Size round and rectangular supply and return ducts using ASHRAE equal friction or velocity methods — with velocity checks and pressure drop for any CFM.
Add rooms or zones to size all supply runs at once. Inherits friction rate and material from the Calculator tab.
Minimum round duct diameter at your selected friction rate, across a range of airflows. The current CFM is highlighted.
Velocity for your CFM at different round duct sizes. The colored band shows the acceptable velocity range for your duct segment type.
Design velocities and friction rates per ASHRAE Handbook of Fundamentals and SMACNA HVAC Duct Construction Standards.
Minimum round duct diameter for standard friction rate. Sheet metal, standard air at 68°F.
CFM per room comes from a Manual J load calculation, which accounts for room size, insulation, windows, and climate. As a rough estimate: 1 ton of cooling ≈ 400 CFM total. Divide proportionally among rooms by their square footage. For example, a 200 sq ft room in a 2,000 sq ft house with a 3-ton (1,200 CFM) system gets about 120 CFM. Use this only for preliminary sizing — Manual J gives accurate results.
The equal friction method sizes every duct section to the same friction rate (in.wg per 100 ft), which helps balance airflow without dampers. It's the standard ASHRAE method for residential design. The velocity method sizes ducts to a target air speed (FPM), which is simpler but can result in unbalanced systems. Equal friction is preferred for new installations; velocity works well for quick checks or renovations.
Flex duct has significantly higher friction than smooth sheet metal — about 1.5× higher when fully extended and stretched tight. Compressed flex duct can have 3× or more pressure drop. This reduces airflow and forces the system to work harder. Flex is acceptable for short runs (under 14 ft) to registers, but main trunk ducts and longer runs should use rigid sheet metal for efficiency and airflow balance.
Yes — use the equivalent diameter formula to convert between round and rectangular ducts. A 12×8 rectangular duct has an equivalent diameter of about 10.5 inches. Rectangular ducts fit better in tight spaces (between joists, in walls), while round ducts have less friction and are easier to seal. Choose based on available space, then verify using the equivalent diameter to confirm pressure drop is similar.
High duct velocity increases noise (air rushing sound at registers), increases pressure drop non-linearly (pressure ∝ V²), and can cause turbulence at fittings. Supply branches over 700–900 FPM will often be audible and can reduce comfort. If your calculated size produces a velocity that exceeds the recommended limit for that duct type, select the next larger standard size even if the friction rate check passes.