This Ductulator uses the Colebrook equation to calculate the smallest duct size acceptable for the selected friction rate and duct type that stays within the selected ACCA Manual D recommended velocities. Unless within 2%, the diameter is rounded up to the nearest inch. Rectangular duct is rounded up to the nearest 1/4 inch. Calculations are for 60°F dry air at sea level, from 5 to 2500 cfm (approximately standard air which is 0.075lb_{da}/ft³).
If "ASHRAE 2013" is selected, Flexible roughness is 0.003 ft when straight and fully stretched tight at 0% compression (adjusted at other compressions), otherwise the original method of constant 0.01 ft roughness is used.

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