Gutter Flow Rate Calculator

Gutter Flow Rate Calculator

The horizontal area draining into the specific gutter section.

Based on your local 5-minute peak rain storm data (often 3″ to 6″).

function calculateGutterFlow() { // Get input values var areaStr = document.getElementById('roofAreaSqFt').value; var intensityStr = document.getElementById('rainfallIntensity').value; var resultDiv = document.getElementById('gutterResult'); // Validate inputs if (areaStr === "" || intensityStr === "") { resultDiv.style.display = "block"; resultDiv.innerHTML = "Please enter both roof area and rainfall intensity."; return; } var area = parseFloat(areaStr); var intensity = parseFloat(intensityStr); if (isNaN(area) || area <= 0 || isNaN(intensity) || intensity <= 0) { resultDiv.style.display = "block"; resultDiv.innerHTML = "Please enter valid positive numbers."; return; } // Calculation Logic // Formula: GPM = (Roof Area (sq ft) * Intensity (in/hr)) / 96.25 // The divisor 96.25 converts square feet-inches/hour into gallons per minute. var flowRateGPM = (area * intensity) / 96.25; // Format results var gpmFormatted = flowRateGPM.toFixed(2); // Display results resultDiv.style.display = "block"; resultDiv.innerHTML = "Required Gutter Flow Rate: " + gpmFormatted + " GPM (Gallons Per Minute)"; }

Understanding Gutter Flow Rate and Drainage Sizing

Properly sizing your gutter system is crucial for protecting your home's foundation, landscaping, and siding from water damage. The primary goal of a gutter system is to handle the volume of water shedding off the roof during a peak rain event. This calculator helps determine the required flow rate, measured in Gallons Per Minute (GPM), that a gutter section must handle based on the roof's surface area and local weather conditions.

How the Calculation Works

The required flow rate is determined by two main factors based on standard plumbing engineering formulas:

1. Roof Footprint Area: This is the horizontal projected area of the roof section that drains into a specific gutter run. It is not the surface area of the shingles, but rather the "shadow" the roof casts on the ground. For a simple gable roof, this is length multiplied by horizontal width.
2. Rainfall Intensity: This is a measure of how hard it rains during a severe storm in your specific geographical location, usually measured in inches per hour (in/hr). Plumbing codes often use a "10-year" or "100-year" storm event, specifically focusing on the peak 5-minute duration intensity.

The formula used converts square feet of catchment area and inches per hour of rain into a standardized flow rate of Gallons Per Minute (GPM). The standard divisor for imperial units is 96.25.

Formula: Flow Rate (GPM) = (Area in Sq. Ft. × Rainfall Intensity in In/Hr) / 96.25

Example Calculation

Let's assume you are sizing a gutter for the back of a house. The roof section draining to this gutter has a footprint of 40 feet by 25 feet, equaling a 1,000 sq. ft. area. You live in an area where the 100-year peak rainfall intensity is recommended to be calculated at 5.0 inches per hour.

Using the calculator:

• Input Roof Area: 1000
• Input Rainfall Intensity: 5.0
• Calculation: (1000 × 5.0) / 96.25 = 51.95 GPM

The result indicates that the gutter and downspout system for that section of the roof needs to be capable of moving approximately 52 Gallons Per Minute without overflowing.

Using the Results

Once you have the required GPM, you can select gutter and downspout sizes that meet this capacity. For example, a standard 5-inch K-style gutter might handle around that 50 GPM threshold depending on the slope and number of downspouts, whereas a 6-inch gutter could handle significantly more. Always ensure your downspouts are sized appropriately to match the gutter's flow capacity.