Return Air Duct Size Calculator

Properly sizing your return air duct is critical for the efficiency, performance, and longevity of your HVAC system. An undersized return can strain your system's blower motor, reduce airflow, increase energy bills, and create uncomfortable temperature imbalances. This calculator helps you determine the appropriate return duct size based on the tonnage of your air conditioning unit.

How Return Duct Sizing Works

The calculation for duct sizing is based on two key factors: the total volume of air your system needs to move, measured in Cubic Feet per Minute (CFM), and the desired velocity of that air, measured in Feet per Minute (FPM).

CFM (Cubic Feet per Minute): A standard industry rule of thumb is that an HVAC system requires 400 CFM of airflow per ton of cooling capacity.
FPM (Feet per Minute): This is the speed of the air moving through the duct. For return ducts, a lower velocity is preferred to minimize noise. A target of 600 FPM is a good standard for quiet operation.

The formula to find the required duct area in square inches is:
Required Area (sq. in.) = (Total CFM / Target FPM) * 144

Manual Calculation Example

Let's say you have a 3-ton HVAC system.

Calculate Total CFM: 3 Tons * 400 CFM/Ton = 1200 CFM
Calculate Required Area: (1200 CFM / 600 FPM) * 144 = 288 square inches
Determine Duct Dimensions:
- For a round duct, the diameter would be approximately 19.1 inches. You would typically use the next standard size up, which is a 20-inch round duct.
- For a square duct, the side length would be the square root of 288, which is ~17 inches. A 17″ x 17″ or 16″ x 18″ duct would be suitable.

Calculate Your Duct Size

HVAC System Size (Tons):

Disclaimer: This calculator provides an estimate based on industry standards. Factors like duct length, number of bends, and filter type can affect required sizing. For a precise assessment, always consult with a qualified HVAC professional.

function calculateDuctSize() { var tonnage = parseFloat(document.getElementById('hvacTonnage').value); var resultDiv = document.getElementById('result'); if (isNaN(tonnage) || tonnage d = 2 * sqrt(Area / PI) var roundDuctDiameter = Math.sqrt(requiredAreaSqIn / Math.PI) * 2; // For Square Duct: Area = side^2 => side = sqrt(Area) var squareDuctSide = Math.sqrt(requiredAreaSqIn); var resultHTML = '

Calculation Results:

' + 'Total Airflow Required: ' + totalCFM.toFixed(0) + ' CFM' + 'Required Duct Area: ' + requiredAreaSqIn.toFixed(1) + ' sq. inches' + '

' + '

Recommended Duct Sizes:

' + 'Round Duct Diameter: ' + roundDuctDiameter.toFixed(1) + ' inches (use next standard size up, e.g., ' + Math.ceil(roundDuctDiameter) + '")' + 'Square Duct Dimensions: ' + squareDuctSide.toFixed(1) + '" x ' + squareDuctSide.toFixed(1) + '" (e.g., ' + Math.ceil(squareDuctSide) + '" x ' + Math.ceil(squareDuctSide) + '")'; resultDiv.innerHTML = resultHTML; resultDiv.style.display = 'block'; }

Calculate Return Air Duct Size