PVC / Plastic (C=150)
New Steel / Copper (C=140)
Cast Iron (New) (C=130)
Galvanized Steel (C=120)
Cast Iron (Old) (C=100)
Corroded Pipe (C=80)
Pressure Drop (PSI):0.00 PSI
Head Loss (Feet of Head):0.00 ft
Flow Velocity:0.00 ft/s
How to Calculate PSI from Flow Rate
Calculating PSI (Pounds per Square Inch) from flow rate involves determining the pressure lost due to friction as fluid moves through a pipe. While flow rate (measured in Gallons Per Minute, or GPM) describes the volume of fluid moving, PSI measures the force exerted by that fluid. These two metrics are connected through the physical properties of the pipe and the fluid itself.
This calculator uses the Hazen-Williams equation, which is the standard method for calculating pressure drop in water piping systems. It helps engineers and DIY enthusiasts determine if a pump is powerful enough to deliver water through a specific length and size of hose or pipe.
The Hazen-Williams Formula
To calculate the friction head loss (which correlates directly to pressure drop), the formula is:
f = Friction head loss in feet of water per 100 feet of pipe
C = Roughness coefficient (depends on pipe material)
Q = Flow rate in GPM (Gallons Per Minute)
d = Inside hydraulic diameter in inches
To convert the Head Loss (ft) to Pressure Drop (PSI), we use the specific gravity of water (2.31 feet of head = 1 PSI):
PSI Drop = Total Head Loss (ft) / 2.31
Example Calculation
Let's say you want to move 20 GPM of water through 200 feet of 1.5-inch PVC pipe.
Flow Rate (Q): 20 GPM
Diameter (d): 1.5 inches
Length: 200 feet
Material (C): 150 (Standard for PVC)
Using the calculator above, you would find that the water velocity is roughly 3.6 ft/s. The friction causes a pressure drop of approximately 3.6 PSI over the 200-foot length. This means your pump must generate at least 3.6 PSI just to overcome the friction in the pipe, not including any elevation changes.
Why Pipe Material Matters
The "C Factor" represents how smooth the inside of the pipe is. Higher numbers indicate smoother pipes. PVC (C=150) creates much less resistance than old corroded steel (C=80). As the pipe gets rougher, the PSI required to maintain the same flow rate increases significantly.
function calculatePSI() {
// 1. Get input values strictly by ID
var flowRateInput = document.getElementById("flowRate");
var diameterInput = document.getElementById("pipeDiameter");
var lengthInput = document.getElementById("pipeLength");
var materialInput = document.getElementById("pipeMaterial");
var resultBox = document.getElementById("resultBox");
// 2. Parse values to floats
var Q = parseFloat(flowRateInput.value); // GPM
var d = parseFloat(diameterInput.value); // Inches
var L = parseFloat(lengthInput.value); // Feet
var C = parseFloat(materialInput.value); // Coefficient
// 3. Validation
if (isNaN(Q) || isNaN(d) || isNaN(L) || isNaN(C) || Q <= 0 || d <= 0 || L <= 0) {
alert("Please enter valid positive numbers for Flow Rate, Diameter, and Length.");
resultBox.style.display = "none";
return;
}
// 4. Calculate Friction Head Loss per 100 ft using Hazen-Williams
// Formula: f100 = 0.2083 * ((100/C)^1.852) * (Q^1.852 / d^4.8655)
var term1 = 0.2083;
var term2 = Math.pow((100 / C), 1.852);
var term3 = Math.pow(Q, 1.852) / Math.pow(d, 4.8655);
var headLossPer100 = term1 * term2 * term3;
// 5. Calculate Total Head Loss over the specific length
var totalHeadLoss = headLossPer100 * (L / 100);
// 6. Convert Head Loss (ft) to PSI
// 1 PSI = 2.31 ft of head
var totalPSI = totalHeadLoss / 2.31;
// 7. Calculate Velocity for reference
// V = (0.4085 * Q) / d^2
var velocity = (0.4085 * Q) / (d * d);
// 8. Display Results
document.getElementById("resultPSI").innerHTML = totalPSI.toFixed(2) + " PSI";
document.getElementById("resultHead").innerHTML = totalHeadLoss.toFixed(2) + " ft";
document.getElementById("resultVelocity").innerHTML = velocity.toFixed(2) + " ft/s";
// Show the result box
resultBox.style.display = "block";
}