PVC / Plastic (C=150)
Copper (C=140)
Steel – New (C=120)
Cast Iron – Old (C=100)
Galvanized Iron (C=110)
Concrete (C=130)
Please enter valid positive numbers for all fields.
Water Velocity:–
Velocity Head (Dynamic Pressure):–
Pressure Drop (Total):–
Pressure Drop (per 100 ft):–
function calculatePressure() {
// 1. Get input values
var flowRate = parseFloat(document.getElementById('flowRate').value);
var diameter = parseFloat(document.getElementById('pipeDiameter').value);
var length = parseFloat(document.getElementById('pipeLength').value);
var materialC = parseFloat(document.getElementById('pipeMaterial').value);
var errorDiv = document.getElementById('errorMsg');
var resultsDiv = document.getElementById('resultsBox');
// 2. Validation
if (isNaN(flowRate) || flowRate <= 0 ||
isNaN(diameter) || diameter <= 0 ||
isNaN(length) || length < 0) {
errorDiv.style.display = "block";
resultsDiv.style.display = "none";
return;
}
errorDiv.style.display = "none";
// 3. Calculation Logic
// A. Velocity Calculation
// Formula: V = (0.4085 * Q) / d^2
// V in ft/s, Q in GPM, d in inches
var velocity = (0.4085 * flowRate) / (diameter * diameter);
// B. Dynamic Pressure (Velocity Head)
// Velocity Head (ft) = V^2 / 2g (where g = 32.174 ft/s^2)
// Dynamic Pressure (PSI) = Head (ft) * 0.433
var velocityHeadFt = (velocity * velocity) / (2 * 32.174);
var dynamicPsi = velocityHeadFt * 0.433;
// C. Pressure Drop (Hazen-Williams Equation)
// Formula: P_drop (psi/100ft) = (4.52 * Q^1.85) / (C^1.85 * d^4.8655)
var qPow = Math.pow(flowRate, 1.85);
var cPow = Math.pow(materialC, 1.85);
var dPow = Math.pow(diameter, 4.8655);
var pressureDropPer100Ft = (4.52 * qPow) / (cPow * dPow);
var totalPressureDrop = pressureDropPer100Ft * (length / 100);
// 4. Display Results
document.getElementById('resVelocity').innerHTML = velocity.toFixed(2) + " ft/s";
document.getElementById('resDynamicPsi').innerHTML = dynamicPsi.toFixed(4) + " PSI";
document.getElementById('resDropTotal').innerHTML = totalPressureDrop.toFixed(2) + " PSI";
document.getElementById('resDropPer100').innerHTML = pressureDropPer100Ft.toFixed(2) + " PSI";
resultsDiv.style.display = "block";
}
How to Calculate Pressure from Flow Rate of Water
Understanding the relationship between water flow rate and pressure is fundamental in fluid mechanics and plumbing. While flow rate (GPM) and pressure (PSI) are distinct physical quantities, they are mathematically linked through pipe geometry and fluid velocity. This guide explains how to calculate dynamic pressure and pressure drop based on the flow rate.
The Relationship Between Flow and Pressure
It is important to distinguish between Static Pressure and Dynamic Pressure. Flow rate alone cannot calculate static pressure (the pressure when water is not moving), as that depends on pumps or elevation (gravity). However, flow rate does determine:
Dynamic Pressure: The kinetic energy of the moving water expressed as pressure.
Pressure Drop (Friction Loss): The amount of pressure lost due to friction as water rubs against the pipe walls.
1. Calculating Water Velocity
Before calculating pressure, you must determine the velocity of the water. As water is forced through a smaller pipe, it must move faster to maintain the same volumetric flow rate.
Velocity (v) = (0.4085 × Flow Rate) / Diameter²
Where Flow Rate is in GPM and Diameter is in inches. Result is in feet per second (ft/s).
2. Calculating Dynamic Pressure
Dynamic pressure represents the momentum of the fluid. It is derived from Bernoulli's principle. This is the pressure exerted by the fluid's motion itself.
3. Calculating Pressure Drop (Hazen-Williams Equation)
The most practical application for plumbers and engineers is calculating how much pressure is lost over a length of pipe. We use the Hazen-Williams equation for water flow.
Pdrop = (4.52 × Q1.85) / (C1.85 × d4.87)
Variables:
Q: Flow Rate in GPM.
C: Roughness Coefficient (150 for PVC, 120 for Steel).
d: Inner Diameter in inches.
Calculation Example
Let's assume you have a 50 GPM flow rate moving through a 2-inch PVC pipe that is 100 feet long.
Metric
Calculation
Result
Velocity
(0.4085 × 50) / 2²
5.11 ft/s
Dynamic Pressure
((5.11)² / 64.34) × 0.433
0.176 PSI
Pressure Loss
Using Hazen-Williams with C=150
0.77 PSI per 100ft
In this example, for every 100 feet of pipe, the system loses approximately 0.77 PSI of pressure due to friction.
Why High Velocity Matters
If the calculated velocity exceeds 5 to 7 ft/s, the risk of water hammer and pipe erosion increases significantly. High flow rates in small pipes create excessive pressure drops, requiring larger pumps to maintain system pressure.