Enter your blower-door CFM50 and conditioned volume to find your home's ACH50 air leakage result at the 50 Pa test pressure.
Advanced options
This is a blower-door test metric. It is not normal daily air exchange, required ventilation, or proof of local code compliance.
Rough normal-condition planning range
Calculated as ACH50 divided by 26. This is not a measured daily rate.
Calculated as ACH50 divided by 14. This is not a ventilation-sizing result.
Table of contents
How to use our Home Air Leakage Calculator
- Find the value labeled CFM50 or airflow at 50 Pa on the blower-door or energy-audit report. Enter it in "Blower door result (CFM50)."
- For "How will you enter conditioned volume?", choose "Use volume from the report" if the report lists conditioned, enclosed, or tested volume.
- If the report does not list volume, choose "Estimate volume from area and height." Enter the total conditioned floor area and the average ceiling height for that same tested space.
- Select "Calculate." The first result, ACH50, is the number of air changes per hour at the 50 Pa blower-door test pressure.
- Check that the displayed conditioned volume matches the home area you intended to use. An unexpectedly large or small volume can change ACH50 a lot.
- Use the normal-condition planning estimates only as rough context. They are not a measured daily air-change rate, a ventilation requirement, or a code result.
Definitions
CFM50: Airflow in cubic feet per minute measured while a blower door creates a 50 pascal pressure difference.
ACH50: Air changes per hour calculated from CFM50 during the 50 Pa blower-door test.
Conditioned volume: The enclosed volume inside the tested air boundary. Conditioned space is generally inside the thermal envelope and heated or cooled. [1]
Normal-condition ACH: A rough estimate of air changes under ordinary weather conditions, not during a 50 Pa blower-door test.
Common mistakes and quick fixes
Mistake: Entering HVAC supply airflow or airflow measured at a pressure other than 50 Pa.
Fix: Copy the blower-door report value specifically labeled CFM50 or airflow at 50 Pa.
Mistake: Including a garage, unconditioned attic, crawlspace, or another space outside the tested air boundary.
Fix: Use only enclosed conditioned space that was inside the blower-door test boundary.
Mistake: Estimating volume with the highest vaulted-ceiling measurement for the whole house.
Fix: Enter an average ceiling height across the conditioned floor area, including lower and taller rooms.
Mistake: Treating ACH50 as the home's normal daily air exchange.
Fix: Read ACH50 as a test-pressure metric. Use the normal-condition range only for rough planning context, not for ventilation sizing or code decisions.
Limitations & Key Assumptions / Boundary Conditions
- ACH50 applies only to a blower-door test at 50 Pa. It does not measure normal daily infiltration.
- The normal-condition planning range uses ACH50 divided by 26 and by 14. Actual infiltration varies with weather, wind, home height, shielding, leak locations, and other conditions.
- The area-times-height path is a volume estimate. It can be less reliable than a report volume, especially with vaulted ceilings, open stairwells, or uneven ceiling heights.
- Airflow and volume must describe the same tested air boundary. Including space outside that boundary changes the ACH50 result.
- This calculation does not size ventilation equipment, predict energy use, or determine local code compliance.
Methodology
Calculation
The calculator uses the conditioned volume from the report. If you choose the estimate path, it multiplies conditioned floor area by average ceiling height.
tested volume = conditioned floor area x average ceiling height
It changes CFM50 from cubic feet per minute to cubic feet per hour by multiplying by 60, then divides by the tested volume.
ACH50 = CFM50 x 60 / tested volume
The secondary planning endpoints use broad divisors of 26 and 14. They are not measured natural air-change rates.
lower planning estimate = ACH50 / 26
upper planning estimate = ACH50 / 14
Worked example
With 1,500 CFM50 and 15,000 cu ft of conditioned volume, the calculation is 1,500 x 60 / 15,000 = 6.00 ACH50. The rough planning endpoints are 6 / 26 = 0.23 ACH and 6 / 14 = 0.43 ACH, shown to two decimal places.