HVAC Duct Sizing Calculator
Size your ductwork by CFM, velocity, and friction rate. Get recommended round and rectangular duct sizes instantly.
HVAC Duct Sizing Chart by CFM
Standard duct sizes for residential and commercial applications at 900 FPM velocity:
| CFM | Round Duct | Rectangular | Flex Duct | Application |
|---|---|---|---|---|
| 50 CFM | 4" | 4" x 4" | 4" | Bathroom exhaust, small vents |
| 100 CFM | 5" | 6" x 4" | 5" | Range hood, small bathroom |
| 150 CFM | 6" | 8" x 4" | 6" | Bedroom supply, small returns |
| 200 CFM | 7" | 8" x 6" | 7" | Large bedroom, office supply |
| 300 CFM | 8" | 10" x 6" | 8" | Living room supply |
| 400 CFM | 10" | 12" x 8" | 10" | Main supply trunk (small home) |
| 600 CFM | 12" | 14" x 10" | 12" | Main trunk (medium home) |
| 800 CFM | 14" | 16" x 12" | 14" | Main trunk (large home) |
| 1,000 CFM | 16" | 20" x 12" | 16" | Return air, commercial supply |
| 1,200 CFM | 18" | 20" x 14" | 18" | Large return, commercial |
| 1,500 CFM | 20" | 24" x 16" | 20" | Commercial main duct |
| 2,000 CFM | 22" | 28" x 18" | 22" | Commercial supply trunk |
| 2,500 CFM | 26" | 30" x 20" | 26" | Large commercial |
| 3,000 CFM | 28" | 36" x 20" | 28" | Industrial ductwork |
| 4,000 CFM | 30" | 36" x 24" | 30" | Industrial main supply |
| 5,000 CFM | 36" | 42" x 28" | 36" | Large industrial, warehouse |
Duct Sizing Methods Explained
- Equal Friction Method (used here): Sizes ducts to maintain consistent friction loss per 100 feet. Most common for residential and light commercial.
- Velocity Method: Sizes ducts based on maximum allowable air velocity to prevent noise and excessive pressure drop.
- Static Regain Method: Used in large commercial systems where pressure recovery at fittings is critical.
- Residential typically uses 0.08 inWG/100ft friction rate and 600-900 FPM velocity.
- Commercial typically uses 0.10-0.15 inWG/100ft friction rate and 1200-1800 FPM velocity.
Common Duct Sizing Rules of Thumb
- Supply registers: 1 CFM per square foot of room area (cooling)
- Return air: 400 CFM per ton of AC capacity
- Flex duct friction loss is 2-3x higher than sheet metal — always size up
- Maximum velocity for residential: 900 FPM (supply), 700 FPM (return)
- Keep aspect ratio of rectangular ducts below 4:1 for efficiency
- Each 90° elbow equals ~30 feet of straight duct for friction calculations
What size duct do I need for 400 CFM?
For 400 CFM at 900 FPM velocity (standard residential), you need a 10" round duct or 12" x 8" rectangular duct. If using flex duct, size up to 10" (or 12" for long runs).
How do I calculate duct size from CFM?
Duct area (sq ft) = CFM / Velocity (FPM). For round duct: Diameter = 2 × √(Area × 144 / π). For 400 CFM at 900 FPM: Area = 400/900 = 0.444 sq ft = 64 sq in. Diameter = 2 × √(64/π) = 9.0" → round up to 10" standard.
What is the difference between flex duct and sheet metal duct sizing?
Flexible duct has 2-3x higher friction loss than sheet metal due to corrugated interior. Always size flex duct 1 inch larger than the equivalent sheet metal duct. For example, a 10" sheet metal duct requires a 12" flex duct for the same airflow.
What friction rate should I use for residential duct design?
The industry standard for residential is 0.08 inWG per 100 feet. For quieter systems, use 0.05. For tighter budgets with smaller ducts, 0.10 is acceptable but may increase noise. Never exceed 0.15 for residential.
How many CFM do I need per room?
For cooling: 1 CFM per square foot of room area. For a 200 sq ft bedroom, you need 200 CFM (7" round duct). For a 400 sq ft living room, 400 CFM (10" round duct). Add 20% for rooms with large windows or high ceilings.
What size return air grill do I need?
Return air grilles should be sized for 400 CFM per ton of AC. A 3-ton system needs 1,200 CFM return air. Use a 20" x 20" return grille (rated ~1,200 CFM at 500 FPM face velocity). Always oversize returns to reduce static pressure.