Perforated Pipe Flow Rate Calculator

Calculate drainage and discharge capacity for perforated piping systems

Hole Diameter (inches)

Hole Spacing (inches center-to-center)

Total Pipe Length (feet)

Average Water Head (feet)

Discharge Coefficient (Cd) 0.61 (Sharp-edged orifice) 0.80 (Short tube) 0.98 (Well-rounded opening) Standard perforated pipes usually range from 0.60 to 0.65.

Calculation Results:

Total Number of Holes:

Total Perforation Area:

0 sq in

Flow Rate (Gallons Per Minute):

0 GPM

Flow Rate (Cubic Ft / Second):

0 CFS

Understanding Perforated Pipe Flow Dynamics

Calculating the flow rate of a perforated pipe is critical for civil engineering, landscaping, and agricultural projects. Whether you are designing a French drain, a septic leach field, or a sub-surface irrigation system, knowing how much water can exit the pipe under a specific pressure head is vital for preventing flooding or system failure.

The Orifice Equation

The calculation is based on the Orifice Equation, which describes the flow of fluid through an opening. The formula used in this calculator is:

            Q = Cd × A × √(2 × g × h)
        

Q: Flow rate (discharge capacity).
Cd: Discharge coefficient (accounts for turbulence and friction at the hole).
A: Total cross-sectional area of all perforations.
g: Acceleration due to gravity (32.174 ft/s²).
h: Head height (the vertical distance from the water surface to the center of the pipe).

Factors Affecting Performance

It is important to note that this calculator assumes the pipe is full and under pressure (head). If the pipe is only partially full, the "head" should be measured from the center of the submerged holes to the water level. Additionally, environmental factors like soil permeability (the ability of the surrounding dirt to absorb the water) often act as a secondary bottleneck that might limit the actual flow below the pipe's theoretical capacity.

Practical Example

Suppose you have a 100-foot French drain using a pipe with 0.5-inch holes spaced every 6 inches. If there is a 1-foot head of water sitting above the pipe:

Number of Holes: (100 ft * 12) / 6 inches = 200 holes.
Area per Hole: π * (0.25)² = 0.196 sq inches.
Total Area: 200 * 0.196 = 39.27 sq inches (0.272 sq feet).
Velocity: √(2 * 32.2 * 1) = 8.02 ft/s.
Total Flow: 0.61 * 0.272 * 8.02 = 1.33 CFS, which is approximately 597 GPM.

function calculateFlowRate() { var holeDiam = parseFloat(document.getElementById('holeDiameter').value); var spacing = parseFloat(document.getElementById('holeSpacing').value); var length = parseFloat(document.getElementById('pipeLength').value); var head = parseFloat(document.getElementById('waterHead').value); var cd = parseFloat(document.getElementById('coeffDischarge').value); if (isNaN(holeDiam) || isNaN(spacing) || isNaN(length) || isNaN(head) || holeDiam <= 0 || spacing <= 0 || length <= 0) { alert("Please enter valid positive numbers for all fields."); return; } // Gravity constant in ft/s^2 var g = 32.174; // 1. Calculate number of holes // Length in inches / spacing var totalHoles = (length * 12) / spacing; // 2. Calculate Area of one hole in square feet // Radius in inches var radiusIn = holeDiam / 2; // Area in sq inches var areaOneIn = Math.PI * Math.pow(radiusIn, 2); // Total Area in sq inches var totalAreaIn = areaOneIn * totalHoles; // Total Area in sq feet var totalAreaFt = totalAreaIn / 144; // 3. Orifice Equation: Q = Cd * A * sqrt(2gh) // Q is in Cubic Feet per Second (CFS) var flowCFS = cd * totalAreaFt * Math.sqrt(2 * g * head); // 4. Convert CFS to GPM (1 CFS = 448.831 GPM) var flowGPM = flowCFS * 448.831; // Display results document.getElementById('totalHolesDisplay').innerHTML = Math.round(totalHoles).toLocaleString(); document.getElementById('totalAreaDisplay').innerHTML = totalAreaIn.toFixed(2) + " sq in"; document.getElementById('gpmDisplay').innerHTML = flowGPM.toFixed(2) + " GPM"; document.getElementById('cfsDisplay').innerHTML = flowCFS.toFixed(4) + " CFS"; document.getElementById('results-box').style.display = 'block'; }