HVAC Duct Sizing Calculator
This free HVAC duct sizing calculator helps engineers, installers, and students quickly calculate the round duct diameter based on airflow and a target air velocity. Whether you work in metric or imperial units, it converts your inputs into the minimum ventilation ductwork dimensions needed to move air efficiently and quietly.
The duct sizing formula
Duct sizing is based on the continuity equation, which relates airflow (Q), air velocity (V), and cross-sectional area (A):
Q = V × A
Rearranging for the required area gives A = Q ÷ V. Because a round duct has a circular cross-section, the diameter (D) is derived from the area of a circle, A = (π ÷ 4) × D²:
D = √(4A ÷ π) = √(4Q ÷ (π × V))
In metric units, airflow is entered in m³/h and velocity in m/s, so the airflow is first converted to m³/s before solving for the diameter in millimetres. In imperial units, airflow is entered in CFM and velocity in FPM (feet per minute), returning the diameter in inches.
Use cases
- Sizing supply and return trunks for residential and commercial HVAC systems.
- Checking whether existing ventilation ductwork dimensions meet a target velocity.
- Performing a fast CFM to m³/h ventilation conversion when working across unit systems.
- Keeping noise low by respecting the maximum air velocity for office ductwork and other quiet spaces.
Choosing the right air velocity
Higher velocities allow smaller ducts but increase noise and pressure drop. As a rule of thumb, main ducts in quiet spaces such as libraries stay around 2.5–3 m/s (500–600 FPM), typical office ductwork runs near 5 m/s (1000 FPM), and industrial systems can exceed 8 m/s (1600 FPM). Selecting the correct maximum air velocity for office ductwork keeps your ventilation system efficient without generating distracting noise.