A preliminary check of a test result against the model code text. Your adopted local code, its amendments and the code official decide what passes. The test itself follows ANSI/RESNET/ICC 380 or ASTM E1554.

Maintained by BuildSolver · Updated October 16, 2026

Calculator · Duct testing

Duct leakage test calculator

A duct tester reading means little until it is divided by the floor area the system serves. A 2,000 sq ft system that leaks 72 CFM25 is at 3.60 CFM25 per 100 sq ft, which passes a 2021 IECC rough-in test with the air handler set and fails the same test without it. Enter your reading to get the number and the limit for your edition. For a blower door test of the whole house, use the air changes per hour calculator instead.

Choose the 2021 or 2024 IECC, total leakage or leakage to outside, and when the test was run.
Type the conditioned floor area the system serves and the duct tester flow at 0.1 inch water gauge (25 Pa).
The calculator returns leakage per 100 sq ft, the allowed CFM25 and whether the reading passes.

Total leakage at 0.1 inch water gauge (25 Pa) against the model 2021 or 2024 IECC. A preliminary result, the adopted local code and the code official come first.

Energy code edition

The edition your jurisdiction adopted. States and cities often amend these limits, so check the local code.

Type of test

Total leakage is the duct tester alone with every register sealed. Leakage to outside runs a blower door at the same time. Both editions set their limits on total leakage.

When the test was run

Used for a total leakage test. Rough-in is before drywall, postconstruction is after the registers are in.

Conditioned floor area this system serves, sq ft

With two systems, enter the floor area each one serves and test them one at a time.

Measured leakage, CFM25

Duct tester flow at 25 Pa (0.1 inch water gauge) in the duct system.

System airflow, CFM (optional)

Design or measured blower airflow. Only used for a reference share, not for pass or fail.

Example result for the default inputs

2021 IECC, total leakage, 2,000 sq ft served by this system.

Leakage per 100 sq ft of floor area

3.60 CFM25

Maximum allowed

80 CFM25

Result

Passes

Calculation details
Measured total leakage	72 CFM25
Limit applied	4.0 CFM25 per 100 sq ft x 2,000 sq ft / 100 (2021 IECC R403.3.6 item 1)
Room under the limit	8 CFM25
Test pressure	0.1 inch water gauge (25 Pa) across the duct system

For estimation purposes only. Consult a licensed engineer for final designs.

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How the leakage check works

The duct tester gives one number, the flow it takes to hold the ducts at 0.1 inch water gauge (25 Pa). The code limit is a flow too, scaled by the floor area the system serves, so the check is a single comparison.

Leakage per 100 sq ft: CFM25 / CFA x 100CFA is the conditioned floor area served by the duct system you tested. With two systems, each one is judged on its own share of the house.
Allowed leakage, 2021 IECC: limit per 100 sq ft x CFA / 100R403.3.6 sets 4.0 at rough-in with the air handler installed, 3.0 at rough-in without it, 4.0 at postconstruction, and 8.0 where all ducts and air handlers are inside the building thermal envelope. All are total leakage, less than or equal to the limit.
Allowed leakage, 2024 IECC: Table R403.3.8 row x CFA / 100, or a fixed CFMAbove 1,000 sq ft the limit is per 100 sq ft and depends on the row and on fewer than 3 or at least 3 ducted returns, from 3 up to 12. At 1,000 sq ft or less the limit is a fixed 30 to 80 CFM25 for the whole system.
Pass or fail: measured CFM25 <= allowed CFM25Both editions word the limit as less than or equal to, so a reading exactly at the limit passes. The comparison uses the unrounded numbers.
Share of airflow (reference): CFM25 / system CFM x 100Optional and not a code test. The leak is measured at 0.1 inch water gauge (25 Pa), and real duct pressures differ along the system, so treat the percent as a rough sense of scale.

The same reading under different rules

Six test results run through the same function the form uses. The limit changes with the edition, the stage of the test, the number of ducted returns and the size of the system.

Duct leakage results against the 2021 and 2024 IECC limits
Test	CFA, sq ft	CFM25	Per 100 sq ft	Allowed	Result
2021, rough-in, air handler set	2,000	72	3.60	80	Passes
2021, rough-in, no air handler yet	2,000	72	3.60	60	Fails
2021, all ducts inside the envelope	2,000	150	7.50	160	Passes
2024, all components, 2 returns	2,000	72	3.60	80	Passes
2024, all components, 3 returns	2,000	110	5.50	120	Passes
2024, small system, 900 sq ft	900	45	5.00	40	Fails

The first two rows are the same 72 CFM25. With the air handler set the 2021 limit is 80, without it 60, so testing before the equipment is in costs 20 CFM of allowance. Under the 2024 table a third ducted return raises the allowance on a 2,000 sq ft system from 80 to 120 CFM25. Small systems work the other way. A 900 sq ft system gets a flat 40 CFM25, about 4.44 per 100 sq ft, so 45 CFM25 fails there.

Standards and data sources

2021 International Energy Conservation Code, R403.3.5 and R403.3.6 - the test methods (ANSI/RESNET/ICC 380 or ASTM E1554, 0.1 inch water gauge across the system) and the total leakage limits of 4.0, 3.0, 4.0 and 8.0 CFM per 100 sq ft of conditioned floor area.
2024 International Energy Conservation Code, R403.3.7, R403.3.8 and Table R403.3.8 - testing of each duct system at 0.1 inch water gauge (25 Pa), and the maximum total leakage by configuration, ducted returns and floor area served.
ANSI/RESNET/ICC 380 and ASTM E1554 - the test procedures both editions point to. The calculator takes their result, it does not replace them.
Building Science Education Solution Center (U.S. DOE), Duct Leakage Testing lessons - the field procedure of reading the flow with the pressure probe in the return and again in a far supply, and averaging the two.

Limitations of this calculator

Limits are the model 2021 and 2024 IECC text. Many states and cities amend them, add a leakage to outside option or keep an older edition. Check the code your jurisdiction adopted before you report a pass.
Leakage to outside gets its per-100 sq ft number but no pass or fail, because neither edition limits it on this path. The 2024 text says a to-outside result is not used for compliance with R403.3.8.
Under 2024 R403.3.4, ductwork buried in attic insulation counts as inside conditioned space only when a rough-in test or a test of leakage to outside shows no more than 1.5 CFM per 100 sq ft. That separate check is not run here.
The 2024 IECC skips testing for systems with no more than 10 ft of ductwork outside the equipment, entirely in conditioned space and with no building-cavity plenums, and for ventilation-only ducts. Decide whether a test is required before you use this page.
Multifamily sampling under 2024 R403.3.9 is not covered.
Exception 3 to 2024 R403.3.7 limits a test without the equipment installed to 3.0 CFM per 100 sq ft, while Table R403.3.8 allows 4 with 3 or more ducted returns. The calculator follows the table. Above 1,000 sq ft with 3 or more ducted returns, a reading that passes only under the table gets a warning in the result. Ask the code official which one they apply.
The calculator trusts the reading. A wrong fan ring setting, a register left open or a hose outside the duct gives a wrong CFM25 that this page cannot spot.
The share of airflow is a reference ratio at test pressure. It is not a code limit and not the leakage at operating conditions.

Common duct test mistakes

Dividing by the whole house when there are two systems. Each system is tested and judged against the floor area it serves. Two systems on a 3,000 sq ft house split that area, and each gets its own limit.
Reporting leakage to outside as the code number. A to-outside test is useful for finding where the air goes, but the 2021 and 2024 limits are on total leakage. Run the total test for the report.
Using the rough-in limit after the air handler is set. Without the air handler the 2021 limit is 3.0 CFM per 100 sq ft, with it the limit is 4.0. Pick the row that matches the day of the test, not the day the permit was pulled.
Counting panned returns as ducted. Table R403.3.8 counts a return as ducted only when sheet metal or flex duct runs from the grille to the air handler. A panned joist or a transfer grille does not move you to the 3 or more column.
Testing with one pressure reading. Duct pressure is not the same in every branch. Reading with the probe in the return and again in a far supply, then averaging the two flows, gives a fairer number than either one.

Common questions about duct leakage testing

What is the allowable duct leakage under the 2021 IECC?

Total leakage at 0.1 inch water gauge (25 Pa) of no more than 4.0 CFM per 100 sq ft of conditioned floor area at rough-in with the air handler installed, 3.0 without it, 4.0 at postconstruction, and 8.0 when all ducts and air handlers are inside the building thermal envelope. For a 2,000 sq ft system at rough-in with the air handler set, that is 80 CFM25.

What changed for duct leakage in the 2024 IECC?

The 2024 edition moved the limits into Table R403.3.8. Above 1,000 sq ft the limit rises with 3 or more ducted returns, for example from 4 to 6 CFM per 100 sq ft with all components installed. Systems serving 1,000 sq ft or less get a fixed limit, 40 CFM25 with all components installed. Leakage is still total leakage at 0.1 inch water gauge (25 Pa).

How do I calculate CFM25 per 100 square feet?

Divide the duct tester reading by the conditioned floor area the system serves and multiply by 100. A 2,000 sq ft system that leaks 72 CFM25 is at 3.60 CFM25 per 100 sq ft.

Is duct leakage to outside allowed for code compliance?

Not under the model 2021 or 2024 IECC prescriptive limits, which are written on total leakage. The 2024 text says a to-outside result can feed the performance calculations of R405 and R406 but is not used for compliance with R403.3.8. Some states amend this, so check your local code.

Do I have to test ducts that are all inside the house?

Under the 2021 IECC yes, with a looser limit of 8.0 CFM per 100 sq ft. The 2024 IECC skips the test only when the system is entirely in conditioned space, has no more than 10 ft of ductwork outside the equipment and has no plenums built from building cavities or drywall.

What duct leakage do small systems get under the 2024 IECC?

A system serving 1,000 sq ft or less gets a fixed CFM limit whatever the number of returns, from 30 CFM25 with the equipment not installed to 80 CFM25 with everything installed in conditioned space. A 900 sq ft system tested under the 2021 rules at rough-in with the air handler set would get 36 CFM25 instead.

Why average two readings in a duct leakage test?

Duct pressure is not the same in every branch, so one probe location can overstate or understate the leak. The DOE Building Science Education Solution Center lesson has you read with the probe in the return, read again in a far supply, and average. Readings of 40 and 120 CFM average to 80 CFM and point to the supply side as the bigger leak.

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