Blower Door ACH50 & Natural Air Changes Calculator

Turns a blower door CFM50 reading into ACH50, estimates natural air changes per hour, compares ACH50 with the 2021 IECC limit for your climate zone, and gives the ASHRAE 62.2 total ventilation rate.

For homeowners, energy auditors and builders.

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Results will appear here once you enter your values.

How the numbers are calculated

ACH50 is CFM50 x 60 / volume. Natural ACH is ACH50 / N. CFM50 per sq ft of floor is CFM50 divided by floor area. The IECC check compares ACH50 with 5 (zones 1-2) or 3 (zones 3-8). The 62.2 total is 0.03 x floor area + 7.5 x (bedrooms + 1).

The defaults are 1,800 CFM50, 16,000 cubic feet, 2,000 sq ft, climate zone 4, two stories and 3 bedrooms. ACH50 is 6.75. N is 14.8, so natural ACH is 0.46. Leakage is 0.90 CFM50 per sq ft of floor. The zone 4 limit is 3 ACH50, so the result shows 'Above limit', and 62.2 total ventilation is 90 CFM.

  • Stories: 1 to 3.
  • Climate zone: 1 to 8.
  • Bedrooms: whole numbers from 1.

What this tool is not

This is not a code inspection or a certified test report. A passing number depends on a properly performed blower door test by a qualified tester, correct volume measurement and the exact code edition your jurisdiction enforces. Sealing leaks can also affect combustion appliance safety, so a tight home needs a combustion safety check by a professional.

  • Be careful with gas appliances: sealing a home can change how furnaces and water heaters vent.
  • Ask the building department: limits and test timing vary.
  • Hire a rater: for an official test or code compliance letter.

Sources and next steps

Sherman and Grimsrud, Lawrence Berkeley Laboratory, N-factor method for estimating natural infiltration from blower door data. ENERGY STAR Home Sealing Specification (2001), Table 4. International Code Council, 2021 International Energy Conservation Code, R402.4.1.2. ASHRAE Standard 62.2-2022, Ventilation and Acceptable Indoor Air Quality in Residential Buildings, Section 4.1.1.

Where the leaks are matters as much as the total. For airflow for a single room, use the CFM calculator. For next steps on heat loss, try the attic insulation calculator and the heating load calculator.

Frequently Asked Questions

Common questions about the Blower Door ACH50 & Natural Air Changes Calculator

ACH50 equals CFM50 times 60 divided by the conditioned volume in cubic feet. CFM50 is the airflow a blower door needs to hold the house at 50 Pascals of pressure difference. For 1,800 CFM50 in a 16,000 cubic foot house, the result is 1,800 x 60 / 16,000 = 6.75 air changes per hour at 50 Pa. Use the volume inside the air barrier, including conditioned basements.

Sources & References

International System of Units (SI): energy, work and heat

Energy, work and heat is measured in the joule (J); 1 cal = 4.184 J. Conversions between SI and other units use exact, internationally agreed factors maintained by NIST.
→Source: NIST — International System of Units (SI)

International System of Units (SI)

Authoritative definitions for energy, work and heat, from the BIPM SI Brochure (9th edition), the defining reference for the SI.
→Source: BIPM — SI Brochure (9th ed.)

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