Calculate allowable duct leakage and pass/fail for
DW/144 (UK, Class A–D) and SMACNA (CL method).
About this duct leakage calculator
This free duct leakage calculator works out the allowable air leakage for a length of ductwork and gives an immediate pass or fail against a measured test result. It supports two methods: BSRIA DW/144 (the UK ductwork specification, leakage Classes A–D) and the SMACNA HVAC Air Duct Leakage Test Manual (the CL method). It is built for UK building-services engineers, ductwork contractors and commissioning engineers who need to size an allowable leakage figure or check a site leakage test on the spot. Enter the duct surface area, the test pressure and your measured leakage in the units you have to hand, m² or ft², L/s, m³/h or cfm, Pa or in.w.g. and the result is shown alongside a full calculation breakdown.
How the allowable leakage is calculated
For DW/144 the permitted leakage rate is q = k · p0.65 in L/s per m² of duct surface, where k is the class constant (Class A = 0.027, B = 0.009, C = 0.003, D = 0.001) and p is the test pressure in Pa. Multiplying that rate by the duct surface area gives the total allowable leakage, and the measured leakage is judged pass or fail against it. For SMACNA the leakage factor is F = CL · P0.65 in cfm per 100 ft², where CL is the leakage class (3, 6, 12, 24, 48 or a custom value) and P is the test pressure in inches of water gauge; the allowable is F × (area / 100), and the tool also back-calculates the measured CL. Methods follow BSRIA DW/144 and the SMACNA HVAC Air Duct Leakage Test Manual.
Frequently asked questions
Which standard should I use, DW/144 or SMACNA?
DW/144 is the UK ductwork specification and uses Classes A to D, so it is the usual choice for UK projects; SMACNA is the US manual and uses CL leakage classes. The calculator supports both, pick the one named in your project specification.
What test pressure should I enter?
Use the pressure called up in your commissioning specification. The DW/144 method offers presets from 125 Pa to 2000 Pa and an editable field for any project-specific value, while the SMACNA method takes the test pressure in inches of water gauge or Pa.
Can I mix units for area and leakage?
Yes. Duct area can be entered in m² or ft², and measured leakage in L/s, m³/h or cfm. The calculator converts internally and reports the allowable and measured figures so you can compare like for like.
For design guidance only. Always verify against your project commissioning requirements and with a qualified engineer.