Making Air-Duct Calculators

Air-duct calculators are very useful tools to use when sizing duct. You can use these calculations to fi nd the proper duct size based on the CFM or FPM. The calculations also make it very easy to fi nd the square equivalent of a round duct or the round equivalent of a square duct.

Air-Duct Sizing Formulas

Use the following formula to size a duct for the proper CFM:
CFM = Area * FPM

Example: 12 * 12 duct at 750 FPM; 12 * 12 = 1 sq. ft.
1 sq ft * 750 FPM = 750 CFM

Use the following formula to change the shape of a duct from round to square:
Area of a circle = πr²
Area / length = width

Example: Convert a 12″ round duct to a 10″ deep square or
rectangular duct.

Area of a 12 IN diameter circle = π6² = 113 sq. IN (rounded)
113 / 10 = 11.3
A 12″ round duct = 10 * 11.3 rectangular duct. Use a 10 * 12
rectangular duct.

Use the following formula to change the shape of a duct from square to round:
2(square root [area / π]) = diameter

Example: Convert a 10 * 10 square duct to an equivalent round
Area = 10 * 10 = 100 sq. IN
2(square root [100/3.14159]) = 2(square root [31.83])
= 2(5.6418) 5 11.28

A 10 * 10 square duct = 11.28″ round duct. Use a standard
12″ diameter duct.

Duct Sizing Guidelines

Calculate the area for square or rectangular using (length * width).

Calculate the area for round using (π * radius² ).

When calculating duct sizes, remember that ducts are not made in 11.5″ round or 12.375″ * 14.24″ rectangular as a standard, so you need to round to the nearest even number for a size, resulting in a 12″ round and a 12 * 14 rectangular duct.

Whenever you change the size of a duct from the size indicated in the engineer’s design, it should be with the intention of improving the constructability. Whenever possible, it is advisable to use the engineer’s design unless you are unable to because of field conditions. If field conditions do not permit an engineered size, then you need to write a request for information (RFI) to inform the engineer about the change being made.


What are the benefits of using air-duct calculators in HVAC system design?
Air-duct calculators provide a quick and accurate way to size ducts based on CFM or FPM, ensuring that the ductwork is properly sized for the HVAC system. This helps to prevent issues such as inadequate airflow, increased pressure drops, and reduced system efficiency. By using air-duct calculators, designers and engineers can also easily convert between round and square duct shapes, making it easier to design and install ductwork that meets the specific requirements of a building or application.
How do I choose the right formula for sizing a duct based on CFM or FPM?

To choose the right formula, you need to determine whether you are working with CFM (cubic feet per minute) or FPM (feet per minute). If you know the CFM, you can use the formula CFM = Area x FPM to calculate the required duct size. If you know the FPM, you can rearrange the formula to solve for Area. It’s essential to understand the relationship between CFM and FPM to ensure accurate duct sizing.

What is the significance of the πr² formula in air-duct calculations?

The πr² formula is used to calculate the area of a circle, which is essential for converting between round and square duct shapes. In the context of air-duct calculations, this formula allows you to calculate the area of a round duct based on its diameter. This value can then be used to determine the equivalent square or rectangular duct size, ensuring that the ductwork is properly sized for the HVAC system.

How do I account for duct fittings and transitions when using air-duct calculators?

Duct fittings and transitions can significantly impact airflow and pressure drop in an HVAC system. When using air-duct calculators, it’s essential to consider the effects of these components on duct sizing. You can do this by using fitting loss coefficients or equivalent lengths to account for the added resistance in the ductwork. This ensures that the duct size is adequate to handle the increased pressure drop and maintain proper airflow.

Can air-duct calculators be used for both residential and commercial HVAC systems?

Yes, air-duct calculators can be used for both residential and commercial HVAC systems. The formulas and calculations remain the same, regardless of the application. However, commercial HVAC systems often require more complex ductwork designs and larger duct sizes, which may necessitate more advanced calculation methods or software tools. In either case, air-duct calculators provide a fundamental understanding of duct sizing principles and can be used as a starting point for more detailed designs.

Are there any limitations or assumptions to consider when using air-duct calculators?

Yes, air-duct calculators are based on idealized conditions and assumptions, such as uniform airflow and negligible friction losses. In reality, ductwork is subject to various sources of pressure drop, including fittings, transitions, and surface roughness. Additionally, air-duct calculators may not account for factors like duct leakage, thermal expansion, or acoustic considerations. It’s essential to understand these limitations and consider them when applying air-duct calculator results to real-world HVAC system designs.