Duct leakage from SMACNA leakage class and test pressure, checked against ASHRAE 90.1 and your spec.
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Duct System Parameters
Rule of thumb: commercial systems carry very roughly 2–5 CFM per ft² of duct surface. The estimate uses 0.5 ft² per CFM (the larger-surface end): —. Leave the area blank to use it.
Leakage Results
Leakage at Class & Pressure
—
CFM
Leakage Rate
—
% of design CFM
90.1 Test Limit, Class 4
—
CFM max at this pressure
vs. 90.1 Class 4
—
tested duct: >3 in wg or outdoors
Spec Allowance
—
CFM max
vs. Spec Allowance
—
your project limit
SMACNA Leakage Class Reference
Class
CFM/100 sf at 1.0 in wg
Typical construction
3
3
Round / flat-oval metal duct, Seal Class A
4
4
ASHRAE 90.1 maximum for ductwork that must be leak-tested
6
6
Rectangular metal duct, Seal Class A
12
12
Rectangular metal duct, Seal Class B
24
24
Rectangular metal duct, Seal Class C
48
48
Unsealed rectangular metal duct (typical; actual leakage scatters widely)
SMACNA seal classes: A = all transverse joints, longitudinal seams and duct-wall penetrations sealed; B = transverse joints and longitudinal seams; C = transverse joints only. Leakage Q = CL × (SA/100) × P0.65. ASHRAE 90.1 requires ductwork to be sealed to Seal Class A (§6.4.4.2.1) and requires ductwork designed above 3 in wg, and ductwork located outdoors, to be leak-tested at CL ≤ 4 with the test pressure equal to the duct’s design pressure class (§6.4.4.2.2). It sets no percentage-of-airflow limit.
Inputs, results, and method — as an Excel workbook or plain CSV.
Doing this for a whole building? See this step inside a connected AIM Works project — loads, ventilation, equipment, and Design Review in one place.
This duct leakage calculator estimates how much air a duct system loses through its seams and joints, using the SMACNA leakage-class method. Enter the design airflow, the duct surface area (or leave it blank for an estimate of about 0.5 ft² per CFM), the SMACNA leakage class, and the static pressure. The tool returns the leakage in CFM and as a percent of design airflow, compares the class with the ASHRAE 90.1 leakage-class-4 limit for ductwork that must be leak-tested, and checks the leakage against the percent-of-airflow allowance from your project specification.
Leakage is more than a code line item: escaped air still had to be moved by the fan, so it raises operating cost. The calculator estimates the fan energy spent on the leaked air from your fan power, operating hours and electricity rate, so you can weigh the value of tighter sealing — moving from a loose class to a sealed one — against the construction effort to achieve it.
CL is the SMACNA leakage class — leakage in CFM per 100 ft² of duct surface at 1 in wg — SA is the duct surface area in square feet, and P is the static pressure in in wg. The 0.65 exponent is the pressure relationship of the SMACNA HVAC Air Duct Leakage Test Manual. Enter the test pressure to check a test; enter the duct’s operating static to estimate in-service leakage. ASHRAE 90.1 states its limit as a leakage class (CL = 4 at a test pressure equal to the duct’s design pressure class, for ductwork designed above 3 in wg or located outdoors), not as a percentage of airflow; the percentage check is against the allowance you enter from your specification. The energy figure is fan energy only and scales linearly with leaked airflow — a rough planning number, not an energy model. Surface area defaults to 0.5 ft² per CFM when left blank.
Frequently Asked Questions
What is a SMACNA duct leakage class?
A SMACNA leakage class (CL) is the leakage in CFM per 100 ft² of duct surface at 1 in wg; at another static pressure P the leakage is CL × P0.65. Lower numbers mean tighter duct: about 3 for sealed round or flat-oval metal duct, 6 for rectangular duct sealed at all joints, seams and penetrations (SMACNA Seal Class A), 12 and 24 for rectangular duct with less sealing (Seal Classes B and C), and around 48 for unsealed rectangular duct.
How is duct leakage CFM calculated?
Leakage flow equals the leakage class times the duct surface area in hundreds of square feet, times the static pressure raised to the 0.65 power: Q = CL × (SA ÷ 100) × P0.65. The 0.65 exponent is the pressure relationship used in the SMACNA HVAC Air Duct Leakage Test Manual. Enter the surface area directly, or leave it blank and the tool estimates 0.5 ft² of duct surface per CFM of design airflow.
What does ASHRAE 90.1 require for duct leakage?
ASHRAE 90.1 does not set a percentage-of-airflow limit. It requires ductwork to be sealed to SMACNA Seal Class A (§6.4.4.2.1) and requires ductwork designed to operate above 3 in wg, and ductwork located outdoors, to be leak-tested; measured leakage may not exceed 4 × P0.65 CFM per 100 ft² — leakage class 4 — with P the test pressure, equal to the duct’s design pressure class (§6.4.4.2.2). This tool shows that class-4 limit at your pressure and whether your selected class is within it. A percent-of-airflow allowance is a project-specification choice; enter yours to check it.
Why does duct leakage cost energy?
Air that escapes through duct seams still had to be moved by the fan, so the fan works harder to deliver design airflow to the rooms. This tool estimates that fan energy as leaked CFM × the system’s fan power in W per CFM × the annual equivalent full-load hours, priced at your electricity rate (defaults: 1.0 W/CFM, 3,000 h, $0.12/kWh). It is a rough planning figure: fan power rises faster than airflow, and duct that leaks outside the conditioned space also wastes the energy used to heat or cool the lost air.
Connect the calculation to the rest of the project.
AIM Works brings peak loads, ASHRAE 62.1 ventilation, equipment and plant sizing, compliance checks, Design Review, and engineering deliverables into one connected mechanical design workspace.
Peak loads (RTS or CLTD/CLF) and ASHRAE 62.1-2019 ventilation
Equipment and plant sizing against ASHRAE 90.1-2022 efficiency floors
Design Review that cites the check behind every finding
An AI co-pilot that drafts changes — you approve each one
Duct and pipe distribution sizing is planned for V2 — at launch, keep using this calculator for that step.
Results are design estimates for preliminary sizing. Verify final designs against applicable codes and standards — engineering judgment and a licensed professional engineer’s review are required.