When to use: Use to size supply and return air ducts using the equal friction method per ACCA Manual D. Enter the required airflow (CFM) for the duct run and select the friction rate and duct material. The recommended friction rate is 0.08–0.10 in.wg/100 ft for residential systems. Maintain trunk duct velocities of 600–900 FPM and branch duct velocities of 400–700 FPM to minimize noise and pressure drop.
This free duct sizing calculator sizes supply and return air ducts from the required airflow using the equal-friction method, following ACCA Manual D and ASHRAE practice. Enter the airflow each run must carry (CFM), pick a design friction rate, and it returns the equivalent round diameter, the nearest standard size, the resulting air velocity, and whether that velocity falls in an acceptable range. It's built for HVAC designers, mechanical contractors, technicians, and students sizing residential or light-commercial ductwork.
Start with the airflow each room or run needs. The required CFM comes from the room's load:
CFM = BTU/h ÷ (1.08 × ΔT)
where ΔT is the supply-to-room temperature difference and 1.08 is the air constant for standard conditions. With CFM known, the equal-friction method sizes every section to the same friction rate — typically about 0.08–0.10 in. w.g. per 100 ft for residential systems — so the design stays balanced.
Velocity follows directly: velocity (FPM) = CFM ÷ duct area (sq ft). Round ducts are sized first; rectangular ducts are sized to the equivalent round diameter that produces the same friction loss.
Worked example: 400 CFM designed at 0.10 in. w.g. per 100 ft works out to roughly a 9-inch round duct (about 0.44 sq ft), giving a velocity near 900 FPM — a reasonable residential trunk value. Dropping to a 0.08 rate would push you toward a 10-inch duct at a quieter velocity.
1. Determine the airflow (CFM) the duct run must carry — from the room load via CFM = BTU/h ÷ (1.08 × ΔT), or from a Manual J / Manual D worksheet. 2. Enter that CFM into the calculator. 3. Choose your design friction rate (0.08–0.10 in. w.g./100 ft is the usual residential target) and the duct material. 4. Read the recommended round diameter and nearest standard size, then check the velocity output — if it flags as too high or too low, adjust the friction rate or step the size up or down.
Friction rate is the pressure lost per 100 ft of duct, and it sets the trade-off. Size a duct too small and the friction rate climbs: velocity rises, static pressure goes up, the blower works harder, and air movement turns into audible noise and whistling at registers.
Size it too large and you waste sheet metal and space, registers lose throw and mixing, and low velocities can let supply air dump short of the room. Designing to a consistent 0.08–0.10 in. w.g./100 ft, with trunk velocities around 600–900 FPM and lower velocities on branches and returns, keeps the system quiet, efficient, and balanced.
Find the airflow each run needs (CFM = BTU/h ÷ (1.08 × ΔT) from the room load), then size every section to a constant friction rate using the equal-friction method per ACCA Manual D. The friction rate plus CFM give the equivalent round diameter, which you round up to the nearest standard size and check against a target velocity.
It is a duct-sizing approach, used in ACCA Manual D, that sizes every duct section to maintain the same friction rate — the same pressure loss per 100 feet — throughout the system. Because each section drops pressure at the same rate, the design is inherently balanced and straightforward to lay out.
Residential systems are typically designed to about 0.08–0.10 in. w.g. per 100 ft of duct. The precise value comes from the air handler’s available external static pressure divided by the total effective length (the longest supply plus return path including fittings).
For residential systems, supply trunks generally run about 600–900 FPM, branch runs lower (roughly 400–700 FPM), and returns lower still. Higher velocities increase noise and static pressure; lower velocities cost more material and can reduce throw and mixing.
Match the equivalent round diameter using De = 1.30 × (a × b)^0.625 ÷ (a + b)^0.25, where a and b are the rectangular dimensions. This gives a rectangular duct with the same friction loss as the round duct for a given airflow.
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