Air Duct Size Calculator
Calculate the correct HVAC air duct size based on airflow (CFM), duct shape, and velocity for efficient home heating and cooling.
Properly sizing air ducts is critical for HVAC system efficiency. Ducts that are too small restrict airflow, increase static pressure, and force the blower to work harder — leading to higher energy bills, uneven temperatures, and premature equipment failure. Ducts that are too large waste material and can cause slow air velocities that allow moisture and dust to settle.
Key Concept: CFM (Cubic Feet per Minute)
CFM is the measure of air volume flowing through a duct. Each room requires a certain CFM based on its size and heat load. Typical values:
- Bedrooms: 1 CFM per 1–1.5 sq ft of room area
- Living rooms: 1 CFM per 1.5–2 sq ft
- Kitchens: 1 CFM per 1–1.2 sq ft (more air needed)
Recommended Air Velocity
| Duct Type | Recommended Velocity |
|---|---|
| Main trunk (supply) | 700–900 FPM (feet per minute) |
| Branch ducts | 500–700 FPM |
| Return air ducts | 400–600 FPM |
| Flexible ducts | 400–600 FPM max |
Velocity in FPM = CFM ÷ Duct area in sq ft
Round Duct Formula
For a round duct: Area = π × (diameter/2)² Required diameter = 2 × √(CFM ÷ (π × target velocity / 144))
In simplified form: diameter (inches) = √(CFM × 144 ÷ (0.7854 × velocity))
Rectangular Duct Equivalent
The hydraulic diameter of a rectangular duct is: D_h = 1.3 × (a × b)^0.625 ÷ (a + b)^0.25
Where a and b are the width and height of the rectangular duct in inches.
Common Round Duct Sizes and Capacity (at 700 FPM)
| Duct Diameter | Max CFM at 700 FPM |
|---|---|
| 4 inch (10 cm) | 61 CFM |
| 6 inch (15 cm) | 137 CFM |
| 8 inch (20 cm) | 244 CFM |
| 10 inch (25 cm) | 381 CFM |
| 12 inch (30 cm) | 549 CFM |
| 14 inch (35 cm) | 748 CFM |
Note: This calculator provides estimates for planning purposes. HVAC system design should be verified by a qualified HVAC professional, especially for whole-house systems.