ResDuct - Residential Air Duct Calculator v1.4 Enter and adjust the parameters in the blue boxes. Click the "Calculate" button for the results.
Header image
      Friction Rate   "wc/100ft                          
 
  Supply     Return         Manual D Velocities  
Duct Type      Supply Return  
    Rigid Flex Rigid or Flex  
Airflow - CFM        Trunks 700 600 600  
Enter airflows, and click the "Calculate" button for results.   Branches 600 600 400  
    Maximums 900 700 700  
  Exceeding maximums increase restriction, noise, and renders fitting equivalent lengths inaccurate.
 
  Allow to exceed maximums by: fpm  
       
     
       
    Velocity Calculator  
Flexible          
Compression Side(a)   N/A     Diameter CFM      
Select 4" to 10" x x     @ = 1019 fpm
           
11" to 17" x x Side(a) Side(b) CFM      
           
18" to 24" x x x @ = 1200 fpm

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 ACCA Manual D maximum 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. 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.