How to Calculate Fan Air Flow Rate

Fan Air Flow (CFM) Calculator

Determine the required Cubic Feet per Minute (CFM) for your space based on room dimensions and desired air changes.

Recommended Fan Capacity

0 CFM

function calculateFanCFM() { var length = parseFloat(document.getElementById('roomLength').value); var width = parseFloat(document.getElementById('roomWidth').value); var height = parseFloat(document.getElementById('roomHeight').value); var ach = parseFloat(document.getElementById('airChanges').value); var resultArea = document.getElementById('fan-result-area'); var cfmOutput = document.getElementById('cfmOutput'); var details = document.getElementById('calcDetails'); if (isNaN(length) || isNaN(width) || isNaN(height) || length <= 0 || width <= 0 || height <= 0) { alert("Please enter valid positive numbers for all room dimensions."); return; } // Calculation: CFM = (Volume * ACH) / 60 var volume = length * width * height; var requiredCFM = (volume * ach) / 60; // Rounding up for safety in ventilation var finalCFM = Math.ceil(requiredCFM); cfmOutput.innerHTML = finalCFM + " CFM"; details.innerHTML = "Based on a room volume of " + volume.toLocaleString() + " cubic feet and " + ach + " air changes per hour."; resultArea.style.display = "block"; }

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How to Calculate Fan Air Flow Rate (CFM)

Understanding air flow rate is essential for maintaining indoor air quality, controlling moisture, and ensuring comfort. The standard unit of measurement for fan performance is CFM (Cubic Feet per Minute). This metric tells you how many cubic feet of air the fan can move every sixty seconds.

The Formula for Sizing a Fan

To calculate the required CFM for any room, you need two primary pieces of information: the total volume of the space and the number of air changes required per hour (ACH). The mathematical formula is:

CFM = (Room Volume × Air Changes per Hour) / 60

Step-by-Step Calculation Guide

1. Measure the Volume: Multiply the Length × Width × Height of the room to get the total cubic feet.
2. Select the ACH: Choose the appropriate Air Changes per Hour based on the room type (e.g., a bathroom typically needs more air turnover than a bedroom).
3. Calculate: Multiply the volume by the ACH, then divide by 60 to convert from hourly to per-minute rates.

Recommended Air Changes per Hour (ACH) Reference

Room Type	Recommended ACH
Bathroom	8 – 10 ACH
Kitchen	6 – 8 ACH
Bedroom/Living Room	2 – 4 ACH
Home Workshop	12 – 15 ACH

Practical Example

Imagine you have a bathroom that is 8 feet long, 10 feet wide, and 8 feet high. You want to clear steam effectively, so you aim for 8 Air Changes per Hour.

• Volume: 8 × 10 × 8 = 640 cubic feet.
• Calculation: (640 × 8) / 60 = 85.33 CFM.
• Result: You should look for a fan rated for at least 90 CFM to ensure proper ventilation.

Important Considerations

• Static Pressure: If your fan is connected to long ducts or has many bends, the actual air flow will be lower than the rated CFM. Always choose a slightly higher CFM than your calculation suggests to account for this resistance.
• Noise Level (Sones): While CFM measures power, "Sones" measure noise. A lower Sone rating means a quieter fan.
• Energy Efficiency: Look for Energy Star certified fans to save on electricity costs, especially for fans that run continuously.