What does CFM25 mean?

CFM25 is the airflow, in cubic feet per minute, that the duct blaster fan must move to hold the duct system at 25 Pascals of pressure relative to the house. It is the standard measure of total duct leakage in a code-compliance test.

How does IECC 2021 measure duct leakage?

IECC 2021 expresses the limit as CFM25 per 100 square feet of conditioned floor area, not as a percentage of airflow. The total CFM25 reading is divided by the floor area and multiplied by 100 to get the metric that is compared to the code threshold.

What are the IECC 2021 thresholds?

Post-construction and rough-in tests with the air handler installed must be 4.0 CFM25 per 100 ft² or less. A rough-in test before the air handler is installed must be 3.0 CFM25 per 100 ft² or less. Tighter local amendments may apply.

Why also show percentage of system airflow?

ACCA Manual D and many commissioning specs target leakage as a share of system airflow, where roughly 6% or less is considered a tight residential system. It is a useful diagnostic even though IECC compliance is judged per 100 ft².

What is the difference between total and leakage-to-outside?

Total leakage tests the whole duct system to anywhere. Leakage-to-outside tests only the leakage that escapes the conditioned envelope, using a blower door alongside the duct blaster. Energy-wise, leakage to outside is what wastes conditioned air.

What is the Duct Leakage Test Calculator?

Takes duct blaster CFM25 test results, system airflow, and conditioned floor area to compute total duct leakage as CFM25 per 100 ft² and as a percentage of system airflow, flagging pass or fail against IECC 2021 code thresholds. For HVAC contractors and energy auditors commissioning systems. It runs free in your browser on Gera Tools, with nothing uploaded.

Duct Leakage Test Calculator

Name: Duct Leakage Test Calculator
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

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This calculator turns a duct blaster reading into the two numbers that matter for commissioning and energy-code compliance: leakage per 100 ft² of conditioned area (the IECC 2021 metric) and leakage as a percentage of system airflow (the Manual D diagnostic). It then flags pass or fail against code.

How it works

A duct blaster pressurises the duct system to a reference 25 Pascals and reports the fan flow needed to hold that pressure — the CFM25 reading. From that number, two rates are derived:

leakage per 100 ft² = CFM25 ÷ conditioned area × 100
% of system airflow  = CFM25 ÷ system airflow × 100

IECC 2021 judges compliance on the per 100 ft² figure alone. For a system with 80 CFM25 leakage serving 2,000 ft²:

80 ÷ 2000 × 100 = 4.0 CFM25 / 100 ft²

That exactly meets the 4.0 limit for a final, air-handler-installed test.

IECC 2021 code thresholds

Test type	IECC 2021 limit
Post-construction (final, air handler installed)	≤ 4.0 CFM25 / 100 ft²
Rough-in, air handler installed	≤ 4.0 CFM25 / 100 ft²
Rough-in, no air handler	≤ 3.0 CFM25 / 100 ft²

The rough-in-only test (air handler not yet installed) has a tighter limit of 3.0 because the duct system cannot be fully pressurised through the air handler connections, so some of the total leakage is not captured — the stricter threshold compensates.

The Manual D percentage metric

While IECC uses per-100-ft² for code compliance, ACCA Manual D commissioning practices use percentage of system airflow as a design and diagnostic target. A common target in residential systems is 6% or less for a tight installation. This figure is useful for diagnosing whether leakage is affecting system performance and comfort, independent of the code pass/fail result.

For example, a system with 100 CFM25 leakage and a 1,200 CFM nominal airflow has a leakage rate of 8.3% — acceptable under IECC 2021 if the floor area is large enough, but poor performance from a comfort and efficiency perspective.

Where leakage typically comes from

Failing a duct test almost always traces to a small number of locations. The most common are:

Disconnected boots where the supply or return register collar separates from the duct
Unsealed plenum seams — the large supply or return box around the air handler
Leaky return chases constructed from framing cavities rather than sheet metal
Unsealed penetrations where ducts pass through framing
Duct joints that were not mastic-sealed or foil-taped

Seal suspect areas with mastic (preferred) or UL 181-rated foil tape and re-test before finalising. Local code amendments may tighten the IECC thresholds further, so confirm the adopted version in your jurisdiction before reporting compliance.