Determine the water flow required based on spillway dimensions.
This is the thickness of the water sheet passing over the lip.
Flow Rate (GPM):–
Flow Rate (GPH):–
Flow Rate (Liters/Hour):–
Estimated Visual Effect:–
function calculateFlow() {
// 1. Get input values
var widthInput = document.getElementById("spillwayWidth").value;
var depthInput = document.getElementById("waterDepth").value;
var resultDiv = document.getElementById("result");
// 2. Validate inputs
if (widthInput === "" || depthInput === "" || isNaN(widthInput) || isNaN(depthInput)) {
alert("Please enter valid numbers for both width and depth.");
return;
}
var widthInches = parseFloat(widthInput);
var depthInches = parseFloat(depthInput);
if (widthInches <= 0 || depthInches <= 0) {
alert("Width and depth must be greater than zero.");
return;
}
// 3. Calculation Logic based on Francis Formula for Rectangular Weirs
// Formula: Q (cfs) = 3.33 * L(ft) * H(ft)^1.5
// Note: This assumes a suppressed weir (no side contractions), typical for artificial waterfalls.
var widthFt = widthInches / 12;
var depthFt = depthInches / 12;
// Calculate Cubic Feet per Second (CFS)
var cfs = 3.33 * widthFt * Math.pow(depthFt, 1.5);
// 4. Unit Conversions
// 1 CFS = 448.831 Gallons Per Minute (GPM)
var gpm = cfs * 448.831;
// Gallons Per Hour
var gph = gpm * 60;
// Liters Per Hour (1 Gallon = 3.78541 Liters)
var lph = gph * 3.78541;
// 5. Determine Visual Effect Description
var visualEffect = "";
// Based on GPH per inch of width
var gphPerInch = gph / widthInches;
if (gphPerInch < 50) {
visualEffect = "Light Trickle / Broken Sheet";
} else if (gphPerInch < 150) {
visualEffect = "Clear, Consistent Sheet";
} else if (gphPerInch < 250) {
visualEffect = "Strong, Turbulent Flow";
} else {
visualEffect = "Heavy 'Niagara' Style Flow";
}
// 6. Output Results
document.getElementById("resGPM").innerHTML = gpm.toFixed(2) + " GPM";
document.getElementById("resGPH").innerHTML = Math.round(gph).toLocaleString() + " GPH";
document.getElementById("resLPH").innerHTML = Math.round(lph).toLocaleString() + " L/hr";
document.getElementById("resVisual").innerHTML = visualEffect;
resultDiv.style.display = "block";
}
Understanding Waterfall Flow Rate Calculations
Designing a backyard pond or a commercial water feature requires precise calculations to ensure the water flows visually as intended. The Waterfall Flow Rate Calculator helps builders and hobbyists determine the necessary pump capacity (Gallons Per Hour) based on the physical dimensions of the spillway and the desired water depth at the crest.
Why is this important? If you purchase a pump that is too small, your waterfall may look like a dripping faucet rather than a cascading sheet. Conversely, an oversized pump can cause splashing, water loss, and excessive noise.
The Math Behind the Flow: The Weir Formula
In fluid mechanics, a waterfall acts as a "weir"—a barrier across the horizontal width of a river that alters the flow characteristics of water. For most artificial waterfalls, we use the standard Francis Formula for rectangular weirs without end contractions (suppressed weirs):
Q = 3.33 × L × H1.5
Where:
Q = Discharge (Flow Rate) in Cubic Feet per Second (CFS).
L = Width of the weir (spillway) in Feet.
H = Head (depth of water) above the weir crest in Feet.
3.33 = A constant accounting for gravity and discharge coefficients.
Visualizing Flow Depth
The "Water Depth" input in the calculator refers to the thickness of the water sheet right as it passes over the lip of the spillway. This thickness dictates the aesthetic of the waterfall. Here is a general guide to interpreting the results:
Water Depth (Inches)
Approximate Effect
Typical Application
0.1″ – 0.25″
Trickle / Broken Sheet
Gentle background noise, zen gardens
0.25″ – 0.5″
Standard Clear Sheet
Most residential backyard ponds
0.5″ – 1.0″
Turbulent / Heavy
Large landscape features, swimming pool descents
1.0″+
Raging River
Commercial displays, large weirs
Selecting the Right Pump
Once you have calculated the required Gallons Per Hour (GPH) using the tool above, you need to select a pump. However, the GPH rating on a pump box is usually its "maximum flow" at 0 feet of lift. To get the correct flow at your waterfall:
Calculate Static Head: Measure the vertical distance from the surface of the pond water to the top of the waterfall spillway.
Account for Friction Loss: Add approximately 1 foot of head for every 10 feet of tubing and every 90-degree elbow fitting.
Check the Pump Curve: Look at the manufacturer's flow chart. Find the flow rate at your calculated "Total Dynamic Head."
For example, if this calculator suggests you need 3,000 GPH for the visual effect, and your waterfall is 5 feet high, you should not buy a pump rated for 3,000 GPH max. You likely need a pump rated for 4,500+ GPH to deliver 3,000 GPH at that specific height.