Bucket Test (Volume & Time)
Pipe Flow (Diameter & Velocity)
Enter the size of the bucket or container you filled.
Use a stopwatch to measure exactly how long it takes to fill.
The internal diameter of the pipe.
Average speed of water moving through the pipe (standard residential is 5-8 ft/s).
Flow Rate: 0.00 GPM
Hourly Flow: 0.00 GPH
function toggleFlowInputs() {
var method = document.getElementById("calcMethod").value;
var bucketDiv = document.getElementById("bucketInputs");
var pipeDiv = document.getElementById("pipeInputs");
if (method === "bucket") {
bucketDiv.style.display = "block";
pipeDiv.style.display = "none";
} else {
bucketDiv.style.display = "none";
pipeDiv.style.display = "block";
}
// Hide results when switching modes
document.getElementById("resultBox").style.display = "none";
}
function calculateFlowRate() {
var method = document.getElementById("calcMethod").value;
var gpm = 0;
var error = false;
if (method === "bucket") {
var volume = parseFloat(document.getElementById("containerVolume").value);
var time = parseFloat(document.getElementById("fillTime").value);
if (isNaN(volume) || isNaN(time) || volume <= 0 || time <= 0) {
alert("Please enter valid positive numbers for Volume and Time.");
return;
}
// Formula: (Gallons / Seconds) * 60 = GPM
gpm = (volume / time) * 60;
} else {
var diameter = parseFloat(document.getElementById("pipeDiameter").value);
var velocity = parseFloat(document.getElementById("waterVelocity").value);
if (isNaN(diameter) || isNaN(velocity) || diameter <= 0 || velocity <= 0) {
alert("Please enter valid positive numbers for Diameter and Velocity.");
return;
}
// 1. Convert Diameter from inches to feet
var diameterFeet = diameter / 12;
// 2. Calculate Radius in feet
var radiusFeet = diameterFeet / 2;
// 3. Calculate Area in square feet (pi * r^2)
var areaSqFt = Math.PI * Math.pow(radiusFeet, 2);
// 4. Calculate Flow in Cubic Feet per Second (CFS) = Area * Velocity
var cfs = areaSqFt * velocity;
// 5. Convert CFS to GPM (1 CFS = 448.831 GPM)
gpm = cfs * 448.831;
}
// Calculate Gallons Per Hour (GPH)
var gph = gpm * 60;
// Display Results
var resultBox = document.getElementById("resultBox");
var gpmDisplay = document.getElementById("gpmResult");
var gphDisplay = document.getElementById("gphResult");
gpmDisplay.innerText = gpm.toFixed(2);
gphDisplay.innerText = gph.toFixed(1);
resultBox.style.display = "block";
}
Understanding Water Flow Rate (GPM)
Water Flow Rate measures the volume of water that passes through a specific point in a system over a set period. It is most commonly expressed in GPM (Gallons Per Minute) in the United States. Knowing your flow rate is critical for sizing pumps, designing irrigation systems, checking well yields, and selecting the right water filtration systems.
Method 1: The Bucket Test
The most accurate and simplest way to measure flow rate at a specific outlet (like a showerhead, garden hose, or faucet) is the "Bucket Test." This physical measurement eliminates guesswork regarding pipe friction or pressure drops.
How to do it:
Get a container of a known volume (e.g., a 5-gallon bucket).
Get a stopwatch or use the timer on your phone.
Turn the water on fully.
Place the bucket under the stream and start your timer simultaneously.
Stop the timer the instant the water reaches the known volume mark.
Input the volume and the seconds into the calculator above.
Formula:(Volume in Gallons / Time in Seconds) × 60 = GPM
Method 2: Pipe Dimensions & Velocity
If measuring the physical water output isn't possible, or if you are designing a system and need to calculate theoretical flow capacity, you can use the pipe's internal diameter and the water velocity.
Diameter: The internal width of the pipe (e.g., 1 inch for standard main lines).
Velocity: The speed at which water travels. For residential plumbing, safe velocities are typically between 5 to 8 feet per second (ft/s) to prevent noise and pipe wear.
Formula:Area × Velocity × 448.8 = GPM.
(Note: Area must be calculated in square feet).
Why GPM Matters for Homeowners
Well Yield: If you rely on well water, your GPM determines if your well can keep up with household demand. A standard home typically requires 6–12 GPM.
Irrigation: Sprinkler heads have specific GPM requirements. If your system's flow rate is too low, sprinkler heads will not pop up or cover the intended radius.
Tankless Water Heaters: These units are sized based on GPM. A heater rated for 4 GPM might not support a shower (2.5 GPM) and a dishwasher (1.5 GPM) running simultaneously.