Choose your calculation method. You can calculate flow rate based on Volume and Time (filling a tank) or Pipe Diameter and Velocity (fluid movement through a conduit).
Volume and Time (Q = V / t)
Pipe Diameter and Velocity (Q = A × v)
function toggleFlowMethods() {
var method = document.getElementById("calcMethod").value;
if (method === "volTime") {
document.getElementById("volTimeSection").style.display = "block";
document.getElementById("areaVelSection").style.display = "none";
} else {
document.getElementById("volTimeSection").style.display = "none";
document.getElementById("areaVelSection").style.display = "block";
}
document.getElementById("flowResult").style.display = "none";
}
function calculateFlowRate() {
var method = document.getElementById("calcMethod").value;
var resultHTML = "";
var flowRateInLps = 0; // standard for conversion: Liters per second
if (method === "volTime") {
var volume = parseFloat(document.getElementById("volumeInput").value);
var volUnit = document.getElementById("volumeUnit").value;
var time = parseFloat(document.getElementById("timeInput").value);
var timeUnit = document.getElementById("timeUnit").value;
if (isNaN(volume) || isNaN(time) || time <= 0) {
alert("Please enter valid positive numbers for volume and time.");
return;
}
// Convert volume to Liters
var volumeInLiters = volume;
if (volUnit === "gallons") volumeInLiters = volume * 3.78541;
else if (volUnit === "m3") volumeInLiters = volume * 1000;
else if (volUnit === "ft3") volumeInLiters = volume * 28.3168;
// Convert time to Seconds
var timeInSeconds = time;
if (timeUnit === "minutes") timeInSeconds = time * 60;
else if (timeUnit === "hours") timeInSeconds = time * 3600;
flowRateInLps = volumeInLiters / timeInSeconds;
} else {
var diameter = parseFloat(document.getElementById("diameterInput").value);
var diaUnit = document.getElementById("diameterUnit").value;
var velocity = parseFloat(document.getElementById("velocityInput").value);
var velUnit = document.getElementById("velocityUnit").value;
if (isNaN(diameter) || isNaN(velocity) || diameter <= 0) {
alert("Please enter valid positive numbers for diameter and velocity.");
return;
}
// Convert diameter to Meters
var dInMeters = diameter;
if (diaUnit === "mm") dInMeters = diameter / 1000;
else if (diaUnit === "cm") dInMeters = diameter / 100;
else if (diaUnit === "inches") dInMeters = diameter * 0.0254;
// Convert velocity to m/s
var vInMps = velocity;
if (velUnit === "fps") vInMps = velocity * 0.3048;
// Area = Pi * r^2
var radius = dInMeters / 2;
var area = Math.PI * Math.pow(radius, 2);
// Q = A * v (m3/s)
var flowInM3ps = area * vInMps;
flowRateInLps = flowInM3ps * 1000;
}
// Calculations for display
var lpm = flowRateInLps * 60;
var gpm = flowRateInLps * 15.8503;
var m3ph = flowRateInLps * 3.6;
resultHTML += "Liters per Minute (L/min): " + lpm.toFixed(2) + " L/min";
resultHTML += "Gallons per Minute (GPM): " + gpm.toFixed(2) + " gal/min";
resultHTML += "Cubic Meters per Hour (m³/h): " + m3ph.toFixed(2) + " m³/h";
resultHTML += "Liters per Second (L/s): " + flowRateInLps.toFixed(4) + " L/s";
document.getElementById("resultOutput").innerHTML = resultHTML;
document.getElementById("flowResult").style.display = "block";
}
How to Calculate Flow Rate: The Essential Guide
Understanding how to calculate flow rate is critical in fields ranging from civil engineering and plumbing to gardening and industrial manufacturing. Flow rate measures the volume of fluid that passes through a specific point in a given amount of time.
The Two Primary Flow Rate Formulas
Depending on the data you have available, you will typically use one of the two following formulas:
1. The Volumetric Flow Formula (Q = V / t)
This is the most straightforward method. Use this if you are filling a container of a known size.
Q = Flow Rate
V = Volume of the fluid
t = Time taken for the fluid to pass or fill the volume
Example: If it takes 2 minutes (120 seconds) to fill a 10-liter bucket, the flow rate is 10 / 120 = 0.083 liters per second.
2. The Pipe Velocity Formula (Q = A × v)
This formula is used when you know the dimensions of the pipe and the speed at which the fluid is moving.
Q = Flow Rate
A = Cross-sectional Area of the pipe ($\pi \times \text{radius}^2$)
v = Velocity (flow speed) of the fluid
Example: In a pipe with a radius of 0.05 meters and a fluid velocity of 2 m/s, the Area is approximately 0.00785 m². The flow rate (Q) would be 0.00785 × 2 = 0.0157 m³/s.
Common Units of Measurement
Flow rate can be expressed in various units depending on the application:
Unit Type
Abbreviation
Common Use
Liters per Minute
LPM
Plumbing and small pumps
Gallons per Minute
GPM
Residential water flow (US)
Cubic Meters per Hour
m³/h
Industrial and municipal supply
Why Accuracy Matters
Calculating flow rate accurately prevents system failures. In irrigation, an incorrect flow rate can lead to over-watering or pump burnout. In chemical processing, precise flow rates ensure the correct mixture of ingredients. Always ensure your pipe diameter measurements are "Inner Diameter" (ID) rather than "Outer Diameter" (OD), as the thickness of the pipe wall does not contribute to the flow area.