Natural Gas (SG 0.60)
Propane (SG 1.50)
Custom Specific Gravity
Volumetric Flow Rate
0 CFH
Cubic Feet per Hour
Energy Capacity
0 BTU/hr
British Thermal Units per Hour
Understanding Gas Pipe Flow Rate
Calculating the flow rate of gas through a piping system is a critical step in designing safe and efficient residential, commercial, and industrial gas systems. This calculator estimates the capacity of a pipe based on the Spitzglass Formula for low-pressure systems (typically less than 1 psi), which is widely used for sizing natural gas and propane lines.
Key Variables in Gas Flow Calculation
Several physical factors influence how much gas can travel through a pipe. Understanding these inputs will help you use the calculator more effectively:
Pipe Internal Diameter: The actual inside diameter of the pipe determines the cross-sectional area available for flow. Note that "nominal" pipe sizes (like 1 inch) often have different actual internal diameters depending on the material (Schedule 40 steel, copper, CSST, etc.).
Pipe Length: Friction increases as the pipe gets longer, reducing the flow rate. The calculation assumes a straight pipe; fittings (elbows, tees) add "equivalent length" which should be added to the total run for accuracy.
Pressure Drop: This is the difference in pressure between the start and end of the pipe segment, usually measured in inches of water column (w.c.). A standard allowance for low-pressure natural gas systems is often 0.3 to 0.5 inches w.c.
Specific Gravity: This compares the density of the gas to air. Natural gas typically has a specific gravity of roughly 0.60, while propane is heavier at roughly 1.50.
The Spitzglass Formula
For low-pressure calculations (pressures under 1 psig), the Spitzglass formula is a standard engineering equation used to relate flow rate to pressure drop and pipe dimensions:
Q = 3550 × √[ (h × d⁵) / (S × L × (1 + 3.6/d)) ]
Where:
Q: Flow rate in Cubic Feet per Hour (CFH)
h: Pressure drop in inches of water column
d: Internal diameter of pipe in inches
S: Specific gravity of gas
L: Length of pipe in feet
Converting CFH to BTU/hr
While the formula outputs the volume of gas (CFH), appliances are rated in energy usage (BTU/hr). To convert:
Natural Gas: 1 Cubic Foot ≈ 1,000 to 1,025 BTUs.
Propane: 1 Cubic Foot ≈ 2,500 BTUs.
This calculator automatically provides the energy capacity estimate based on the selected gas type.
Safety and Compliance
Warning: Gas pipe sizing involves strict safety codes (such as NFPA 54 or the International Fuel Gas Code). Undersized pipes can cause appliance malfunction due to pressure starvation, while incorrect installation poses fire and explosion risks. Always verify calculations with local codes and a licensed professional before installation.
function updateGravity() {
var gasSelect = document.getElementById('gasType');
var gravityInputGroup = document.getElementById('customGravityGroup');
var gravityInput = document.getElementById('specificGravity');
if (gasSelect.value === 'custom') {
gravityInputGroup.style.display = 'block';
} else {
gravityInputGroup.style.display = 'none';
gravityInput.value = gasSelect.value;
}
}
function calculateGasFlow() {
// 1. Get Input Values
var diameter = parseFloat(document.getElementById('pipeDiameter').value);
var length = parseFloat(document.getElementById('pipeLength').value);
var drop = parseFloat(document.getElementById('pressureDrop').value);
var gravity = parseFloat(document.getElementById('specificGravity').value);
var gasTypeSelect = document.getElementById('gasType').value;
var errorDiv = document.getElementById('errorMessage');
var resultsDiv = document.getElementById('results');
// 2. Validation
errorDiv.style.display = 'none';
resultsDiv.style.display = 'none';
if (isNaN(diameter) || diameter <= 0) {
errorDiv.innerHTML = "Please enter a valid Pipe Diameter greater than 0.";
errorDiv.style.display = 'block';
return;
}
if (isNaN(length) || length <= 0) {
errorDiv.innerHTML = "Please enter a valid Pipe Length greater than 0.";
errorDiv.style.display = 'block';
return;
}
if (isNaN(drop) || drop <= 0) {
errorDiv.innerHTML = "Please enter a valid Pressure Drop greater than 0.";
errorDiv.style.display = 'block';
return;
}
if (isNaN(gravity) || gravity <= 0) {
errorDiv.innerHTML = "Please enter a valid Specific Gravity greater than 0.";
errorDiv.style.display = 'block';
return;
}
// 3. Calculation (Spitzglass Low Pressure Formula)
// Q = 3550 * sqrt( (h * d^5) / (S * L * (1 + 3.6/d)) )
var d5 = Math.pow(diameter, 5);
var denominatorTerm = 1 + (3.6 / diameter);
var numerator = drop * d5;
var denominator = gravity * length * denominatorTerm;
var flowCFH = 3550 * Math.sqrt(numerator / denominator);
// 4. Calculate BTUs
// Standard approximations: Natural Gas ~ 1000 BTU/cf, Propane ~ 2500 BTU/cf
var btuMultiplier = 1000;
// Determine multiplier based on gravity if custom is not strictly selected,
// or infer based on range.
if (Math.abs(gravity – 1.50) < 0.1) {
btuMultiplier = 2500; // Propane
} else if (Math.abs(gravity – 0.60) 1.0) ? 2500 : 1000;
}
var flowBTU = flowCFH * btuMultiplier;
// 5. Output Formatting
document.getElementById('resultCFH').innerHTML = Math.round(flowCFH).toLocaleString() + " CFH";
document.getElementById('resultBTU').innerHTML = Math.round(flowBTU).toLocaleString() + " BTU/hr";
// Show Results
resultsDiv.style.display = 'block';
}