HVAC Duct Calculator

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Properly sizing the ductwork in your home is essential to maintaining a comfortable temperature throughout every season.  

An undersized duct can restrict airflow, leading to hot or cold spots in certain rooms. On the other hand, an oversized duct can cause your HVAC system to work harder than necessary, wasting energy and driving up your utility bills.

Fortunately, there are simple ways to calculate the ductwork size you need. All you need is a tape measure and a few minutes. With these tools, you can ensure that your home’s HVAC system operates at peak efficiency.

HVAC Ductwork Calculator (Ductulator)

Our easy-to-use HVAC duct size calculator (ductulator) will help you determine the proper size for your ductwork based on your home’s airflow needs.

To use our duct size calculator, simply enter the maximum CFM of your blower, then click “Calculate.”

For residential applications, the maximum velocity will rarely be above 1,500 FPM and the maximum friction rate should not exceed 0.1 in w.c./100 feet. As such, these values are not adjustable in the calculator.






How to read the results of The Ductwork Sizing Calculator:

The output from the calculator will give you up to 4 duct size options to choose from: 1 round size and up to 3 rectangular size options.

  • The round duct size gives the size in diameter
  • The rectangular duct options give the width and height.

You can choose 1 of the 4 sizes for your home based on what work best for its layout.

Once you’ve used our duct size calculator to determine the proper size for your ductwork, always hire a professional HVAC contractor to help with installation. This is because improperly installed ductwork can cause several problems, including decreased efficiency and uneven temperatures throughout your home.

Key Factors for Calculating HVAC Ductwork Sizing

a couple checking their home’s blueprint

When sizing the ductwork for your home, there are many important factors that you need to consider. Here are the top considerations. 

Home Size

The size of your home will directly impact the size of your ductwork. A larger home will require more ductwork to ensure adequate airflow throughout the entire space. If you don’t know your home’s square footage, check the blueprints or simply measure each room with a tape measure. 

Number of Rooms

The number of rooms in your home will also play a role in determining the size of your ductwork. More rooms will require more ductwork branches and ventilation to maintain comfortable temperatures.

Blower Size

The size of your HVAC system’s blower depends on the size of your home.  A larger home will require a larger blower to ensure adequate airflow. You can determine the maximum CFM of your blower by taking its cooling capacity size in tons or BTUs (12,000 BTUs equals 1 ton) and multiplying it by 400.

For example, if you have a 5-ton AC unit, its maximum CFM will be 5 x 400 = 2,000 CFM blower. 

Maximum Velocity 

The final key factor to consider when designing the duct sizing for your home is the maximum velocity of your system. This is the fastest speed that air can travel through your ductwork without causing damage, loud noises, etc

The maximum velocity is typically around 1,500 feet per minute for most systems. You can calculate the maximum velocity of your system by taking the square root of the product of your blower’s CFM and 4.5. 

For example

If you have a 2,000 CFM blower, the maximum velocity would be the square root of (2,000 x 4.5) = 94.87 feet per minute. 

Total Effective Length (Duct TEL)

The TEL is the total length of all your ductwork runs, branches, and bends. It essentially represents the longest path that air can take through your system. 

To calculate the TEL of your duct simply add up the length of all straight sections of ductwork and multiply by 2. Then, add up the length of all elbow sections of ductwork and multiply by 4. 

For example, if you have 10 feet of straight ductwork and two elbows, your TEL would be (10 x 2) + (2 x 4) = 28 feet. 

Home Duct Sizing Chart

Instead of using one of the many calculators available online (or ours above) or doing complex calculations yourself, you can use our duct size charts

How To Calculate Your HVAC Duct Sizing

An HVAC contractor fitting the ductwork

Now that you understand the key factors to consider when sizing your ductwork, it’s time to put this information to use. 

To properly size your ductwork, you need to know two measurements: 

  • The size of your HVAC system 
  • The total effective length of your ductwork (in feet) 
  • Maximum velocity 
  • Friction rate

Here is how to determine each factor.

HVAC System Size

HVAC system sizes are measured in BTUs and tons of cooling (or heating) capacity. 1 ton equals 12,000 BTUs. You can estimate how many BTUs you need for your home based on its square footage. 

For heating, most HVAC experts will recommend about 35 BTUs in warmer climates like Florida and 50 BTUs for cooler climates like Chicago and Minneapolis. If you need cooling, they will use 20 BTUs per square foot to estimate the HVAC size needed

Or, to get a more accurate HVAC system size, you can use the Manual J Calculation. The Manual J Calculation is a sizing method developed by the Air Conditioning Contractors of America (ACCA)

You can use the Manual J to determine exactly how much heating and cooling your home needs. It factors in everything from Climate Zone, natural shade, the number of windows, insulation,  and more. 

Note: The Manual J Calculation will give you a more accurate HVAC size in BTUs or tons for your home. 

Total Effective Lenght of Ductwork

As we mentioned earlier, the total effective length is the sum of all straight sections of ductwork (multiplied by 2) and all elbow sections of ductwork (multiplied by 4). 

Maximum Velocity

The maximum velocity is the fastest speed that air can travel through your duct without causing damage or loud noises. For most houses, the maximum velocity is around 1,500 feet per minute. Therefore, we keep the velocity at 1,500 FPM in our calculator.

Maximum Friction Rate

The final factor you need to consider when sizing your HVAC ductwork is friction rate or static pressure. This measures the amount of resistance in your duct as air moves through it. 

The higher the friction rate, the more energy it takes for your system to move air through the ductwork. 

You can think of it like trying to blow air through a straw. The narrower the straw, the more resistance (friction) there is, and the harder it is to blow air through it. The same is true for your ductwork. If you have a lot of bends and curves, or if the ductwork sizing is narrow, the friction rate will be higher. 

If you have very long ductwork runs or if the static pressure in your system is high, you may need to use a lower friction rate. 

Note: In general, we recommend using a friction rate of 0.1 inches w.c. per 100 feet. This is why we keep the friction rate at 0.1 in w.c. in our calculator. It is considered a low friction rate and will allow your system to operate efficiently. 


Sizing your HVAC ductwork is necessary for any HVAC installation or replacement project. In this article, we’ve covered everything you need to know about how to size your ductwork.

Now that you understand how to size your HVAC ductwork, it’s time to put this information into action. Use our HVAC Duct Sizing Calculator or get in touch with an HVAC professional for help today!

Do you have any questions relating to this article? Email us at [email protected] or call us on +1 (310) 961-4908

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Jonathon is a mechanical engineer with over ten years of experience in the HVAC industry. He has hands-on experience with all types of HVAC systems.

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