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The specified SMACNA duct leakage classification.
Common ratings range from CL 3 (very tight, e.g. hospital/cleanroom systems) up to CL 24 (typical unsealed low-pressure ductwork), with CL 6 and CL 12 commonly specified for sealed low/medium pressure systems.
The static pressure the duct system is tested at.
This should match the duct system's pressure classification (low, medium, or high pressure) per the project's mechanical specification.
The total external surface area of the ductwork being tested.
Include the surface area of all duct sections, fittings, and plenums within the test section.
Maximum allowable system leakage
47.08 CFM
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 per 100 ft² of duct surface
- 9.42 CFM/100 ft²
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SMACNA Duct Leakage Class Allowable Leakage Calculator: 47.08 CFM — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- 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.
Inputs used
- SMACNA Leakage Class (CL)
- 6
- Test Pressure (in. w.g.)
- 2
- Total Duct Surface Area
- 500 sq ft
Intermediate steps
- Allowable leakage rate per 100 ft² of duct surface
- 9.42 CFM/100 ft²
Confidence note: 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.
What this calculation does not cover
- The allowance rises with whatever area you feed it, so an over-measured test section quietly buys the system a larger budget to leak into. Use the developed sheet area of what is genuinely sealed off and under test — for rectangular duct that is 2 x (width + height) x length, unfolded — and stop at the caps. Carrying in duct beyond the test boundary, or applying the whole system's area to a section-by-section test, is how a failing section passes.
- A pass is a total, not a distribution. The same allowable CFM can be a fine mist of small leaks spread evenly over every seam, or one open joint on a single branch — and the second one starves the rooms on that branch while the system still tests inside its class. The result tells you the ductwork is tight enough overall; it says nothing about whether the air arrived where the design sent it.
- The class applies to the duct, not to the ends of it. Flexible runouts, the connections at terminal boxes and diffusers, access doors and the air handler casing itself are usually outside the tested section and outside this figure, and they are where a great deal of real system leakage lives. An installation that meets its class on paper can still be losing a meaningful share of fan air past the boundary of the test.
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This result is a specification — 47.08 CFM — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-09-06 · in the site-wide review of 2026-09-06 · v1.0.1
Regulatory standards & verification citations1
- 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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