The K-factor is specific to your device, nozzle, or orifice plate.
Calculated Results:
Flow Rate (CFM):0 CFM
Flow Rate (m³/h):0 m³/h
Flow Rate (L/s):0 L/s
How to Calculate Air Flow from Pressure Drop
In fluid dynamics and HVAC engineering, determining the air flow rate ($Q$) through a restriction, such as an orifice plate, nozzle, or venturi meter, is commonly achieved by measuring the pressure drop ($\Delta P$) across that restriction. The relationship follows the square root law, meaning that flow is proportional to the square root of the pressure differential.
The standard formula used in this calculator is:
Q = K × √ΔP
Where:
Q: The Air Flow Rate.
K: The K-factor (Coefficient of Discharge/Flow Factor), which accounts for the geometry and density of the medium.
ΔP: The measured pressure drop across the device.
Why is the K-Factor Important?
The K-factor is a dimensionless or unit-specific constant provided by the manufacturer of the flow measuring station, damper, or filter. It represents the physical characteristics of the device. If the K-factor is provided for units in Pascals but you are measuring in Inches of Water, a conversion is required before applying the formula.
Realistic Example Calculation
Suppose you are measuring the flow across a cooling coil with a known K-factor of 35.0. Your manometer shows a differential pressure of 150 Pascals.
This calculation is vital in several industrial and commercial scenarios:
Filter Loading: Monitoring pressure drop across HEPA filters to determine when they need replacement while maintaining required air exchange rates.
Laboratory Fume Hoods: Ensuring constant volume or variable volume systems are exhausting the correct amount of hazardous air.
Fan Performance: Verifying if a fan is operating on its intended performance curve by measuring the pressure differential at the inlet cone.
Unit Conversion Tips
If your pressure reading is in Inches of Water (inH2O), the flow behaves slightly differently compared to SI units (Pascals). This calculator automatically handles the conversion to ensure that the square root relationship is applied accurately regardless of the input units selected.
function calculateAirFlow() {
var dp = parseFloat(document.getElementById("pressureDrop").value);
var k = parseFloat(document.getElementById("kFactor").value);
var unit = document.getElementById("pressureUnit").value;
var resultArea = document.getElementById("resultArea");
if (isNaN(dp) || isNaN(k) || dp < 0) {
alert("Please enter valid positive numbers for Pressure Drop and K-Factor.");
return;
}
var pressureInPa = dp;
// Convert to Pa if input is inH2O for a standardized calculation approach
// 1 inH2O = 248.84 Pa
if (unit === "inh2o") {
pressureInPa = dp * 248.84;
}
// Standard flow formula Q = k * sqrt(dP)
// Usually, manufacturers provide K factors designed for specific units.
// For this calculator, we assume the K factor is provided for the units selected.
// However, if we assume K is a generic constant:
var flowRate = k * Math.sqrt(dp);
var cfm, m3h, ls;
if (unit === "inh2o") {
// If input was inH2O, we treat flowRate as CFM (common US standard)
cfm = flowRate;
m3h = cfm * 1.69901;
ls = cfm * 0.471947;
} else {
// If input was Pa, we treat flowRate as m3/h (common Metric standard)
m3h = flowRate;
cfm = m3h / 1.69901;
ls = m3h / 3.6;
}
document.getElementById("cfmResult").innerHTML = cfm.toLocaleString(undefined, {minimumFractionDigits: 2, maximumFractionDigits: 2});
document.getElementById("m3hResult").innerHTML = m3h.toLocaleString(undefined, {minimumFractionDigits: 2, maximumFractionDigits: 2});
document.getElementById("lsResult").innerHTML = ls.toLocaleString(undefined, {minimumFractionDigits: 2, maximumFractionDigits: 2});
resultArea.style.display = "block";
}