CFM Formula and Rules of Thumb
The standard rule of thumb for residential HVAC sizing is 1 CFM per square foot for cooling and 1.2 CFM per square foot for heating. These figures assume standard 8–9 ft ceilings. For taller ceilings, multiply by ceiling height / 9.
Once you have CFM, the duct cross-sectional area (in square feet) is: area = CFM / velocity (FPM). Convert to a round diameter with: diameter (in) = √(4 × area / π) × 12, then round up to the next standard size.
Standard round duct sizes are: 4, 5, 6, 7, 8, 9, 10, 12, 14, 16, 18, and 20 inches. Always round up — a duct that is slightly oversized moves air quietly and efficiently; an undersized duct creates noise, pressure loss, and uneven comfort.
Standard Duct Sizes and Typical Uses
| Round Diameter | Approx. CFM @ 800 FPM | Typical Use |
|---|---|---|
| 4" | 70 CFM | Small closet or bathroom |
| 5" | 110 CFM | Small bathroom or half bath |
| 6" | 160 CFM | Bathroom, small bedroom |
| 7" | 215 CFM | Bedroom (up to ~215 sq ft) |
| 8" | 280 CFM | Medium bedroom, home office |
| 9" | 355 CFM | Large bedroom, small living room |
| 10" | 436 CFM | Living room, master suite |
| 12" | 628 CFM | Large living/dining area |
| 14" | 855 CFM | Open-plan great room |
| 16" | 1,117 CFM | Main trunk line, large spaces |
Frequently Asked Questions
What size duct do I need?
It depends on the CFM your room requires and your target air velocity. A 200 sq ft room needs about 200 CFM for cooling. At 800 FPM, that requires a 7-inch round duct (duct area ≈ 0.25 sq ft). Use the calculator for your exact dimensions and system type.
What is CFM in HVAC?
CFM (Cubic Feet per Minute) is the volumetric flow rate of air through a duct. Every room has a required CFM based on its heat load. Too little CFM means poor comfort; too much wastes energy and creates noise. The rule of thumb is 1 CFM/sq ft for cooling, 1.2 CFM/sq ft for heating.
Supply vs. return duct sizing?
Supply ducts are typically sized for 600–900 FPM. Return ducts move the same volume of air but at lower velocity (400–600 FPM) to avoid noise and pressure drop, so they are usually one or two sizes larger than the corresponding supply duct. A single return plenum serving multiple rooms is often significantly larger than any individual supply run.
What velocity should HVAC ducts run at?
Residential supply ducts: 600–900 FPM. Use the lower end (600–700 FPM) near bedrooms and living areas where quiet operation matters. 800–1,000 FPM is acceptable for mechanical rooms or main trunk lines. Return ducts: 400–600 FPM. Higher velocities cause turbulence noise and reduce system efficiency.