Duct Size Calculator

Airflow (CFM) divided by velocity (feet per minute, fpm) gives required duct cross-sectional area in square feet. Multiply by 144 to convert to square inches for layout. Round duct diameter comes from area = π × (d/2)² — this calculator solves for diameter and also shows an equivalent square duct for rectangular trunk planning.

Lower velocity is quieter but needs larger ducts; higher velocity saves space but increases noise and friction losses. Residential supply trunks often target 700–900 fpm; branches commonly run 500–700 fpm. Verify with your design standards and equipment static pressure limits — this is a geometry calculator, not full duct design software.

Friction loss grows with velocity, duct length, fittings, and surface roughness. A duct sized only on velocity may still fail if equivalent length is long or filters are dirty. Manual D and manufacturer tables account for those factors; use this tool for initial diameter picks before detailed loss calculations. Insulated flex in unconditioned attics still needs the same internal free area — compression during install is a common field mistake.

Worked example with defaults: 400 CFM at 700 fpm → area = 400 ÷ 700 ≈ 0.571 sq ft, or about 82.3 sq in. Round duct diameter ≈ 10.2 inches — a 10-inch round or equivalent rectangular size is the planning bracket. Upsize one inch if noise or pressure drop is a concern on long runs.

Square equivalent side length is √(area in sq in) — useful when ceiling space favors a flat trunk. Maintain aspect ratio limits on rectangular ducts (avoid extremely flat boxes) per SMACNA guidance. Flex duct must be fully extended; compressed flex shrinks effective diameter and raises velocity and noise. Note the calculated diameter on your takeoff sheet before ordering — round snap-lock and spiral ducts are not always stocked in half-inch increments.

Return ducts often run at lower velocity than supply for quieter operation. Size supply and return separately — do not assume one diameter serves both. Transfer grilles and open doorways affect return paths in residential layouts. When upgrading equipment, verify existing duct can carry the new CFM at acceptable velocity — undersized legacy duct drives noise and high static pressure.

Limits: elbows, transitions, tees, and dampers add equivalent length and pressure drop not modeled here. Size ducts with proper HVAC design tools for permitted installs. Local code may require sealed and insulated duct in unconditioned spaces — insulation does not change required free area for a given CFM and velocity.