Calculate water velocity and pressure drop for 1/2 inch PEX tubing.
*Calculations based on standard 1/2″ CTS PEX with an average Inside Diameter (ID) of 0.485″. Hazen-Williams C-factor used: 150.
function calculatePexFlow() {
var gpm = parseFloat(document.getElementById('flowRateGpm').value);
var length = parseFloat(document.getElementById('pipeLengthFt').value);
var resultDiv = document.getElementById('pexResult');
if (isNaN(gpm) || gpm <= 0) {
alert("Please enter a valid Flow Rate greater than zero.");
return;
}
if (isNaN(length) || length <= 0) {
length = 100; // Default to 100 if invalid
}
// 1/2" PEX average inside diameter is 0.485 inches
var innerDiameter = 0.485;
// 1. Calculate Velocity (fps)
// Formula: V = (GPM * 0.408) / d^2
var velocity = (gpm * 0.4085) / (innerDiameter * innerDiameter);
// 2. Calculate Pressure Drop (PSI per 100 ft) using Hazen-Williams
// Pressure Drop (psi/100ft) = 0.433 * (head loss in feet)
// Formula: Delta P = (4.52 * Q^1.852) / (C^1.852 * d^4.8655)
var cFactor = 150;
var pressureDropPer100 = (4.52 * Math.pow(gpm, 1.852)) / (Math.pow(cFactor, 1.852) * Math.pow(innerDiameter, 4.8655));
// 3. Total Pressure Drop
var totalPressureDrop = (pressureDropPer100 * length) / 100;
// Display results
resultDiv.style.display = 'block';
var output = "Water Velocity: " + velocity.toFixed(2) + " ft/s";
output += "Pressure Drop per 100ft: " + pressureDropPer100.toFixed(2) + " psi";
output += "Total Pressure Loss (" + length + " ft): " + totalPressureDrop.toFixed(2) + " psi";
// Performance warnings
var warning = "";
if (velocity > 8) {
warning = "⚠️ HIGH VELOCITY: Exceeding 8 fps can cause excessive noise, water hammer, and potential pipe erosion. Consider 3/4\" PEX.";
} else if (velocity > 5) {
warning = "⚠️ MODERATE VELOCITY: For hot water systems, it is generally recommended to stay below 5 fps to prevent long-term degradation.";
} else {
warning = "✓ OPTIMAL: Flow velocity is within safe residential limits.";
}
resultDiv.innerHTML = output + warning;
}
Understanding 1/2″ PEX Flow Rates
When designing a residential plumbing system, 1/2″ PEX (Cross-linked Polyethylene) is the most common tubing size used for individual branch lines leading to fixtures like sinks, showers, and toilets. Understanding the relationship between flow rate, velocity, and pressure drop is critical for ensuring a quiet and efficient system.
The Physics of 1/2″ PEX
Unlike copper pipe, PEX has a thicker wall, which means its Inside Diameter (ID) is smaller than copper of the same nominal size. For 1/2″ PEX, the average internal diameter is approximately 0.485 inches. Because the opening is smaller, water must travel faster to deliver the same amount of GPM compared to copper, leading to higher friction loss (pressure drop).
Key Metrics Explained
Flow Rate (GPM): This is the volume of water moving through the pipe per minute. A standard bathroom faucet usually requires about 1.5 to 2.2 GPM, while a showerhead typically pulls 2.5 GPM.
Velocity (FPS): This is the speed of the water in Feet Per Second. Plumbing codes generally recommend a maximum velocity of 8 fps for cold water and 5 fps for hot water to prevent "wire draw" erosion and noisy pipes.
Pressure Drop (PSI): As water rubs against the inside walls of the PEX tube, it loses energy. This is measured as a loss in pounds per square inch (PSI). The longer the run and the higher the flow rate, the more pressure you lose at the fixture.
Typical Flow Rate Limits for 1/2″ PEX
Flow Rate (GPM)
Velocity (FPS)
PSI Loss / 100ft
Suitability
1.0
1.74
1.7
Excellent
2.0
3.47
6.2
Good
3.0
5.21
13.1
Max for Hot
4.0
6.95
22.3
Max for Cold
Design Tips for PEX Systems
1. Home Runs: If using a manifold system (home run), 1/2″ PEX is ideal because each fixture has its own dedicated line, meaning you rarely exceed 2-3 GPM per pipe.
2. Trunk and Branch: If you are using a trunk and branch system, do not use 1/2″ PEX for the main trunk. A 1/2″ pipe cannot effectively supply a shower and a toilet simultaneously without a significant drop in pressure and a spike in noise.
3. Fitting Restrictions: Remember that PEX insert fittings (crimped) have a smaller ID than the pipe itself. If your run has many 90-degree elbows, your actual pressure drop will be higher than the calculated value for straight pipe.