Rainfall Rate Calculator

Rainfall Rate & Volume Calculator

Calculation Results

Rainfall Intensity: –

Total Water Volume: –

Metric Volume: –

function calculateRainfall() { var depth = parseFloat(document.getElementById('rainDepth').value); var duration = parseFloat(document.getElementById('rainDuration').value); var area = parseFloat(document.getElementById('catchmentArea').value); var coeff = parseFloat(document.getElementById('runoffCoeff').value); if (isNaN(depth) || isNaN(duration) || isNaN(area) || duration <= 0) { alert("Please enter valid positive numbers for depth, duration, and area."); return; } // Calculate Intensity (inches per hour) var intensity = depth / duration; // Calculate Total Volume in Gallons // Formula: Area (sq ft) * Depth (in) * (1 ft / 12 in) * (7.48 gallons / 1 cubic ft) * efficiency var totalGallons = (area * depth / 12) * 7.48052 * coeff; // Convert to Liters var totalLiters = totalGallons * 3.78541; document.getElementById('resIntensity').innerText = intensity.toFixed(2) + " inches / hour"; document.getElementById('resGallons').innerText = totalGallons.toLocaleString(undefined, {maximumFractionDigits: 2}) + " Gallons"; document.getElementById('resLiters').innerText = totalLiters.toLocaleString(undefined, {maximumFractionDigits: 2}) + " Liters"; document.getElementById('rainResults').style.display = 'block'; }

Understanding Rainfall Rates and Catchment

A Rainfall Rate Calculator is an essential tool for hydrologists, gardeners, and homeowners planning rainwater harvesting systems. It helps you determine how quickly water is falling and exactly how much volume you can expect to collect from a specific surface area, such as a roof or a driveway.

The Importance of Rainfall Intensity

Rainfall intensity is measured by the depth of rain falling over a specific period, usually expressed in inches per hour. High intensity can lead to flash flooding and soil erosion, while consistent low intensity is ideal for garden absorption. If you are designing a drainage system, knowing the intensity helps you size your gutters and downspouts correctly.

How to Calculate Rainwater Volume

To find out how much water hits your roof, you can use the following standard formula:

Volume (Gallons) = Area (sq. ft.) × Rainfall (inches) × 0.623

The 0.623 factor represents the number of gallons in one inch of water covering one square foot. However, not all water is caught; we use a Runoff Coefficient to account for evaporation and absorption. A metal roof might have a coefficient of 0.95, whereas a grassy lawn might only have a coefficient of 0.10 to 0.20.

Real-World Example

Imagine you have a 2,000 square foot roof and you experience a 1-inch rainstorm over 4 hours.

• Intensity: 1 inch / 4 hours = 0.25 inches per hour.
• Volume: 2,000 sq ft × 1 inch × 0.623 × 0.90 (efficiency) = 1,121.4 Gallons.

This data reveals that even a modest storm can quickly fill several rain barrels, providing a significant source of free water for your landscape.