Duct Size Calculator (Online Ductulator) – HVAC Ductwork Sizing

Correct duct sizing ensures the right amount of conditioned air reaches each zone while keeping fan energy, noise levels, and pressure drop within acceptable limits. Undersized ducts create excessive velocity (noise, high pressure drop, energy waste); oversized ducts waste sheet metal cost and ceiling space. The two standard methods are the equal friction method (maintaining constant pressure drop per metre) and the velocity method (setting duct velocity based on application).

SMACNA (Sheet Metal and Air Conditioning Contractors National Association) duct construction standards are the globally accepted reference for HVAC ductwork. The equal friction method - typically 0.8 Pa/m for commercial systems - balances the duct network automatically and simplifies balancing damper adjustment. For high-velocity systems in critical spaces (hospitals, clean rooms), the velocity method is preferred to control noise and ensure LAM (laminar air movement).

Example: An AHU serves a large open-plan office needing 1200 L/s (1.2 m³/s) of supply air through a main round duct, with a maximum velocity of 6 m/s per SMACNA guidelines. Required area A = Q/v = 1.2/6 = 0.2 m², giving a calculated diameter D = √(4×0.2/π) = 0.505 m. Rounding up to the nearest standard 25 mm increment gives a 525 mm diameter duct. Checking actual velocity at this size: v = 1.2/(π/4×0.525²) = 5.54 m/s, comfortably within the 5–8 m/s SMACNA range for main ducts.