How To Determine HVAC Duct Size

Determining the size of your HVAC ducts is very important before starting the installation
  • Author: Amanda Arnold
  • Posted On: October 29, 2022
  • Updated On: August 21, 2023

Your home’s ducting must be properly sized if you want to keep it at a reasonable temperature all year round. A too-small duct might block airflow, creating hot or cold patches in particular rooms. On the other hand, a large duct can force your HVAC system to work harder than necessary, wasting energy and raising your power costs.

Fortunately, determining the required ductwork size may be done straightforwardly. A tape measure and a few minutes are all you need. Using these tools, you can ensure that your home’s HVAC system runs as efficiently as possible.

To calculate the size of a duct, engineers typically use straightforward software or a ductulator. However, non-experts can use the round duct friction loss table to determine the round duct size and, if necessary, translate it to the size of the rectangle duct.

Things To Consider Before Calculating HVAC Duct Size

Area’s Size

There are numerous significant considerations to consider while sizing your house’s ducting. The amount of your ductwork will directly depend on the size of your house. A bigger house will need more ductwork to ensure enough airflow throughout the whole thing.

Check the plans or measure each room with a tape measure if you are unsure of your home’s square footage. The size of your ductwork will also depend on how many rooms there are in your house.

More ducting branches and ventilation will be needed to keep larger spaces at a comfortable temperature.

HVAC Duct Size

Your home’s size will determine the blower size for your HVAC system. To provide appropriate airflow, a larger residence will need a larger blower. By calculating the cooling capacity of your blower in tonnes or BTUs, you may find its maximum CFM.

HVAC Velocity

The maximum velocity of your system should be considered the last important aspect when sizing the ducting for your home. The air can move through your ductwork at this pace without causing harm or making much noise.

By multiplying the product of the CFM of your blower by the square root, you may determine the maximum velocity of your system.

Steps To Calculate HVAC Duct Size

It’s time to use what you’ve learned about the important aspects to consider when sizing your ducting.

Determine HVAC System Size

In tonnes of cooling capacity, HVAC system sizes are expressed. According to your home’s square footage, you can calculate how many BTUs you require. Most HVAC professionals advise a heating capacity of 35 BTUs in warmer climes.

You can make manual calculations to determine the size of the HVAC system with better precision. Determine the precise amount of heating and cooling your home need with the manual calculator. Climate Zone, natural shade, the number of windows, insulation, and many other aspects are all considered.

Length And Velocity Of Ductwork

As we previously stated, the sum of all ductwork elbow portions and straight ductwork sections determines the total effective length. The maximum velocity is the quickest speed at which air can move through your ductwork without creating damage or obtrusive noise. The maximum speed for most homes is a little over 1,500 feet per minute.

Determine Static Pressure

Static pressure or friction rate is the final factor when sizing your HVAC ductwork. By measuring the resistance to airflow in your ductwork, you may find out how much resistance there is.

For your system to move air through the ductwork, more energy is required the greater the friction rate. That is how it feels if you try to blow air through a straw—the difficulty of blowing air through a straw increase with its narrowness due to the increased resistance.

Your ductwork is similarly true. The friction rate will be higher if your ducting has many twists and curves or is narrow. You should choose a lower friction rate if your system has long duct lengths or high static pressure.


Any HVAC construction or replacement job requires properly sizing your HVAC ducting. Everything you require to know about sizing your ductwork has been addressed in this post. It’s time to put your newly acquired knowledge of HVAC ductwork sizing into practice.

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Author: Amanda Arnold

Amanda has been working with ConstructionHow since 2021. Her experience spans over 5 years in the creative niche such as home decor and trends, landscaping, renovations, and custom architectural values. As a home designer expert, she has a keen eye for the latest home improvement trends with accurate facts that readers find impossible to ignore. Being invested in home-building trends is how she has gained her lucrative expertise exploring more to bring a positive ambiance for all homeowners (and even tenants!). Currently, she lives in a beautiful beach home, a source of fascination for her.