HVAC Duct Calculator

Proper duct sizing balances airflow, friction loss, velocity, available space, and system requirements. The AnyHVAC Duct Calculator uses airflow and friction rate to determine a calculated round duct diameter and helps evaluate practical round and rectangular duct sizes.

Enter the required airflow (CFM) and friction rate (in. w.g. per 100 ft) to calculate the duct size. The results also show air velocity and the actual friction rate associated with the selected nominal duct size.

Calculated Diameter

The calculated diameter is the theoretical round duct diameter required for the entered airflow and friction rate.

Because manufactured duct sizes typically use standard nominal dimensions, the calculated diameter may fall between available sizes. The calculator therefore also provides a suggested nominal round duct size.

Air Velocity

Air velocity indicates how quickly air moves through the duct and is expressed in feet per minute (FPM).

For the same airflow, a smaller duct produces a higher velocity while a larger duct produces a lower velocity. Velocity can affect system pressure loss, noise, air distribution, and overall system performance.

The calculator displays velocity so the selected duct size can be evaluated as part of the overall design rather than based on duct diameter alone.

Actual Friction Rate

When the calculated diameter is converted to a nominal duct size, the resulting friction rate may differ slightly from the original design friction rate.

AnyHVAC calculates the actual friction rate of the selected nominal size, allowing you to see how the practical duct selection compares with the original design target.

Round duct is commonly used because of its efficient airflow characteristics and relatively low surface area for a given cross-sectional area. However, building conditions do not always allow round ductwork.

Rectangular duct is often required where ceiling height, structure, equipment, or other building systems limit available space.

The AnyHVAC Duct Calculator provides rectangular duct recommendations based on the required equivalent duct performance. A fixed dimension can be used when one side of the duct must fit within a specific space.

For example, when vertical clearance is limited, a designer may establish the maximum duct height and determine the corresponding width required to provide suitable airflow performance.

Rectangular alternatives should still be reviewed for velocity, friction loss, aspect ratio, available space, and project requirements before final selection.

Friction rate represents the pressure loss caused by air moving through ductwork and is commonly expressed as:

in. w.g. per 100 ft of duct

As air travels through a duct, friction between the moving air and the duct surface creates resistance. Duct size, airflow, duct material, geometry, fittings, and system configuration all influence total pressure loss.

A lower friction rate generally requires a larger duct for the same airflow, while a higher friction rate generally results in a smaller duct and higher air velocity.

The appropriate design friction rate depends on the system and project requirements. It should not be selected solely because a particular value is commonly used.

Airflow is expressed in cubic feet per minute (CFM) and represents the volume of air moving through the duct.

Air velocity depends on both airflow and duct cross-sectional area:

Velocity (FPM) = Airflow (CFM) ÷ Duct Area (ft²)

For example, increasing airflow through the same duct increases velocity. Increasing the duct size while maintaining the same airflow decreases velocity.

This relationship is one reason duct sizing should consider more than simply finding a duct capable of carrying the required CFM.

Suppose a duct must carry:

Airflow:

5,000 CFM

Design friction rate:

0.10 in. w.g./100 ft

Using the AnyHVAC Duct Calculator produces a calculated round diameter of approximately:

25.6 inches

A practical nominal selection would therefore be:

26-inch round duct

At that nominal size, the calculator indicates approximately:

Air velocity:

1,356 FPM

Actual friction rate:

0.092 in. w.g./100 ft

This illustrates why the calculator provides both the theoretical calculated diameter and the performance of the practical nominal duct selection.

The final duct size should still be evaluated within the complete HVAC system design.

The equal friction method sizes ductwork around a consistent friction rate while maintaining the airflow required by the system.