Duct Velocity Calculator

Measuring air speed inside ductwork is one of the most important steps in HVAC design. A Duct Velocity Calculator helps you find the airflow speed based on flow rate and duct size. With the right velocity, you can design quieter, more efficient, and properly balanced systems.

We developed this calculator so users can easily enter their airflow and duct dimensions to get the velocity instantly. Whether you are following ASHRAE duct velocity standards or simply checking a rule of thumb, this tool gives you the accurate numbers you need.

How to Calculate Velocity in Ductwork

The fundamental formula for duct velocity is:

Velocity = Flow Rate ÷ Duct Area

For a round duct:

Area = π × (Diameter ÷ 2)²

For a rectangular duct:

Area = Width × Height

If you know the mass flow rate instead of volume flow, use:

Velocity = Mass Flow ÷ (Density × Area)

If you are measuring with a pitot tube or velocity pressure device, you can use:

Velocity (ft/min) = 4005 × √(Velocity Pressure in. wg)

and convert to m/s by multiplying by 0.00508.

Recommended Duct Velocity (Rule of Thumb)

Engineers often follow ASHRAE guidelines and common design practice when choosing duct velocity:

• Main supply ducts: 900 – 1200 ft/min (4.5 – 6 m/s)
• Branch supply ducts: 600 – 900 ft/min (3 – 4.5 m/s)
• Return ducts: 500 – 700 ft/min (2.5 – 3.5 m/s)

Keeping within these ranges reduces noise and pressure loss.

How to Use the Duct Velocity Calculator

1. Enter the flow rate in CFM, L/s, or m³/h.
2. Select the duct shape: round or rectangular.
3. Input the duct size in inches, feet, millimeters, or meters.
4. Click calculate to see the velocity in m/s and ft/min.

This calculator supports both imperial and metric units, so you can work with CFM or m³/s depending on your project.

Final Verdict

The Air Duct Velocity Calculator is a practical tool for HVAC engineers, technicians, and even students. It takes the guesswork out of airflow design and ensures your ductwork meets efficiency and comfort standards. Whether you are checking round duct velocity, comparing with ASHRAE standards, or verifying airflow on-site, this calculator makes it simple.

FAQs

What is the rule of thumb for duct velocity?

A common rule of thumb is 700 – 900 ft/min for supply air in medium velocity systems.

What is the recommended velocity for air ducts?

ASHRAE recommends keeping velocities between 500 and 1200 ft/min depending on the duct type and location.

Where to measure duct velocity?

Velocity is best measured in a straight section of duct, away from bends, fans, or transitions. A pitot tube or vane anemometer can be used.

How do you calculate velocity in ductwork?

Divide the airflow rate (CFM or m³/s) by the duct cross-sectional area (ft² or m²). This gives velocity in ft/min or m/s.