SMACNA Duct Leakage Calculator
For tab technicians running a SMACNA leakage test on new duct before the system is handed over — enter the duct surface area and test pressure, pick the seal class and duct shape, and read the allowable CFM you must stay under during the duct-blaster test.
Duct Leakage Calculation
-- CFM
SMACNA Leakage Class (CL) Values
| Seal Class | Rectangular Duct | Round or Flat Oval Duct |
|---|---|---|
| A | 6 | 3 |
| B | 12 | 6 |
| C | 24 | 12 |
How to Use This Calculator
- Enter the duct surface area in sq ft for the section being tested.
- Enter the test pressure in in. w.g. (e.g. 1.0 or 2.0).
- Choose the seal class (A, B or C) and the duct shape (rectangular, or round or flat oval).
- Read the max allowable leakage in CFM, or type a custom CL in the override field to use a specified leakage class instead.
FAQ
- Q: What formula does SMACNA use for duct leakage?
- A: F = CL × P^0.65, expressed in CFM per 100 sq ft, where P is the test pressure in in. w.g.; total allowable leakage = CL × (Area ÷ 100) × P^0.65.
- Q: What CL value do I use for rectangular versus round duct?
- A: For rectangular duct the leakage class is 6, 12 or 24 for seal classes A, B and C; for round or flat oval duct it is 3, 6 or 12.
- Q: Why is the leakage rate per 100 sq ft instead of total CFM?
- A: Because F = CL × P^0.65 is defined per 100 sq ft of duct surface, so the calculator multiplies it by Area ÷ 100 to get the allowable CFM for your actual duct area.
- Q: What happens if I enter zero area or zero pressure?
- A: The calculator returns an error and no CFM value, because the formula needs an area and a pressure greater than zero to produce a meaningful allowable leakage.