Duct Leakage Test-and-Balance

SMACNA leakage class from measured-vs-design CFM at test pressure.

Run the calculator

Example

You enter

Design CFM 1000
Measured CFM at registers 990
Duct surface area (ft²) 300
Test pressure (in WC) 1
Design class 4

You get

Leak at 1inwc 10
Leak per 100ft2 3.333
Effective class 4
Target label Class 4 - ASHRAE 90.1 limit (all duct)
Pass Yes

Checked against published worked examples from: SMACNA HVAC Air Duct Leakage Test Manual + ASHRAE 90.1; ACCA Manual D + SMACNA HVAC Duct Construction Standards leakage classes.

Details, formula, and sources

ScopePass/fail against the design class, including the ASHRAE 90.1 class 4 limit and the 2012 SMACNA classes (2/4/8/16) alongside the 1985 ones (3/6/12/24/48).

Formula or tableleakage_cfm = design_cfm - measured_cfm. leak_at_1inwc = leakage_cfm / (test_pressure_inwc)^0.65 (SMACNA leakage-class flow exponent). leak_per_100ft2 = leak_at_1inwc / duct_surface_ft2 × 100. Effective class = smallest SMACNA class (2, 3, 4, 6, 8, 12, 16, 24, 48) whose limit ≥ leak_per_100ft2; above every class it reports past every class.

Source and editionSMACNA HVAC Air Duct Leakage Test Manual (2nd ed., 2012) by name. ASHRAE 90.1-2022 (Energy Standard for Sites and Buildings Except Low-Rise Residential Buildings) §6.4.4.2.2, leakage class 4 for tested duct.

Free accessSMACNA standards licensed; class-system overview free at smacna.org outreach. Free read-only access to ASHRAE standards at ashrae.org (Technical Resources, read-only standards).

What governsEstimate. AHJ and licensed mechanical contractor govern. ACCA Manual J / D / S supersede rules of thumb.

API field namesdesign_cfm, measured_cfm, duct_surface_ft2, test_pressure_inwc, design_class, leak_at_1inwc, leak_per_100ft2, effective_class, target_label, pass

Leakage scales with P^0.65 SMACNA leakage-class flow exponentSMACNA Duct Leakage Test Manual
Leakage classes 2 / 4 / 8 / 16 (2012) and 3 / 6 / 12 / 24 / 48 (1985) cfm per 100 ft² at 1 in WC; ASHRAE 90.1 limit is class 4SMACNA HVAC Air Duct Leakage Test Manual 2012 / ASHRAE 90.1-2022 §6.4.4.2.2