How to Calculate Water Flow Rate Through Pipe

Pipe Flow Rate Calculator

Enter the internal diameter, not the outer diameter.

Typical household velocity is 5-8 ft/sec. Industrial can be higher.

function calculateWaterFlow() { // Get input values var diameterInches = parseFloat(document.getElementById('pipeInnerDiameter').value); var velocityFtSec = parseFloat(document.getElementById('waterVelocity').value); // Validation if (isNaN(diameterInches) || diameterInches <= 0 || isNaN(velocityFtSec) || velocityFtSec < 0) { alert("Please enter valid positive numbers for both Diameter and Velocity."); return; } // Calculation Logic // 1. Convert Diameter from inches to feet var diameterFeet = diameterInches / 12; // 2. Calculate Radius in feet var radiusFeet = diameterFeet / 2; // 3. Calculate Cross-Sectional Area (A = π * r²) in square feet var areaSqFt = Math.PI * Math.pow(radiusFeet, 2); // 4. Calculate Flow Rate in Cubic Feet per Second (Q = A * v) // Q (cfs) = Area (sq ft) * Velocity (ft/s) var flowCFS = areaSqFt * velocityFtSec; // 5. Convert CFS to Gallons Per Minute (GPM) // 1 Cubic Foot = 7.48052 Gallons // 1 CFS = 7.48052 * 60 = 448.831 GPM var flowGPM = flowCFS * 448.83117; // 6. Convert GPM to Liters Per Minute (LPM) // 1 Gallon = 3.78541 Liters var flowLPM = flowGPM * 3.78541; // 7. Convert LPM to Cubic Meters per Hour (m³/h) // 1 LPM = 0.06 m³/h var flowM3H = flowLPM * 0.06; // Display Results document.getElementById('resultGPM').innerText = flowGPM.toFixed(2) + " GPM"; document.getElementById('resultCFS').innerText = flowCFS.toFixed(4) + " ft³/s"; document.getElementById('resultLPM').innerText = flowLPM.toFixed(2) + " L/min"; document.getElementById('resultM3H').innerText = flowM3H.toFixed(2) + " m³/h"; // Show results container document.getElementById('wfrResults').style.display = "block"; }

How to Calculate Water Flow Rate Through a Pipe

Calculating the water flow rate through a pipe is a fundamental task in fluid dynamics, plumbing, and irrigation engineering. Understanding the relationship between the pipe's size (diameter) and the speed of the water (velocity) allows you to determine exactly how much volume is being moved over time.

The flow rate determines whether your system can supply enough water for showers, irrigation sprinklers, or industrial processes. While pressure is important, flow rate is the actual measure of volume delivery.

The Flow Rate Formula

The most basic equation for calculating water flow rate inside a pipe is the continuity equation:

Q = A × v

Where:

• Q = Flow Rate (e.g., Cubic Feet per Second)
• A = Cross-sectional Area of the pipe
• v = Velocity of the fluid

Step-by-Step Calculation Guide

If you are calculating this manually without the tool above, follow these steps:

1. Measure the Inner Diameter (ID): It is critical to use the inner diameter, not the outer diameter (OD). For example, a 2-inch Schedule 40 PVC pipe has an actual inner diameter of approximately 2.047 inches, while Schedule 80 is thicker with a smaller ID.
2. Convert to Consistent Units: If your velocity is in feet per second, your diameter must be converted to feet to calculate the area in square feet.
   Formula: Diameter (ft) = Diameter (in) ÷ 12
3. Calculate the Area: Use the formula for the area of a circle: A = π × (d/2)².
4. Multiply by Velocity: Multiply the area (ft²) by the velocity (ft/sec) to get Cubic Feet per Second (CFS).
5. Convert to GPM: Multiply your CFS result by 448.83 to get Gallons Per Minute.

Example Calculation

Let's say you have a pipe with a 3-inch inner diameter and water flowing at a velocity of 6 feet per second.

• Diameter in feet: 3 ÷ 12 = 0.25 ft
• Radius in feet: 0.25 ÷ 2 = 0.125 ft
• Area: 3.14159 × (0.125)² ≈ 0.0491 sq ft
• Flow (CFS): 0.0491 sq ft × 6 ft/s = 0.2945 CFS
• Flow (GPM): 0.2945 × 448.83 ≈ 132.18 GPM

Estimating Water Velocity

If you do not know the velocity, you can often estimate it based on system design standards:

• Gravity Flow: Often 1 to 3 ft/sec.
• Suction Lines (Pump): Typically 2 to 5 ft/sec.
• Discharge Lines (Pressure): Typically 4 to 10 ft/sec.

Keep in mind that higher velocities cause significantly higher friction loss (pressure drop) and can lead to water hammer issues.