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.

Duct Leakage Test Calculator

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.

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:

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. 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 — a pass.

Code thresholds

Test type	IECC 2021 limit
Post-construction (final)	≤ 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

Notes

The percentage-of-airflow figure is shown for ACCA Manual D commissioning reference, where 6% or less is a common “tight system” target. Always seal, then re-test: most failures come from disconnected boots, unsealed plenum seams, and leaky return chases. Local code amendments can tighten these limits, so confirm the adopted version in your jurisdiction.