Duct Leakage Rate Calculator
Calculate duct leakage airflow (CFM) based on duct surface area, leakage class, and test pressure differential. Uses SMACNA and ASHRAE 90.1 standard leakage equations.
Formula
FL = CL × (A / 100) × Pn
- FL — Duct leakage airflow (CFM)
- CL — Leakage class (CFM per 100 ft² of duct surface area at 1 in. w.g.)
- A — Total duct surface area (ft²)
- P — Test static pressure differential (in. w.g.)
- n — Pressure exponent (0.65 per SMACNA; range 0.5–0.75)
Leakage percentage: % = (FL / Qsystem) × 100
Assumptions & References
- Formula per SMACNA HVAC Air Duct Leakage Test Manual and ASHRAE Standard 90.1.
- Leakage class (CL) values are defined at a reference pressure of 1.0 in. w.g.
- Standard pressure exponent n = 0.65 is used per SMACNA for typical sheet metal duct joints and seams.
- ASHRAE 90.1 requires duct leakage ≤ 4% of system design airflow for sealed duct systems.
- For pressurization testing, typical test pressures are 0.5–2.0 in. w.g. depending on duct pressure class.