# How to Convert Duct Area to Square Feet

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HVAC ducts must be sized accurately to deliver the airflow needed for effective heating and cooling. Ducts with a cross-sectional area that is too small won't provide enough air, while those with an area that is too large will cause your HVAC equipment to overwork and wear out more quickly.

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Duct size is determined by the velocity of airflow needed to supply the part of the building the duct is servicing. Airflow is measured in cubic feet per minute (CFM), but duct dimensions are typically measured in inches, which calls for a conversion factor to convert a cross-sectional area from square inches to square feet. Even though most pros and homeowners routinely refer to charts rather than doing the calculations themselves, it's good to know this conversion factor and how to use it.

## How to Convert to Square Feet

Area is always measured in square units because it's calculated by multiplying one dimension by another. This is most obvious when calculating the area of a rectangle, which you do by multiplying the length of the rectangle by the width. If the length is L inches and the width is W inches, the area is L x W square inches. To calculate the area of a circle, you multiply the radius by itself (in other words, you square the radius) and multiply by π (pi = 3.14), which expressed mathematically is πr2. If you're using oval-shaped ducts with a long radius r1 and a radius of r2, the area is (π x r1 x r2).

The problem for ductwork installers is that dimensions are usually measured in inches, but you need square footage for airflow calculations. To convert from square inches to square feet, all you need to remember is that there are 12 inches in a foot. Therefore, 1 square foot is 12 x 12 = 144 square inches and 1 square inch = 1/144 = 0.0069 square feet.

## Transitions and Sample Calculations

Ductwork installations often call for transitioning from rectangular to round or oval ductwork or vice versa because of space limitations. When changing shapes, it's important to maintain the same cross-sectional area. To see how this is done, consider transitioning from a square 6 x 6 duct to a circular one.

The cross-sectional area of the square duct is 8 x 8 = 64 square inches. To find a round duct with the same area, consider that πr2 = 64 and solve for r: r2 = 64/3.14 = 20.4 square inches, so r = √20.4 = 4.5 inches. Since radius is half the diameter of a circle and you always measure round duct by diameter, you need 9-inch round duct to make the transition.