Enter the Brake Horsepower (BHP) rating of your pump motor.
Typical centrifugal pumps are 50% – 85% efficient.
Vertical distance + friction loss in pipes (in feet).
Gallons Per Minute (GPM):–
Liters Per Minute (LPM):–
Cubic Meters Per Hour (m³/h):–
Water Horsepower (WHP):–
How to Calculate Water Flow Rate of a Pump
Calculating the water flow rate of a pump is essential for ensuring your system is sized correctly for applications such as irrigation, pools, or industrial plumbing. While flow rate is often provided by the manufacturer's pump curve, you can calculate the theoretical flow rate if you know the pump's power, efficiency, and the total head it needs to overcome.
The Pump Flow Rate Formula
The standard equation relating Horsepower (HP), Head, and Flow Rate (GPM) for water is derived from the hydraulic power formula:
GPM = (3960 × HP × Efficiency) / Head
Where:
GPM: Flow rate in Gallons Per Minute.
HP: The Brake Horsepower of the motor.
Efficiency: The efficiency of the pump head (entered as a decimal in math, or percentage in our tool).
Head: The Total Dynamic Head in feet (vertical lift + friction loss).
3960: A conversion constant for specific gravity of water (1.0).
Understanding the Variables
Total Dynamic Head (TDH): This is arguably the most critical factor. It represents the total resistance the pump must overcome. It includes the vertical distance you are lifting the water (Static Head) and the resistance caused by fluid flowing through pipes and fittings (Friction Head). As Head increases, Flow Rate decreases.
Pump Efficiency: No pump converts 100% of motor energy into water movement. Energy is lost to heat, vibration, and mechanical friction. Most standard centrifugal pumps operate between 50% and 85% efficiency depending on their design and age.
Example Calculation
Let's say you have a 2 HP pump. You estimate the pump's efficiency at 75% (0.75). You need to pump water up a vertical height of 30 feet, but with pipe friction, your total dynamic head is 50 feet.
Using the formula:
GPM = (3960 × 2 × 0.75) / 50
GPM = (5940) / 50
Flow Rate = 118.8 GPM
Why Calculated Flow Differs from Actual Flow
The result provided by this calculator is a theoretical maximum based on the power available. Real-world flow rates can be affected by:
Suction Lift: If the pump is located above the water source, gravity fights the intake.
Wear and Tear: Impellers erode over time, reducing efficiency.
Voltage Drops: If the motor isn't receiving full voltage, RPM and HP drop.
For critical applications, always cross-reference these calculations with the manufacturer's specific pump curve chart.
function calculatePumpFlow() {
// Get input values
var hpInput = document.getElementById('pumpHP').value;
var effInput = document.getElementById('pumpEff').value;
var headInput = document.getElementById('totalHead').value;
// Clean values
var hp = parseFloat(hpInput);
var eff = parseFloat(effInput);
var head = parseFloat(headInput);
// Validation
if (isNaN(hp) || isNaN(eff) || isNaN(head)) {
alert("Please enter valid numbers for all fields.");
return;
}
if (hp <= 0 || eff <= 0 || head 100) {
alert("Efficiency cannot exceed 100%.");
return;
}
// Logic: GPM = (3960 * HP * (Eff/100)) / Head
// Note: The constant 3960 is used when Water Specific Gravity is 1.0
// Calculate Water Horsepower (Power actually delivered to water)
var decimalEff = eff / 100;
var waterHP = hp * decimalEff;
// Calculate GPM
var gpm = (3960 * waterHP) / head;
// Metric Conversions
// 1 GPM = 3.78541 Liters per Minute (LPM)
var lpm = gpm * 3.78541;
// 1 LPM = 0.06 Cubic Meters per Hour (m3/h)
var m3h = lpm * 0.06;
// Update DOM
document.getElementById('resGPM').innerHTML = gpm.toFixed(2) + " GPM";
document.getElementById('resLPM').innerHTML = lpm.toFixed(2) + " L/min";
document.getElementById('resM3H').innerHTML = m3h.toFixed(2) + " m³/h";
document.getElementById('resWHP').innerHTML = waterHP.toFixed(2) + " HP";
// Show result section
document.getElementById('resultDisplay').style.display = "block";
}