How to Calculate Air Flow Rate of Exhaust Fan

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Exhaust Fan CFM Calculator

Bathroom (8 ACH) Kitchen (15 ACH) Garage (6 ACH) Bedroom/Living Room (4 ACH) Basement (10 ACH) Commercial Kitchen (20 ACH)

Results:

0 CFM

function calculateCFM() { 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('achValue').value); if (isNaN(length) || isNaN(width) || isNaN(height) || length <= 0 || width <= 0 || height <= 0) { alert("Please enter valid positive numbers for dimensions."); return; } // Formula: CFM = (Volume * ACH) / 60 // Volume = L * W * H var volume = length * width * height; var cfm = (volume * ach) / 60; // Bathroom Rule of Thumb check: Minimum 50 CFM for bathrooms if (ach === 8 && cfm < 50) { cfm = 50; } document.getElementById('finalCfm').innerText = Math.ceil(cfm); document.getElementById('recommendation').innerText = "For a " + length + "x" + width + " room, you need an exhaust fan rated for at least " + Math.ceil(cfm) + " Cubic Feet per Minute (CFM). It is often recommended to size up slightly to account for duct resistance."; document.getElementById('cfmResult').style.display = 'block'; }

How to Calculate Air Flow Rate for Exhaust Fans

Choosing the right exhaust fan is critical for maintaining indoor air quality, removing moisture, and preventing mold growth. The air flow rate of a fan is measured in CFM (Cubic Feet per Minute), which indicates how much air the fan can move out of the room every sixty seconds.

The Standard Formula

To calculate the required CFM for any room, use the following steps:

  1. Calculate Room Volume: Multiply Length × Width × Height (in feet).
  2. Determine ACH (Air Changes per Hour): This is how many times the air in the room should be completely replaced per hour.
  3. The Equation: CFM = (Volume × ACH) ÷ 60

Recommended Air Changes per Hour (ACH)

Room Type Recommended ACH
Bathrooms 8 times per hour
Kitchens 15 times per hour
Garages 6 times per hour
Basements 10 times per hour

Real-World Example

Suppose you have a bathroom that is 8 feet wide, 10 feet long, and has an 8-foot ceiling.

  • Volume: 8 × 10 × 8 = 640 cubic feet.
  • Required ACH: 8 (Standard for bathrooms).
  • Calculation: (640 × 8) ÷ 60 = 85.33 CFM.

In this scenario, you should look for an exhaust fan with a rating of at least 90 CFM to ensure efficient moisture removal.

Factors That Affect Fan Performance

It is important to remember that the CFM rating on the box is the "rated capacity." Real-world performance may be lower due to:

  • Duct Length: Longer ducts create more static pressure, slowing down the air.
  • Bends and Elbows: Every 90-degree turn in your ductwork significantly reduces actual air flow.
  • Vent Caps: Small or clogged external vent hoods can restrict air movement.

Expert Tip: If your ductwork is long or has many turns, select a fan with a CFM rating 20-30% higher than your calculated requirement.

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