Calculate the correct duct size (round or rectangular) based on airflow (CFM) and target velocity. Instant, accurate results for any HVAC system.
Enter your required airflow in CFM (cubic feet per minute) and your target air velocity in FPM (feet per minute). Choose between round duct, rectangular duct (with adjustable aspect ratio), or use the comparison tab to see multiple sizes side by side. Hit Calculate to instantly get the recommended duct dimensions, actual velocity, and an efficiency rating.
Duct sizing is one of the most consequential decisions in HVAC design — and one of the most frequently botched. An undersized duct creates excessive velocity, producing noise, pressure drop, and higher energy bills. An oversized duct wastes money on materials and can cause poor air distribution and comfort complaints.
For a typical 2,000 sq ft house, the main trunk duct might carry 1,600–2,000 CFM. Branch ducts to individual rooms typically range from 100 to 600 CFM. A 400 CFM branch duct should be sized to carry air at 700–900 FPM — which works out to about a 10" round duct. Using an 8" round instead would push velocity above 1,100 FPM, creating audible whistling and causing the system to work harder.
HVAC contractors, mechanical engineers, building inspectors, and advanced DIYers all use duct sizing calculations on nearly every project. Even a simple bathroom exhaust fan upgrade benefits from proper duct sizing to ensure adequate CFM delivery.
All duct sizing uses the fundamental airflow equation:
CFM = Area (ft²) × Velocity (FPM)
Rearranged: Area = CFM ÷ Velocity
For a round duct: Area = π × (D/2)² → solve for D: D = 2 × √(CFM ÷ (π × Velocity)) × 12 inches
For a rectangular duct: Area = W × H. Given an aspect ratio R = W/H, then H = √(Area ÷ R) and W = H × R. The equivalent diameter (for comparing to round ducts) uses the ASHRAE formula: De = 1.3 × (a×b)^0.625 ÷ (a+b)^0.25
A general rule of thumb is 1 CFM per square foot of conditioned space for standard 8-foot ceilings. A 200 sq ft bedroom would need approximately 200 CFM, though a proper Manual J load calculation will give you a more precise number based on insulation, windows, and climate.
Above 900 FPM in supply ducts and 700 FPM in return ducts, occupants typically begin to notice airflow noise. For bedrooms and quiet spaces, staying below 700 FPM is recommended. Commercial spaces tolerate up to 1,600 FPM without significant comfort complaints.
Use the equivalent diameter formula (ASHRAE 2009): De = 1.3 × (a×b)^0.625 ÷ (a+b)^0.25, where a and b are the duct width and height in inches. This gives you a round duct diameter with the same pressure drop characteristics as the rectangular duct — not the same cross-sectional area.
Both. The main trunk carries total system CFM and should be sized at commercial velocities (1,000–1,200 FPM) to keep it compact. Branch ducts carry individual room CFM and should be sized at residential velocities (600–900 FPM) for quiet operation. Trunk-and-branch systems need the trunk to step down in size after each branch takeoff.