Estimate water flow capacity (GPM) based on pipe size, type, and velocity.
1/2″
3/4″
1″
1 1/4″
1 1/2″
2″
2 1/2″
3″
4″
Type K (Thickest Wall / Heavy Duty)
Type L (Standard / Residential)
Type M (Thinnest Wall / Heating)
Type L is most common for interior plumbing.
Standard max recommended: Cold Water (8 ft/s), Hot Water (5 ft/s).
Calculated Internal Diameter:0.000 inches
Flow Rate (GPM):0.00 GPM
Flow Rate (LPM):0.00 LPM
Cross-Sectional Area:0.0000 sq ft
function calculateFlowRate() {
// Retrieve inputs
var sizeSelect = document.getElementById('nominalSize');
var typeSelect = document.getElementById('pipeType');
var velocityInput = document.getElementById('flowVelocity');
var size = parseFloat(sizeSelect.value);
var type = typeSelect.value;
var velocity = parseFloat(velocityInput.value);
// Validation
if (isNaN(velocity) || velocity { K: ID, L: ID, M: ID }
var pipeDims = {
"0.5": { "K": 0.527, "L": 0.545, "M": 0.569 },
"0.75": { "K": 0.745, "L": 0.785, "M": 0.811 },
"1": { "K": 0.995, "L": 1.025, "M": 1.055 },
"1.25": { "K": 1.245, "L": 1.265, "M": 1.291 },
"1.5": { "K": 1.481, "L": 1.505, "M": 1.527 },
"2": { "K": 1.959, "L": 1.985, "M": 2.009 },
"2.5": { "K": 2.435, "L": 2.465, "M": 2.495 },
"3": { "K": 2.907, "L": 2.945, "M": 2.981 },
"4": { "K": 3.857, "L": 3.905, "M": 3.935 }
};
// Get Internal Diameter (ID) in inches
var internalDiameterInches = 0;
if (pipeDims[size] && pipeDims[size][type]) {
internalDiameterInches = pipeDims[size][type];
} else {
// Fallback estimation if exact data missing (should not happen with fixed select)
internalDiameterInches = size;
}
// Calculations
// 1. Convert ID to Feet
var internalDiameterFeet = internalDiameterInches / 12;
// 2. Calculate Area in Square Feet: A = pi * r^2 = pi * (d/2)^2
var radiusFeet = internalDiameterFeet / 2;
var areaSqFt = Math.PI * Math.pow(radiusFeet, 2);
// 3. Calculate Flow Rate in Cubic Feet per Second (CFS): Q = A * V
var flowCFS = areaSqFt * velocity;
// 4. Convert CFS to Gallons Per Minute (GPM)
// 1 CFS = 448.831 GPM
var flowGPM = flowCFS * 448.831;
// 5. Convert GPM to Liters Per Minute (LPM)
// 1 GPM = 3.78541 LPM
var flowLPM = flowGPM * 3.78541;
// Display Results
document.getElementById('resID').innerHTML = internalDiameterInches.toFixed(3) + " inches";
document.getElementById('resGPM').innerHTML = flowGPM.toFixed(2) + " GPM";
document.getElementById('resLPM').innerHTML = flowLPM.toFixed(2) + " LPM";
document.getElementById('resArea').innerHTML = areaSqFt.toFixed(4) + " ft²";
// Show result container
document.getElementById('result-container').style.display = 'block';
}
Guide to Calculating Copper Pipe Flow Rates
Understanding the flow rate of copper piping is essential for plumbing design, whether you are sizing a main water line for a residential home, designing a hydronic heating system, or ensuring adequate pressure for irrigation. This calculator helps determine the volumetric flow rate (GPM) based on the physical properties of ASTM B88 copper tubing.
Why Pipe Type Matters: K vs. L vs. M
Not all copper pipes of the same "nominal size" have the same internal capacity. The nominal size refers to the approximate outer diameter, but the internal diameter varies based on wall thickness.
Type K (Green Label): Has the thickest walls. It is used for underground service lines and heavy-duty commercial applications. Because the walls are thick, the internal diameter is the smallest, resulting in slightly lower flow capacity for the same outer size.
Type L (Blue Label): The industry standard for interior residential and commercial plumbing. It offers a balance between durability and flow capacity.
Type M (Red Label): Has the thinnest walls. It is primarily used for residential heating systems and low-pressure applications. It has the largest internal diameter, allowing for the highest flow rate among the three types, but is less durable against high pressure or acidic water.
The Flow Rate Formula
The calculation performed by this tool uses the continuity equation for fluids:
Q = A × V
Where:
Q is the Flow Rate (Cubic Feet per Second).
A is the Cross-Sectional Area of the pipe's interior (Square Feet).
V is the Velocity of the fluid (Feet per Second).
To get Gallons Per Minute (GPM), we convert the result using the factor: 1 cubic foot ≈ 7.48 gallons, and convert seconds to minutes.
Recommended Water Velocities
While a pipe can carry water at very high speeds, doing so is dangerous for the plumbing system. High velocity causes hydraulic shock (water hammer), erosion-corrosion (wearing away the copper), and excessive noise.
Application
Recommended Max Velocity
Cold Water Systems
8 ft/s (2.4 m/s)
Hot Water Systems (>140°F)
5 ft/s (1.5 m/s)
Hot Water Recirculation
2-4 ft/s (0.6-1.2 m/s)
Example Calculation
Let's say you are running a main water line using 1″ Type L Copper Pipe and you want to maintain a safe velocity of 6 ft/s.
Determine ID: 1″ Type L copper has an actual internal diameter of 1.025 inches.
Calculate Area: Radius = 1.025″ / 2 = 0.5125″
Area = π × (0.5125)^2 ≈ 0.825 square inches
Convert to sq ft ≈ 0.00573 sq ft.