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SMACNA Duct Leakage Class Allowable Leakage Calculator

Calculate the maximum allowable duct system leakage rate for a given SMACNA Leakage Class, test pressure, and duct surface area.

Computed in your browser — nothing you enter is uploaded. Figures are presented for United States against IRC 2024, and every formula is cited under regulatory standards below.

Last verified 2026-08-26 · v1.0.0

Market
Imperial · sales tax

Maximum allowable system leakage

47.12 CFM

High confidence

The specified Leakage Class and test pressure must match the project's mechanical specification and SMACNA duct pressure classification (low/medium/high pressure) — this calculator applies the SMACNA formula to whatever class and pressure you supply, it does not select the required class for you.

Allowable leakage rate
9.42 CFM/100ft²

For the dimensions entered, expect a maximum allowable system leakage of 47.1 CFM. Note that total duct surface area sits at the low end of the range this calculator was checked against, so treat the output as indicative rather than settled. Set for United States against IRC 2024. The market selector changes both the units and the code cited.

Add the equipment this sizes

This result is a specification — 47.124 CFM — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.

Preliminary estimate, not certified engineering. This tool produces an indicative quantity calculation for planning purposes only — it is not a certified structural analysis, a guaranteed material takeoff, or a substitute for building department approval. Always verify measurements on-site and have a licensed contractor or structural engineer review any load-bearing, code-sensitive, or safety-critical work before purchasing materials or starting construction. Spotted an arithmetic or standards error? Report it to contact@craftquantities.com with your inputs — confirmed fixes become pinned regression tests.

[Schema Verified] Computed in alignment with American Concrete Institute (ACI 318-19) formulas and International Residential Code (IRC 2024) spatial boundaries.

Regulatory standards & verification citations

  • Allowable leakage (CFM per 100 ft² of duct surface) = Leakage Class (CL) × (Test Pressure, in. w.g.)^0.65, per the SMACNA HVAC Air Duct Leakage Test Manual. Total allowable leakage = that rate × (total duct surface area ÷ 100). Common Leakage Class ratings range from CL 3 (very tight, e.g. hospital/cleanroom systems) to CL 24 (typical unsealed low-pressure ductwork), with CL 6 and CL 12 commonly specified for sealed low/medium pressure systems.

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Frequently asked questions

What does SMACNA Leakage Class actually rate?
It rates how airtight the duct construction and sealing must be — Leakage Class ratings range from CL 3 (very tight, e.g. hospital/cleanroom systems) to CL 24 (typical unsealed low-pressure ductwork), with CL 6 and CL 12 commonly specified for sealed low/medium pressure systems. A lower class number means a tighter (less leaky) duct system.
How does test pressure affect the allowable leakage rate?
Allowable leakage scales with test pressure raised to the 0.65 power, per the SMACNA HVAC Air Duct Leakage Test Manual — so a higher test pressure allows a higher leakage rate per 100 ft² of duct surface for the same Leakage Class, since more air naturally escapes through the same size opening at higher pressure.
Does this calculator tell me which Leakage Class my project requires?
No — the specified Leakage Class and test pressure must match the project's mechanical specification and SMACNA duct pressure classification (low/medium/high pressure). This calculator only applies the SMACNA formula to whatever class and pressure you supply; it does not select the required class for you.