HVAC Duct Sizing Calculator
Duct sizing is important in building services because it ensures that air is distributed efficiently and evenly throughout HVAC systems while maintaining comfort, energy efficiency, and acceptable noise levels. Properly sized ducts help avoid issues such as excessive pressure drop, high fan energy consumption, poor airflow at terminal units, and uneven temperature distribution in occupied spaces.
Duct sizing is typically carried out using methods such as the equal friction method or the velocity reduction method. These approaches balance airflow rate, duct dimensions, and allowable pressure losses to determine suitable duct sizes for different sections of a system. Engineers use airflow (CFM or L/s), velocity limits, and friction charts to select duct sizes that maintain efficient system performance while minimising noise and energy use. It is important to consider the pressure drop caused by the ductwork and associated components. The Darcy-Weisbach equation can be used to calculate these losses. The engineer must consider the impact of these pressure drops when designing a duct network.
According to CIBSE Guide B - HVAC & Refrigeration 2005, the maximum recommended duct velocities for low pressure ductwork systems, depending on acoustic criteria and project type can be seen below. A comparison of the calculated values against the values in this table can be used to determine if CIBSE standards are met.
| Maximum Air Velocity (m/s) | ||||
| Typical Applications | Typical Noise Ratings (NR) | Main Ducts | Branch | Runouts |
| Domestic Building (bedrooms) | 25 | 3 | 2.5 | 2 |
| Theaters, Concert Halls | 20-25 | 4 | 2.5 | 2 |
| Auditoria, Lecture Halls, Cinemas | 25-30 | 4 | 3.5 | 2 |
| Bedrooms (non-domestic) | 20-30 | 5 | 4.5 | 2.5 |
| Private offices, libraries | 30-35 | 6 | 5.5 | 3 |
| General offices, restaurants, banks | 35-40 | 7.5 | 6 | 3.5 |
| Department stores, supermarkets, shops | 40-45 | 9 | 7.5 | 4.5 |
| Industrial Buildings | 45-50 | 10 | 8 | 5 |