Using Pipe Diameter & Velocity
Using Volume & Time
Inches
Millimeters
Meters
Feet
Meters/Second (m/s)
Feet/Second (ft/s)
Liters
US Gallons
Cubic Meters
Cubic Feet
Seconds
Minutes
Hours
Calculation Results
Flow Rate (Cubic Meters/Hour):–
Flow Rate (Liters/Minute):–
Flow Rate (US Gallons/Minute – GPM):–
Flow Rate (Cubic Feet/Minute – CFM):–
Base SI Rate (m³/s):–
Understanding Fluid Flow Rate
Whether you are designing a plumbing system, managing an HVAC setup, or calculating irrigation needs, understanding the fluid flow rate is essential. This calculator allows you to determine the volumetric flow rate of a liquid or gas moving through a pipe or channel using two distinct methods: the Velocity-Area method and the Volume-Time method.
Method 1: Velocity and Pipe Diameter
This method determines the flow rate based on the physical dimensions of the pipe and the speed at which the fluid is traveling. It relies on the Continuity Equation.
Q = A × v
Where:
Q = Volumetric Flow Rate
A = Cross-sectional Area of the pipe (calculated from the diameter)
v = Average Velocity of the fluid
To use this method, you need the internal diameter of the pipe. The calculator first determines the cross-sectional area using the formula A = π × (d/2)², where d is the diameter, and then multiplies it by the velocity.
Method 2: Volume and Time
This is the empirical method often used in field tests (bucket tests). If you can capture a specific volume of fluid over a known period, you can easily calculate the average flow rate.
Q = V / t
Where:
Q = Volumetric Flow Rate
V = Volume of fluid collected
t = Time taken to collect that volume
Common Flow Rate Units
Depending on the industry, flow rate is expressed in various units. This calculator provides conversions for the most common metrics:
GPM (Gallons Per Minute): Standard in the US for plumbing, pumps, and irrigation.
L/min (Liters Per Minute): Common in Europe and scientific applications.
m³/hr (Cubic Meters per Hour): Used for large scale water treatment or industrial processes.
CFM (Cubic Feet per Minute): Often used for air flow in HVAC systems.
Example Calculation
Scenario: You have a 4-inch water pipe, and the water is flowing at a velocity of 2 meters per second. What is the flow rate?
Convert 4 inches to meters: 4″ ≈ 0.1016 meters.
Calculate Radius: 0.1016 / 2 = 0.0508 meters.
Calculate Area: π × 0.0508² ≈ 0.008107 m².
Calculate Flow (Q): 0.008107 m² × 2 m/s = 0.016214 m³/s.
Convert to GPM: 0.016214 × 15,850 ≈ 257 GPM.
function toggleMode() {
var mode = document.getElementById('calcMode').value;
var velSection = document.getElementById('velocityInputSection');
var volSection = document.getElementById('volumeInputSection');
if (mode === 'velocity') {
velSection.classList.remove('hidden');
volSection.classList.add('hidden');
} else {
velSection.classList.add('hidden');
volSection.classList.remove('hidden');
}
// Hide results when mode changes until recalculated
document.getElementById('resultDisplay').style.display = 'none';
}
function calculateFlowRate() {
var mode = document.getElementById('calcMode').value;
var flowRateM3S = 0; // Base unit: Cubic meters per second
var isValid = false;
if (mode === 'velocity') {
// Get inputs
var diameter = parseFloat(document.getElementById('pipeDiameter').value);
var diamUnit = document.getElementById('diameterUnit').value;
var velocity = parseFloat(document.getElementById('flowVelocity').value);
var velUnit = document.getElementById('velocityUnit').value;
if (isNaN(diameter) || isNaN(velocity) || diameter <= 0 || velocity < 0) {
alert("Please enter valid positive numbers for Diameter and Velocity.");
return;
}
// Convert Diameter to Meters
var diameterInMeters = 0;
if (diamUnit === 'inch') diameterInMeters = diameter * 0.0254;
else if (diamUnit === 'mm') diameterInMeters = diameter / 1000;
else if (diamUnit === 'ft') diameterInMeters = diameter * 0.3048;
else diameterInMeters = diameter; // already meters
// Convert Velocity to Meters/Second
var velocityInMS = 0;
if (velUnit === 'fts') velocityInMS = velocity * 0.3048;
else velocityInMS = velocity; // already m/s
// Calculate Area (A = pi * r^2)
var radius = diameterInMeters / 2;
var area = Math.PI * Math.pow(radius, 2);
// Calculate Flow Rate (Q = A * v)
flowRateM3S = area * velocityInMS;
isValid = true;
} else {
// Volume / Time Method
var volume = parseFloat(document.getElementById('volumeAmount').value);
var volUnit = document.getElementById('volumeUnit').value;
var time = parseFloat(document.getElementById('timeDuration').value);
var timeUnit = document.getElementById('timeUnit').value;
if (isNaN(volume) || isNaN(time) || volume < 0 || time <= 0) {
alert("Please enter valid positive numbers for Volume and Time.");
return;
}
// Convert Volume to Cubic Meters
var volumeInM3 = 0;
if (volUnit === 'liters') volumeInM3 = volume / 1000;
else if (volUnit === 'gallons') volumeInM3 = volume * 0.00378541; // US Gallons
else if (volUnit === 'ft3') volumeInM3 = volume * 0.0283168;
else volumeInM3 = volume; // already m3
// Convert Time to Seconds
var timeInSeconds = 0;
if (timeUnit === 'min') timeInSeconds = time * 60;
else if (timeUnit === 'hr') timeInSeconds = time * 3600;
else timeInSeconds = time; // already seconds
// Calculate Flow Rate (Q = V / t)
flowRateM3S = volumeInM3 / timeInSeconds;
isValid = true;
}
if (isValid) {
// Conversions from m3/s
var m3perHour = flowRateM3S * 3600;
var litersPerMin = flowRateM3S * 60000; // (m3/s * 1000 L/m3 * 60 s/min)
var gpm = flowRateM3S * 15850.32314; // US Gallons per minute
var cfm = flowRateM3S * 2118.88; // Cubic Feet per Minute
// Update DOM
document.getElementById('resM3S').innerText = flowRateM3S.toFixed(6) + " m³/s";
document.getElementById('resM3H').innerText = m3perHour.toFixed(4);
document.getElementById('resLPM').innerText = litersPerMin.toFixed(2);
document.getElementById('resGPM').innerText = gpm.toFixed(2);
document.getElementById('resCFM').innerText = cfm.toFixed(2);
document.getElementById('resultDisplay').style.display = 'block';
// Scroll to results
document.getElementById('resultDisplay').scrollIntoView({behavior: 'smooth'});
}
}