How to Convert an HVAC Duct Size From Round to Square & Rectangular

Find round-to-square duct size equivalents by computing the cross-sectional area of your round duct. To find the cross-sectional area, use a calculator to multiply the circular duct’s radius by itself. Multiply the resulting product by Pi, or 3.1416. Use the calculator to find the square root of the round duct’s cross-sectional area.

Round the square root up to the nearest whole number. This gives you the side of a square duct equivalent in size to your round duct. For example, if you had a round duct 12 inches in diameter, divide 12 by 2 to get a radius of 6. Multiply 6 by 6 to get 36, and then multiply 36 by 3.1416 to get 113.1.

Figure the square root of 113.1 to get 10.63. Round 10.63 up to 11. Your 12-inch round duct will be roughly equivalent to a square duct that is 11 inches on each side.

Find a rectangular duct that’s roughly equivalent in size to the diameter of your round duct by multiplying the length of a rectangular duct’s long side by the length of the short side. Divide that number by .625. Divide the result by the sum of the short and long sides. Multiply that result by 1.4 to get the approximate circular duct equivalent.

If you have a rectangular duct 12 inches by 10 inches, multiply 12 by 10 to get 120. Divide 120 by .625 to get 192. Add 10 and 12 to get 22. Divide 192 by 22 to get 8.73. Multiply 8.73 by 1.4 to get an equivalent circular duct diameter of 12.2 inches.

If you had a 12-inch round duct, a 10-by-12-inch rectangular duct would be its rough equivalent.