Calculate the precipitation rate of your sprinkler system and determine how long to run your zones.
Total gallons per minute for the zone or sprinkler head.
Total area being watered by the flow above.
How much water do you want to apply? (Standard is often 1.0 inch/week).
Application Rate (Precipitation Rate):0.00 in/hr
Time to Apply Target Depth:0 minutes
How to Calculate Irrigation Rate
Understanding your irrigation rate (often called the precipitation rate) is crucial for efficient water management. It defines how quickly your sprinkler system applies water to a specific area. If you apply water faster than the soil can absorb it, you cause runoff and waste. If you apply it too slowly, you may need to run your system for excessively long periods.
The Irrigation Rate Formula
The standard formula used by landscape professionals to calculate the application rate in inches per hour is:
Application Rate (in/hr) = (96.3 × Total GPM) ÷ Total Area (sq. ft.)
96.3: A conversion constant derived to convert gallons per minute and square feet into inches per hour.
Total GPM: The total flow rate in Gallons Per Minute. This can be for a single sprinkler head or an entire zone.
Total Area: The square footage covered by that specific flow.
Why This Calculation Matters
Most lawns require approximately 1 to 1.5 inches of water per week during the growing season. Without knowing your precipitation rate, you are merely guessing how long to run your sprinklers.
For example, if your system delivers water at a rate of 0.5 inches per hour, you would need to run the system for exactly 2 hours (120 minutes) to apply 1 inch of water. However, if your system delivers 2.0 inches per hour, running it for 2 hours would result in severe overwatering and potential root rot.
Steps to Measure Your Inputs
Determine Flow Rate (GPM): You can check the manufacturer specifications for your sprinkler nozzles based on your water pressure (PSI), or perform a "bucket test" by timing how long it takes to fill a measured container.
Measure the Area: Calculate the square footage (Length × Width) of the zone being watered.
Input into the Calculator: Enter these figures above to see your exact rate and required run time.
Note: Efficiency varies based on system uniformity. It is often recommended to audit your irrigation system annually to ensure even coverage.
function calculateIrrigationRate() {
// Get input elements by ID strictly
var flowInput = document.getElementById('irrigation_flow');
var areaInput = document.getElementById('irrigation_area');
var targetInput = document.getElementById('irrigation_target');
var resultsDiv = document.getElementById('irrigation-results');
var resRateDisplay = document.getElementById('res_rate');
var resTimeDisplay = document.getElementById('res_time');
// Parse values
var gpm = parseFloat(flowInput.value);
var area = parseFloat(areaInput.value);
var targetDepth = parseFloat(targetInput.value);
// Validation
if (isNaN(gpm) || gpm <= 0) {
alert("Please enter a valid Flow Rate greater than 0.");
return;
}
if (isNaN(area) || area 0) {
timeMinutes = (targetDepth / rate) * 60;
}
// Display Logic
resultsDiv.style.display = "block";
resRateDisplay.innerHTML = rate.toFixed(2) + " in/hr";
if (timeMinutes > 0) {
// Format time nicely
var hours = Math.floor(timeMinutes / 60);
var mins = Math.round(timeMinutes % 60);
var timeString = "";
if (hours > 0) {
timeString += hours + " hr ";
}
timeString += mins + " min";
// Add raw minutes in parens if over an hour
if (hours > 0) {
timeString += " (" + Math.round(timeMinutes) + " total min)";
}
resTimeDisplay.innerHTML = timeString;
} else {
resTimeDisplay.innerHTML = "N/A (Enter target depth)";
}
}