Ductwork is where a correctly sized system goes wrong. The equipment can deliver its rated airflow only if the trunk and branches carry it at a friction rate the blower can overcome, and the most common comfort complaint after a replacement is a new system on old ducts that never could. This page is for the estimator or technician in a US HVAC contracting business using a duct calculator, on paper, in an app or the trunk line calculator on this site: what it computes, what it needs from you, how the trunk and the branches are sized, and the reading that checks the result once the system runs.
What goes in: airflow and friction rate
The calculator needs the design airflow, from the load calculation or from the equipment's rated airflow, and a friction rate: the pressure the blower can spend per hundred feet of duct after the coil, filter, registers and fittings have taken their share of the available static. Choosing the friction rate honestly, from the equipment's rated static minus the components, is the step most often skipped, and skipping it produces ducts that look right on paper and starve the system on site.
Sizing the trunk
The trunk carries the total airflow from the equipment to the branch takeoffs, and the calculator gives its size in round diameter or in a rectangular equivalent for the height the ceiling allows. The trunk line calculator on this site does exactly this from the airflow and friction rate you enter, and shows the velocity so you can hold it below the point where the trunk becomes the loudest thing in the house. Reducing trunks step down as branches take airflow off; size each section for the airflow it carries.
Sizing the branches
Each branch carries the room's airflow from the room-by-room load, at the same friction rate, and its size is read the same way. Flexible duct behaves differently from sheet metal, and the calculator or its tables should say which it assumes; flex that is compressed or sagging carries far less than its diameter suggests. Returns are sized for the same airflow as the supplies they serve, and undersized returns are the commonest single cause of high static in existing homes.
Fittings, and why the drawing lies
Elbows, takeoffs, transitions and boots all consume pressure, and a duct system with many fittings needs a lower friction rate for the straight runs or a bigger blower. Count them. A duct calculator sizes straight duct; the equivalent length of the fittings is the estimator's responsibility, and the total is what the blower has to overcome.
The reading that checks the design
Once the system runs, total external static pressure measured across the equipment tells you whether the ducts are doing what the calculator said. High static with low airflow means the ducts, the filter or the coil are restricting more than the design allowed. Record it on the installation checklist with the airflow method used. The calculator is the design; the static reading is the proof.
Questions people ask about hvac ductwork calculator
Can a HVAC ductwork calculator size ducts for an existing house?
It sizes what the ducts should be. Measuring the existing ducts and the static pressure tells you how far the house is from that, which is the replacement conversation about ductwork.
Round or rectangular?
Round carries airflow with less friction for the same area; rectangular fits the ceiling. The calculator on this site gives both so the estimator can choose by the space.
What friction rate should we use?
The one the equipment can actually deliver after the coil, filter, registers and fittings have taken their share. Work it from the equipment's rated static, not from a habit.