Sand Filter Flow Rate Calculation

15 (Standard Residential) 20 (High Rate Sand Filter) 12 (Public Pool/Commercial)

Results

Filter Surface Area:

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Recommended Flow Rate:

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Hourly Flow Rate:

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Understanding Sand Filter Flow Rates

In pool maintenance, the sand filter flow rate is a critical metric that determines how much water passes through your filter media in a specific timeframe. Calculating this correctly ensures that your pump isn't overpowering your filter (which causes "channeling") and that your water is being adequately cleaned.

How the Calculation Works

The flow rate is determined by two primary factors: the surface area of the sand bed and the filtration design rate. Most residential high-rate sand filters are designed to handle 20 Gallons Per Minute (GPM) per square foot of surface area.

The Math Behind the Calculator:

• Step 1: Find the Radius. Divide the diameter in half and convert to feet (Inches / 2 / 12).
• Step 2: Calculate Surface Area. Use the formula π × r² to find the square footage.
• Step 3: Calculate GPM. Multiply the Surface Area by the Filtration Rate (typically 15-20 GPM).

Example: 24-Inch Sand Filter

If you have a standard 24-inch diameter sand filter:

1. Radius: 12 inches = 1 foot.
2. Surface Area: 3.14 × 1² = 3.14 sq. ft.
3. Flow Rate (at 20 GPM/sq ft): 3.14 × 20 = 62.8 GPM.

Why You Should Never Exceed the Flow Rate

Exceeding the manufacturer's recommended flow rate can lead to several pool system failures:

• Channeling: High pressure creates "tunnels" in the sand, allowing dirty water to bypass the filtration media entirely.
• Filter Damage: Excessive pressure can crack the internal laterals or the filter tank itself.
• Poor Water Clarity: If water moves too fast, small debris particles are forced through the sand bed rather than being trapped by it.

function calculateFlowRate() { var diameter = parseFloat(document.getElementById('filterDiameter').value); var filtrationRate = parseFloat(document.getElementById('filtrationRate').value); var resultsDiv = document.getElementById('calcResults'); if (isNaN(diameter) || diameter <= 0) { alert("Please enter a valid filter diameter."); return; } // Calculate Radius in feet var radiusInFeet = (diameter / 2) / 12; // Calculate Surface Area (sq ft) var surfaceArea = Math.PI * Math.pow(radiusInFeet, 2); // Calculate Flow Rate (GPM) var flowRateGPM = surfaceArea * filtrationRate; // Calculate Flow Rate (GPH) var flowRateGPH = flowRateGPM * 60; // Update UI document.getElementById('surfaceAreaDisplay').innerText = surfaceArea.toFixed(2) + " sq. ft."; document.getElementById('flowRateDisplay').innerText = flowRateGPM.toFixed(1) + " GPM"; document.getElementById('gphDisplay').innerText = Math.round(flowRateGPH).toLocaleString() + " GPH"; resultsDiv.style.display = "block"; // Smooth scroll to results resultsDiv.scrollIntoView({ behavior: 'smooth', block: 'nearest' }); }