final Flow Rate (GPM)
var flowGPM = Math.pow(base, (1 / 1.852));
// 3. Calculate Velocity (V in ft/sec)
// V = (0.4085 * Q) / d^2
// d in inches, Q in GPM
var velocity = (0.4085 * flowGPM) / (diameterInches * diameterInches);
// 4. Calculate Area (sq inches)
var radius = diameterInches / 2;
var area = Math.PI * radius * radius;
// Display Results
document.getElementById('resultGPM').innerText = flowGPM.toLocaleString('en-US', { minimumFractionDigits: 2, maximumFractionDigits: 2 }) + " GPM";
document.getElementById('resultVelocity').innerText = velocity.toFixed(2) + " ft/s";
document.getElementById('resultHeadLoss').innerText = headLossFeet.toFixed(2) + " ft";
document.getElementById('resultArea').innerText = area.toFixed(2) + " in²";
// Show Result Card
document.getElementById('pfcResults').style.display = 'block';
}
Understanding Pipe Flow Capacity
Determining the flow capacity of a pipe is a critical step in designing plumbing, irrigation systems, and industrial piping networks. The flow capacity represents the volume of fluid (usually water) that can pass through a pipe within a specific timeframe, typically measured in Gallons Per Minute (GPM) or Liters Per Minute (LPM).
Unlike simple static volume calculations, flow capacity is dynamic. It depends heavily on the friction generated inside the pipe, the pressure pushing the fluid, and the physical dimensions of the conduit. This Pipe Flow Capacity Calculator utilizes the industry-standard Hazen-Williams equation to provide accurate estimates for water flow.
Key Factors Affecting Flow Rate
When calculating how much water a pipe can handle, three primary variables come into play:
Internal Diameter: The most influential factor. A small increase in diameter results in a massive increase in flow capacity. For example, doubling the diameter of a pipe can increase its flow capacity by nearly 6 times, assuming pressure remains constant.
Pipe Roughness (C-Factor): The interior surface of the pipe affects how easily water slides through it. Smoother materials like PVC have a high C-Factor (around 150), allowing for higher flow rates. Older, pitted cast iron or corroded steel has a lower C-Factor (100 or less), creating drag and reducing capacity.
Pressure Drop (Head Loss): Fluid moves because of a pressure difference. The "Pressure Drop" input in the calculator represents the energy available to push the water from the start of the pipe to the end. Higher available pressure drop results in higher velocities and flow rates.
The Hazen-Williams Equation
This calculator uses the Hazen-Williams formula, which is the preferred method for Civil and Mechanical Engineers when designing water distribution systems. The formula is empirically derived and specifically tuned for water at ambient temperatures.
Q = 0.442 × C × D2.63 × (ΔP / L)0.54
Note: The calculator logic applies a variation of this formula adjusted for standard units (GPM, PSI, Inches).
Interpreting Fluid Velocity
While maximizing flow (GPM) is often the goal, you must also pay attention to the Fluid Velocity output.
Safe Range: For most water systems, a velocity between 5 to 10 feet per second is recommended.
Too High (>10 ft/s): excessive velocity can cause water hammer (hydraulic shock), noise, and rapid erosion of pipe fittings.
Too Low (<2 ft/s): very low velocity may allow sediment to settle in the pipe, leading to blockages over time.
Common Pipe Material C-Factors
Select the correct material in the calculator to ensure accuracy: