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Duct Leakage Calculator

Turn a duct leakage test result into the metrics programs and codes use — CFM25 per 100 ft² of floor area and percent of system airflow — and compare it with your target.

Method reviewed October 26, 2026. Version 1.0.0. How it works

Your inputs

Unit system

Use the limit that applies to you (program, code edition or contract). ENERGY STAR’s new-homes program uses 4 CFM25 per 100 ft² (or 40 CFM25 if larger) for total leakage.

Some programs allow a fixed minimum for small homes; enter 0 if none.

Result

Exceeds your target

Leakage per 100 ft² of floor area
6CFM25
Leakage as a share of system airflow
10%

6 CFM25 per 100 ft² (10 % of airflow) — exceeds your target of 80 CFM25.

  • Total leakage (to inside and outside) and leakage to outdoors are different tests with different limits. Compare like with like.
Allowed by your target
80CFM
Show the working
  1. Per 100 ft² = CFM25 ÷ floor area × 100
    120 ÷ 2,000 × 100 = 6
  2. Share of airflow = CFM25 ÷ system CFM
    10 %
  3. Allowed = larger of target × area ÷ 100 and the minimum
    80 CFM25

Reading a duct leakage test

A duct leakage test pressurises the duct system to 25 pascals and measures the airflow needed to hold it there: the leakage in CFM25. To compare houses of different sizes, programs divide by the conditioned floor area and express it per 100 ft².

There are two common tests. Total leakage counts every leak, inside and outside the house; leakage to outdoors counts only leaks to unconditioned space. They have different limits, so make sure the target you enter matches the test you ran.

Targets

Limits depend on the program, the code edition your jurisdiction adopted and the test type. As one example, ENERGY STAR’s new-homes program uses 4 CFM25 per 100 ft² of floor area, or 40 CFM25 if that is larger, for the total-leakage provision it describes. Enter the limit that applies to your project.

Worked example

These are the example values the calculator opens with, and the result the engine calculates for them.

Inputs

Measured leakage at 25 Pa (CFM25)
120 CFM
Conditioned floor area served
2,000 ft²
System airflow
1,200 CFM
Target leakage (CFM25 per 100 ft² of floor area)
4
Minimum allowance (CFM25)
40 CFM

Result

Leakage per 100 ft² of floor area
6 CFM25
Leakage as a share of system airflow
10 %
Allowed by your target
80 CFM

6 CFM25 per 100 ft² (10 % of airflow) — exceeds your target of 80 CFM25.

Assumptions

  • CFM25 is the leakage measured with the ducts pressurised to 25 Pa, as reported by the test equipment.

Limitations

  • Limits differ by program, code edition and test type; enter the one that applies.

Frequently asked questions

What is CFM25?

The airflow, in cubic feet per minute, that leaks from the ducts when they are held at 25 pascals of pressure during the test.

Is percent of airflow the same as per 100 ft²?

No. Percent of airflow relates leakage to how much air the system moves; per 100 ft² relates it to the size of the home. Programs usually specify one or the other.

How can duct leakage be reduced?

Sealing joints and connections with mastic or approved tape, sealing the air handler cabinet and boots, and moving ducts inside the conditioned space all reduce leakage.

Sources

  1. Duct Leakage to Outdoors Test Exemptions (SFNH Rev 11 / MFNC Rev 02) — ENERGY STAR (U.S. EPA). Duct leakage expressed in CFM25 per 100 ft² of conditioned floor area; total duct leakage ≤ 4 CFM25 per 100 ft² or 40 CFM25, whichever is larger, for the cited ENERGY STAR provision.

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