Use this calculator to determine the Gallons Per Minute (GPM) flow rate of water or other fluids. We provide two methods of calculation: the "Bucket Test" (Volume/Time) for measuring existing outlets, and the "Pipe Velocity" method for theoretical flow based on pipe size.
Calculate Water Flow Rate
Method 1: Bucket Test (Volume vs Time)
Best for measuring garden hoses, faucets, or showerheads.
Gallons
Seconds
Flow Rate Results
Flow Rate (GPM):–
Flow Rate (GPH):–
Liters per Minute:–
Method 2: Pipe Dimension & Velocity
Best for calculating theoretical flow in plumbing systems.
Inches
Feet per Second (ft/s)
Pipe Flow Results
Flow Rate (GPM):–
Cubic Ft/Sec (CFS):–
Cross-Section Area:–
function calculateBucketFlow() {
// Get inputs
var volume = parseFloat(document.getElementById('bucket_volume').value);
var timeSeconds = parseFloat(document.getElementById('bucket_time').value);
var resultsDiv = document.getElementById('bucket_results');
// Validation
if (isNaN(volume) || isNaN(timeSeconds) || timeSeconds <= 0) {
alert("Please enter a valid volume and a time greater than 0.");
resultsDiv.style.display = 'none';
return;
}
// Calculation Logic
// GPM = Volume (Gallons) / (Time (Seconds) / 60)
var gpm = volume / (timeSeconds / 60);
var gph = gpm * 60; // Gallons per Hour
var lpm = gpm * 3.78541; // Liters per Minute
// Display Results
document.getElementById('res_bucket_gpm').innerHTML = gpm.toFixed(2) + " GPM";
document.getElementById('res_bucket_gph').innerHTML = gph.toFixed(0) + " GPH";
document.getElementById('res_bucket_lpm').innerHTML = lpm.toFixed(2) + " L/min";
resultsDiv.style.display = 'block';
}
function calculatePipeFlow() {
// Get inputs
var diameter = parseFloat(document.getElementById('pipe_diameter').value);
var velocity = parseFloat(document.getElementById('flow_velocity').value);
var resultsDiv = document.getElementById('pipe_results');
// Validation
if (isNaN(diameter) || isNaN(velocity) || diameter <= 0) {
alert("Please enter a valid pipe diameter and velocity.");
resultsDiv.style.display = 'none';
return;
}
// Calculation Logic
// Formula: GPM = 2.448 * d^2 * v
// where d is diameter in inches, v is velocity in ft/sec
var gpm = 2.448 * Math.pow(diameter, 2) * velocity;
// Calculate Cubic Feet per Second (CFS)
// CFS = GPM / 448.831
var cfs = gpm / 448.831;
// Calculate Area in sq inches
var radius = diameter / 2;
var area = Math.PI * Math.pow(radius, 2);
// Display Results
document.getElementById('res_pipe_gpm').innerHTML = gpm.toFixed(2) + " GPM";
document.getElementById('res_pipe_cfs').innerHTML = cfs.toFixed(4) + " ft³/s";
document.getElementById('res_pipe_area').innerHTML = area.toFixed(2) + " in²";
resultsDiv.style.display = 'block';
}
Understanding Gallons Per Minute (GPM)
GPM, or Gallons Per Minute, is the standard unit of measurement for liquid flow rate in the United States. It quantifies the volume of liquid that passes through a given point in a system within one minute. Understanding your GPM is crucial for various applications, including:
Irrigation Systems: Ensuring sprinkler heads receive adequate water pressure and volume.
Plumbing: Sizing tankless water heaters and filtration systems.
Industrial Pumps: Verifying pump efficiency and system requirements.
Fire Fighting: Calculating water delivery capability for hoses and hydrants.
How to Calculate Flow Rate
There are two primary ways to determine GPM, depending on the information available to you.
1. The Bucket Test Method
This is the most practical method for measuring flow from an open outlet, such as a garden hose or bathtub faucet. You simply measure how long it takes to fill a container of known volume.
Formula:
GPM = Container Volume (Gallons) ÷ (Time to Fill (Seconds) ÷ 60)
Example: If it takes 15 seconds to fill a 5-gallon bucket:
GPM = 5 ÷ (15 ÷ 60) = 5 ÷ 0.25 = 20 GPM
2. The Pipe Diameter & Velocity Method
This method is used in engineering and design when you cannot physically measure the water output but know the pipe specifications and fluid velocity. The velocity of water in residential piping typically ranges from 5 to 10 feet per second (ft/s) to prevent noise and erosion.