HVAC Duct Sizing Calculator
Calculate the required diameter or dimensions of HVAC ducts based on air flow rate and design velocity. Use this when designing ventilation systems to balance air delivery with noise and pressure loss constraints.
About this calculator
Duct sizing ensures that conditioned air reaches each zone at the correct flow rate without excessive velocity (which causes noise) or undersized cross-sections (which cause high pressure drop). For a round duct, the diameter is derived from the area equation: D = 2 × √(Q × 144 / (π × V)), where Q is the airflow rate (CFM) and V is the design velocity (FPM). The factor 144 converts square feet to square inches. For a rectangular duct with a given aspect ratio (AR), the equivalent width is: W = √((Q × 144 × AR) / (2 × (AR + 1) × V)). Typical design velocities range from 500–800 FPM for low-velocity residential systems to 1,500–2,500 FPM for commercial supply ducts. Pressure loss per 100 ft of duct (in. WG) guides duct layout optimisation — 0.08–0.10 in. WG/100 ft is a common design target.
How to use
Example: Round duct, Air flow rate (Q) = 400 CFM, Design velocity (V) = 800 FPM. Apply the formula: D = 2 × √(400 × 144 / (π × 800)). Inside the square root: (400 × 144) / (3.1416 × 800) = 57,600 / 2,513.3 = 22.91. √22.91 ≈ 4.79. D = 2 × 4.79 ≈ 9.58 inches. Round up to the nearest standard duct size, 10 inches, to ensure adequate airflow without exceeding the design velocity.
Frequently asked questions
What is the recommended air velocity for HVAC duct sizing in residential and commercial buildings?
For residential systems, supply duct velocities are typically kept between 500 and 900 FPM to minimise noise and maintain comfort. Main trunk ducts in commercial buildings commonly run at 1,000–2,000 FPM, with branch ducts at 600–1,200 FPM. Velocities above 2,500 FPM can generate objectionable noise and significantly increase pressure drop, raising fan energy consumption. The design velocity should be selected based on acoustic requirements, available static pressure from the fan, and the friction rate target for the system.
How does aspect ratio affect the size and performance of rectangular HVAC ducts?
Aspect ratio is the ratio of the longer side to the shorter side of a rectangular duct. Ratios closer to 1:1 (square) are hydraulically most efficient, producing the lowest pressure drop for a given cross-sectional area. High aspect ratios (e.g. 4:1 or 6:1) are used when vertical space is limited, such as in ceiling plenums, but they increase the wetted perimeter relative to the area, raising friction losses. As a general rule, aspect ratios above 4:1 should be avoided unless space constraints make them necessary, and the increased pressure loss must be accounted for in fan selection.
Why is pressure loss per 100 feet used as a design criterion in HVAC duct systems?
Pressure loss per 100 ft (in. WG/100 ft) is a normalised friction rate that allows designers to size ducts consistently along the entire distribution network. Using a uniform friction rate (typically 0.08–0.10 in. WG/100 ft) ensures that all duct branches experience similar resistance, making it easier to balance airflow without excessive damper throttling. If the friction rate is too high, fan energy consumption increases and noise problems arise; too low, and ducts become unnecessarily large and expensive. This approach, called the equal friction method, is one of the most widely used duct sizing techniques in HVAC engineering.