HVAC calculators
Size supply and return ducts from airflow and friction rate, the way a ductulator does, with the formula and assumptions shown.
Duct size calculator
Round and rectangular duct sizes from airflow and friction rate, with a CFM chart for every size.
Static pressure calculator
Total external static pressure from your readings, plus the friction rate for duct sizing.
AC tonnage calculator
Cooling capacity estimate by floor area and climate zone, with an honest check against real load calculations.
Sizing ductwork in the right order
- Airflow per room. Each room's CFM comes from its heating and cooling load, usually a Manual J calculation. Supply registers and branch ducts are sized for that airflow.
- Friction rate. Divide the static pressure the blower can spend on duct friction by the total effective length of the longest run, then multiply by 100. Residential systems usually land between 0.05 and 0.10 in. w.c. per 100 ft.
- Duct size. With airflow and friction rate known, each section's size follows. Trunks carry the sum of the branches downstream, so they are sized section by section as branches leave.
Velocity is the check on the result. Residential supply trunks are usually kept under about 900 fpm and branches lower, because faster air is louder at the registers.