HVAC Duct Size Calculator

An HVAC duct size calculator helps engineers determine the correct duct dimensions required to deliver a specific airflow in heating, ventilation, and air conditioning systems.

Proper duct sizing ensures:

• Balanced air distribution
• Reduced energy loss
• Lower noise levels
• Efficient HVAC performance

If ducts are too small, airflow resistance increases and reduces system efficiency.
If ducts are too large, installation costs increase unnecessarily.

This HVAC duct sizing tool calculates rectangular duct dimensions based on airflow and velocity.

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HVAC Duct Sizing Formula

The fundamental airflow equation used in HVAC design is:

Q = V × A

Where:

Symbol	Description
Q	Airflow rate
V	Air velocity
A	Duct cross-sectional area

Rearranging the formula:

A = Q / V

Where:

• A = duct area (m²)
• Q = airflow (m³/s)
• V = air velocity (m/s)

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Example HVAC Duct Size Calculation

Given:

• Airflow = 750 CFM
• Velocity = 900 FPM

Step 1 Convert airflow to m³/s

1 CFM = 0.0004719 m³/s

750 CFM = 0.354 m³/s

Step 2 Convert velocity to m/s

900 FPM = 4.57 m/s

Step 3 Calculate duct area

A = Q / V
A = 0.354 / 4.57
A = 0.0775 m²

Step 4 Determine duct dimensions

Possible rectangular duct sizes:

Width (mm)	Height (mm)
400	200
350	225
300	250

These dimensions deliver the required airflow.

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Recommended HVAC Air Velocity

Typical duct velocity ranges:

Duct Type	Velocity
Main supply duct	6–10 m/s
Branch duct	4–6 m/s
Return duct	3–5 m/s

Maintaining proper velocity reduces noise and pressure losses.

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HVAC Duct Friction Loss

Friction loss occurs when air flows through ducts due to:

• duct roughness
• bends and fittings
• air turbulence

Typical friction rate used in HVAC design:

0.6 – 1.0 Pa/m

Lower friction improves system efficiency.

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Rectangular vs Round Duct

Rectangular Duct

Advantages:

• Fits in limited ceiling space
• Easier installation in buildings

Disadvantages:

• Higher friction loss

Round Duct

Advantages:

• Lowest air resistance
• Better airflow efficiency

Disadvantages:

• Requires more installation space

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Free HVAC Duct Size Calculator

Use the BrickMate HVAC duct calculator to instantly determine:

• rectangular duct sizes
• airflow velocity
• required duct area

The calculator supports:

• CFM
• m³/h
• L/s

This makes it useful for both ASHRAE and international HVAC projects.

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Download HVAC Duct Size Calculator in excel

Engineers can also download the HVAC duct sizing Excel sheet for quick calculations during design and site work.

Download HVAC Duct Size Excel Calculator

This Excel tool automatically calculates:

• airflow conversion
• duct area
• rectangular duct dimensions
• airflow velocity

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HVAC Standards for Duct Design

Common standards used in duct sizing include:

• ASHRAE Handbook – HVAC Systems
• SMACNA Duct Design Guide
• IS 655 – Metal Air Duct Standards

Following these standards ensures proper system performance.

FAQ

What is duct sizing in HVAC?

Duct sizing is the process of determining the correct duct dimensions required to deliver the desired airflow efficiently in HVAC systems.

How do you calculate HVAC Duct Size Calculator

HVAC duct size is calculated using the airflow equation:

Area = Airflow / Velocity

The calculated area is then used to determine suitable duct dimensions.

What is the recommended air velocity in HVAC ducts?

Typical air velocity ranges between 3 m/s and 10 m/s depending on duct type.

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