Use the Air Duct Calculator to quickly determine the required airflow, velocity, or the dimensions of a rectangular duct based on the other known variables. This tool utilizes the fundamental relationship between Airflow (CFM), Velocity (FPM), and Duct Cross-Sectional Area (Sq Ft).
Air Duct Calculator
Air Duct Calculator Formula
The primary formula governing airflow, velocity, and duct size is:
$$Q = V \times A$$Where:
$$A = \frac{W \times H}{144} \quad (\text{Area in Square Feet})$$Rearranging the formula to solve for the missing variables:
- Velocity: $$V = \frac{Q}{A}$$
- Area: $$A = \frac{Q}{V}$$
- Width: $$W = \frac{A \times 144}{H}$$
- Height: $$H = \frac{A \times 144}{W}$$
Variables Explained
- Airflow (Q – CFM): The volume of air moving through the duct per unit of time. This is often the design target for a space.
- Velocity (V – FPM): The speed at which the air is moving inside the duct. Higher velocities can lead to increased noise and friction loss.
- Duct Width (W – Inches): The horizontal dimension of the rectangular duct.
- Duct Height (H – Inches): The vertical dimension of the rectangular duct.
Related Calculators
Explore these related HVAC and engineering tools:
- Duct Friction Loss Calculator
- Cooling Load Calculator
- Fan Power Calculator
- Psychrometric Chart Analysis
What is Air Duct Calculator?
An air duct calculator is an essential tool for HVAC (Heating, Ventilation, and Air Conditioning) professionals and engineers. It uses fundamental fluid dynamics principles to establish the relationship between the three key parameters of air distribution: the required volume of air (Airflow, Q), the speed of the air (Velocity, V), and the size of the duct (Area, A).
Proper duct sizing is critical for system efficiency and occupant comfort. If a duct is too small for a given airflow, the air velocity will be excessively high, leading to excessive noise, vibrations, and high energy costs due to increased static pressure loss. Conversely, if the duct is too large, the system may struggle to deliver the air effectively, leading to inefficient distribution and higher initial material costs.
This calculator provides a simple way to quickly verify duct sizing during the design phase or troubleshoot performance issues in existing systems by solving for any one variable when the other three are known.
How to Calculate Air Duct Parameters (Example)
Suppose you need to find the required duct height (H) given the following known values:
- Identify Knowns: Airflow ($Q$) = 1,200 CFM; Velocity ($V$) = 900 FPM; Duct Width ($W$) = 18 inches.
- Calculate Required Area (A): Use the formula $A = Q / V$. $$A = \frac{1200 \text{ CFM}}{900 \text{ FPM}} = 1.333 \text{ Square Feet}$$
- Convert Area to Square Inches: Multiply the area in square feet by 144 (12 inches $\times$ 12 inches). $$A_{\text{in}} = 1.333 \times 144 = 192 \text{ Square Inches}$$
- Solve for Height (H): Use the formula $H = A_{\text{in}} / W$. $$H = \frac{192 \text{ sq in}}{18 \text{ inches}} = 10.67 \text{ Inches}$$
- Conclusion: The duct requires a cross-sectional area of $18 \times 10.67$ inches to maintain 900 FPM velocity at 1,200 CFM.
Frequently Asked Questions (FAQ)
What is the maximum recommended air velocity for residential ducts?
For residential supply ducts, velocities typically range from 700 to 900 FPM. Velocities above 1,000 FPM often result in noticeable noise issues.
Does this calculator account for friction loss?
No, this is a simplified calculator focused purely on the volumetric flow (Q=V*A). Friction loss, which affects the required fan power, requires a more complex calculation involving the duct roughness, length, and shape (aspect ratio).
What is the typical aspect ratio limit for ducts?
To minimize excessive friction loss and material cost, the aspect ratio (W/H) should ideally be kept below 4:1 (e.g., 20″ wide x 5″ high). An aspect ratio close to 1:1 (square duct) is most efficient, but rectangular ducts are often used due to space constraints.
Can I use this calculator for circular ducts?
While this calculator uses rectangular dimensions (W and H), you can calculate the Area (A) and convert it to an equivalent diameter ($D = \sqrt{4A / \pi}$) if needed, but the primary inputs are for rectangular ducts.