Free HVAC Duct Sizing Calculator 

How does this HVAC duct size calculator work?

Our free HVAC duct sizing calculator takes the info you have about your HVAC installation project and helps you determine the correct size and specs for ductwork.  To use the calculator, input your duct material, the airflow requirements (CFM), desired air velocity (FPM), and available static pressure. Using these inputs, the duct sizing calculator determines the appropriate duct size to ensure the required airflow is delivered efficiently.

Why HVAC duct size matters

Before you begin any HVAC installation project, you need to know the right size ductwork to use. We’ll dive into how to calculate this below.

• Energy efficiency: Properly sized ducts ensure that the HVAC system operates efficiently, minimizing energy consumption. Undersized ducts can restrict airflow, which can make the system work harder and use more energy. On the other hand, oversized ducts can lead to inefficiencies and increased costs.
• Comfort: Correct duct sizing ensures even and consistent airflow throughout a building. That way, comfortable temperatures and air quality are maintained. Inadequate ductwork can result in hot or cold spots, insufficient ventilation, and poor air distribution.
• System performance and longevity: Proper duct design helps maintain optimal system performance and extends the lifespan of HVAC equipment. Poorly designed ducts can cause the system to overwork. This means wear and tear and the potential for costly repairs or replacements.
• Noise reduction: The right duct size and design help minimize noise from the HVAC system. High air velocities and improperly installed ducts can create noise and vibration issues.
• Cost management: Accurate duct sizing and design help in estimating material and installation costs. This prevents overspending on unnecessary materials and labor. This will also help your customers avoid future expenses related to inefficient system operation or repairs.
• Compliance with building codes and standards: Properly calculating and designing HVAC ductwork ensures compliance with local building codes and industry standards. This is vital for safety, insurance, and resale value.
• Indoor air quality: Correct ductwork design helps maintain good indoor air quality. It ensures adequate ventilation and prevents issues like poor air circulation, humidity control problems, and the buildup of pollutants or contaminants.

How do you calculate HVAC duct size?

To calculate the correct HVAC duct size for your project, you’ll have to consider several factors, including airflow rate, duct material, duct length and configuration, and static pressure, and velocity. We’ll walk you through the process.

1. Calculate the square footage It’s important to know how much space you’re talking about. Make sure you start by calculating the square footage of the entire home or building. You’ll also need the square footage of each individual room. All you need to do for this is multiply the length and width of each room.
2. Determine heating and cooling loads Calculating the HVAC capacity for the whole building should be done using a recognized method, such as ACCA Manual J or something similar. This will also help you determine what size equipment you need.
3. Determine airflow requirements (CFM)  ext, you’ll calculate the required airflow (CFM) for each room or zone based on the heating and cooling loads and equipment CFM. Use the equation below to get this number: 
Room CFM = (Room load/Whole house load) x Equipment CFM
4. Consider the duct material Whether metal or flexible, the duct material will be based on the project's requirements. The material you use will affect the friction loss and sizing calculations.
5. Select desired air velocity Next, decide the appropriate air velocity for the ducts. Typical velocities for main ducts are 600-900 feet per minute (FPM), while they’re 400-600 FPM for branch ducts. These ranges balance efficiency and noise levels. 
You can reference the Recommended Velocity chart below to find the right air velocity for your project.
   
6. Use a duct sizing chart Now that you have your airflow (CFM) and desired air velocity (FPM), you can consult a duct sizing chart to figure out the corresponding duct diameter or cross-sectional dimensions. Or you can use our free HVAC duct size calculator to input the friction loss (in. of water), duct airflow (CFM), and duct length (feet).

• Static pressure: Another important factor is static pressure, which is the resistance to airflow in the ductwork. Static pressure controls how fast air moves through the ducts. The amount of pressure is influenced by the size of your duct. Think of it like water in a garden hose. If you narrow the opening of the garden hose with your thumb, the water pressure increases. Therefore, a smaller duct diameter will increase static pressure, while a larger duct decreases pressure. Finding the right static pressure is key for efficiency. It also impacts noise levels and the long-term health of your ductwork.
• Total friction loss: Another factor you’ll need to consider is the total friction loss. This calculation depends on the duct material, total equivalent length, and desired airflow. It’s important this loss doesn’t exceed the available external static pressure, as specified by the HVAC system’s fan.

How does this HVAC duct size calculator work?

Our free HVAC duct sizing calculator takes the info you have about your HVAC installation project and helps you determine the correct size and specs for ductwork. To use the calculator, input your duct material, the airflow requirements (CFM), desired air velocity (FPM), and available static pressure. Using these inputs, the duct sizing calculator determines the appropriate duct size to ensure the required airflow is delivered efficiently.