function calculateFlowRate() {
// 1. Get input elements
var diameterInput = document.getElementById('pipeDiameter');
var diameterUnitSelect = document.getElementById('diameterUnit');
var velocityInput = document.getElementById('flowVelocity');
var velocityUnitSelect = document.getElementById('velocityUnit');
var resultsDiv = document.getElementById('results');
// 2. Parse values
var diameter = parseFloat(diameterInput.value);
var velocity = parseFloat(velocityInput.value);
var diameterUnit = diameterUnitSelect.value;
var velocityUnit = velocityUnitSelect.value;
// 3. Validation
if (isNaN(diameter) || isNaN(velocity) || diameter <= 0 || velocity < 0) {
alert("Please enter valid positive numbers for diameter and velocity.");
resultsDiv.style.display = "none";
return;
}
// 4. Normalize inputs to Standard SI units (Meters for length, Meters/Second for velocity)
// Convert Diameter to Meters
var diameterInMeters = 0;
if (diameterUnit === 'mm') {
diameterInMeters = diameter / 1000;
} else if (diameterUnit === 'in') {
diameterInMeters = diameter * 0.0254;
} else {
diameterInMeters = diameter;
}
// Convert Velocity to Meters/Second
var velocityInMS = 0;
if (velocityUnit === 'fts') {
velocityInMS = velocity * 0.3048;
} else {
velocityInMS = velocity;
}
// 5. Calculate Area (A = pi * r^2) or (A = pi * (d/2)^2)
var radius = diameterInMeters / 2;
var areaM2 = Math.PI * Math.pow(radius, 2);
// 6. Calculate Flow Rate (Q = A * v) in Cubic Meters per Second (m³/s)
var flowM3s = areaM2 * velocityInMS;
// 7. Convert Results
// m³/s to m³/h
var flowM3h = flowM3s * 3600;
// m³/s to Liters/min (1 m³ = 1000 L, per sec to min = *60)
var flowLPM = flowM3s * 1000 * 60;
// m³/s to US GPM (1 m³/s ≈ 15,850.3231 GPM)
var flowGPM = flowM3s * 15850.3231;
// 8. Update DOM
document.getElementById('resM3h').innerText = flowM3h.toFixed(2);
document.getElementById('resGPM').innerText = flowGPM.toFixed(2);
document.getElementById('resLPM').innerText = flowLPM.toFixed(2);
document.getElementById('resArea').innerText = areaM2.toFixed(6);
// Show results
resultsDiv.style.display = "block";
}
Understanding Total Flow Rate Calculation
Calculating the total volumetric flow rate is a fundamental task in fluid dynamics, hydraulic engineering, and process control. Whether you are designing an HVAC system, managing a water treatment plant, or sizing industrial piping, understanding how to determine the flow rate ($Q$) based on pipe size and fluid velocity is essential for system efficiency.
The Flow Rate Formula
The most common method to calculate volumetric flow rate is using the continuity equation. The total flow depends on the cross-sectional area of the pipe and the average velocity of the fluid moving through it.
Q = A × v
Where:
Q = Volumetric Flow Rate (e.g., m³/s, GPM)
A = Cross-Sectional Area of the pipe (e.g., m²)
v = Average Fluid Velocity (e.g., m/s)
How to Calculate Pipe Area
Since pipes are generally cylindrical, the area ($A$) is calculated using the internal diameter ($D$):
A = π × (D / 2)² OR A = (π × D²) / 4
It is critical to ensure that the units for diameter match the units used for velocity. For example, if you measure diameter in inches and velocity in meters per second, you must convert the diameter to meters before calculating the area.
Factors Affecting Flow Rate
While this calculator provides the theoretical flow rate based on geometry and velocity, real-world total flow can be influenced by several factors:
Friction Loss: As fluid moves through a pipe, friction against the walls reduces pressure and potential flow.
Viscosity: Thicker fluids (like oil) resist flow more than thinner fluids (like water).
Pipe Roughness: Older or corroded pipes create turbulence, affecting the effective velocity profile.
Example Calculation
Let's say you have a water pipe with an internal diameter of 4 inches and the water is moving at a velocity of 5 feet per second.
Convert Diameter: 4 inches ≈ 0.1016 meters.
Calculate Area: A = π × (0.1016 / 2)² ≈ 0.008107 m².
Convert Velocity: 5 ft/s ≈ 1.524 m/s.
Calculate Flow ($Q$): 0.008107 m² × 1.524 m/s ≈ 0.01235 m³/s.
Convert Output: This equals approximately 195.8 GPM (Gallons Per Minute).