Input logic is unit-neutral (GPM, L/min, m³/hr, etc.)
Please enter valid numeric values for all fields. RPM must be greater than 0.
New Flow Rate (Q2):–
Speed Change:–
Flow Factor:–
function calculatePumpFlow() {
// 1. Get input elements
var ratedFlowInput = document.getElementById("ratedFlow");
var ratedRpmInput = document.getElementById("ratedRpm");
var targetRpmInput = document.getElementById("targetRpm");
var resultsBox = document.getElementById("resultsBox");
var errorMsg = document.getElementById("errorMessage");
// 2. Parse values
var q1 = parseFloat(ratedFlowInput.value);
var n1 = parseFloat(ratedRpmInput.value);
var n2 = parseFloat(targetRpmInput.value);
// 3. Validation
if (isNaN(q1) || isNaN(n1) || isNaN(n2) || n1 <= 0 || n2 < 0) {
errorMsg.style.display = "block";
resultsBox.style.display = "none";
return;
}
// Hide error if valid
errorMsg.style.display = "none";
// 4. Calculation Logic (Affinity Law 1: Q2/Q1 = N2/N1)
// Therefore Q2 = Q1 * (N2 / N1)
var ratio = n2 / n1;
var q2 = q1 * ratio;
// Calculate percentage change
var percentChange = ((n2 – n1) / n1) * 100;
// 5. Display Results
document.getElementById("resultFlow").innerHTML = q2.toFixed(2) + " (Units match input)";
var sign = percentChange >= 0 ? "+" : "";
document.getElementById("resultPercent").innerText = sign + percentChange.toFixed(2) + "%";
document.getElementById("resultFactor").innerText = ratio.toFixed(4) + "x";
resultsBox.style.display = "block";
}
How to Calculate Pump Flow Rate from RPM
Calculating the change in flow rate resulting from a change in pump speed (RPM) is a fundamental task in fluid dynamics and mechanical engineering. This relationship is governed by the Pump Affinity Laws.
The First Affinity Law
The first affinity law states that the flow rate (Q) is directly proportional to the shaft speed (N). This implies a linear relationship: if you double the speed of the pump, you double the flow rate (assuming the impeller diameter remains constant).
Formula: Q₂ = Q₁ × (N₂ / N₁)
Where:
Q₁ = Initial Flow Rate (GPM, L/min, etc.)
Q₂ = New Flow Rate
N₁ = Initial Speed (RPM)
N₂ = New Speed (RPM)
Why is this important?
Understanding this calculation is critical for operators using Variable Frequency Drives (VFDs). A VFD allows you to adjust the frequency (Hz) supplied to the motor, which changes the RPM. By manipulating the RPM, you can precisely control the flow rate to meet process requirements without using throttling valves, which wastes energy.
Example Calculation
Let's say you have a centrifugal pump rated for 500 GPM at 1750 RPM. You install a VFD and increase the motor speed to 2000 RPM.
Calculate the ratio: 2000 / 1750 = 1.1428
Multiply original flow by ratio: 500 × 1.1428 = 571.4 GPM
The new flow rate is approximately 571.4 GPM.
Important Considerations
While Flow Rate changes linearly with speed, other parameters do not:
Head Pressure changes with the square of the speed (N²).
Power Consumption (BHP) changes with the cube of the speed (N³).
This means a small increase in RPM results in a massive increase in power requirement. Always verify your motor's horsepower rating before increasing pump speed significantly above its rated RPM.